Lynn Olson's terrific article about the basics of speaker design and construction.

PART 1

This is the first section of an article published in the Vol. 3, Issue 6 ofPositive Feedback magazine. It is my *personal* view of the art ofdesigning high-end loudspeakers, and is not intended as any kind of "bible"for folks new to high-end audio. Since I've gotten a lot of questions aboutthe kind of thing that appears in Positive Feedback, it's probably simplestto demonstrate by reposting a complete article in this forum.------------------snip-----------------

* Looking Over My Shoulder, Part I *

by Lynn Olson

* Introduction

Where did I come from?Where am I going?Who am I?

These ancient questions of philosophy are the first questions you must askyourself if you are serious about designing audio equipment. Thesequestions repeat themselves in only slightly altered form:

What is the history of the art of sound?What is is the true potential of the future?And what, really, do you want to do?

* The Future

If you relax and make a mental journey to the far future, it is easy toimagine the perfect loudspeaker. It would made of an immense number of tinypoint sources that would create a true acoustic wavefront (or soundfield).Resonances due to massive drivers, cabinets, or frames would be a thing ofthe distant past. A myriad of waveform distortions (harmonic,intermodulation, crossmodulation, frequency, phase, and group delay) wouldbe utterly absent ... the sound would be literally as clear as air itself.

This perfect loudspeaker would be made of trillions of microscopic coherentlight and sound transducers, integrated with signal processing circuits alloperating in parallel. (This is similar to present-day militaryphased-array radars, with tens of thousands of tiny antennas withintegrated electronics subsystems.) It would be constructed by acombination of nanotechnology and genetic engineering and operate at themolecular level, appearing simply as a very thin film when not inoperation.

Just for a moment, imagine a thin-film mirror for 3-dimensional sound andimages with a near-infinite random-access memory that is also connected toall other mirrors; it would be "transparent" in a lot more ways than one.

* The Present

Whew! Now that we've glimpsed perfection it shows just how far we have togo in 1994. Here's a partial list of the major problems we face now:

* 2-speaker stereo falls woefully short of a true acoustic wavefront,producing a phasey, unrealistic image of small size that causes listeningfatigue for many people (particularly non-audiophiles). The virtual imageis unstable with respect to listener position, spectral energydistribution, and room characteristics.

Even a simple central mono image has been shown to suffer from deepcomb-filter cancellation nulls between between 1kHz and 4 kHz, which is whya solo vocalist sounds different coming from a single mono speaker and aconventional stereo pair. It now appears that 2-channel stereo requires aminimum of 3 speakers to faithfully represent the tonal quality ofcentrally located sound sources, such as vocalists.

* Large amounts of harmonic, intermodulation, and crossmodulationdistortions combine with mechanical driver resonances to concentratespectral energy at certain frequencies. Driver damping techniques usuallyimprove spectral characteristics (the frequency response curve looks betteras a result) but do not provide much improvement for the underlying breakupmodes, so the distortions may actually be spread over a much broaderfrequency range. The narrowband nature of resonant distortions inloudspeakers is why a single-frequency THD or IM measurement is useless; ittakes an expensive tracking-generator type of measuring system in order to

create a usable frequency vs. harmonic distortion graph.

In order to eliminate these resonant distortions, the driver diaphragmneeds to have a density equal to air and absolutely uniform accelerationover the entire surface at all frequencies. As you can imagine, we arenowhere close to meeting this criterion. As a result, all speakers havetonal colorations ranging from subtle to gross, with some types ofcolorations present at all times, and other types of colorations appearingonly at high or low levels. A reviewer's preferences in music can easilymask the presence of these problems, so make sure to get a second (andthird) opinion.

* Standing-wave resonant energy is stored in drivers (all types, except for"massless" exotics), cabinets, and in the listening room itself. Theunwanted mechanical energy must be quickly removed in two ways: rigid,low-loss mechanical links to the earth itself (a rigid path from the magnetto stand to floor to ground), and also dissipated as heat energy inhigh-loss, amorphous materials such as lead, sand, sorbothane, etc. Theenergy that is not removed is slowly released from every single mechanicalpart of the speaker, each of which has its own resonant signature.

In any real speaker system, regardless of operating principle, there arehundreds of standing-wave resonances at any one time, which are graduallyreleased over times ranging from milliseconds to several seconds. Theseresonances continually overlay the actual structure of the music and alterthe tonal color, distort and hide the reverberent qualities of the originalrecording, and deform and blur the stereo image.

In speakers that measure "textbook-perfect", this type of "hidden"resonance is the dominant source of coloration. This is also the reasonthat 1/3 octave pink-noise measurement techniques have fallen out of favor,being replaced by much more sensitive techniques such as TDS, FFT, MLSSA,and others.

* Radiation patterns shift dramatically with frequency, and change sharplyat crossover points; in addition, the radiation pattern is further deformedby diffractive re-radiation at every sharp cabinet edge (regardless ofcabinet size or type - this includes planar types of loudspeakers).

Diffraction, which occurs at every sharp cabinet boundary, creates delayed,

reverse-phase phantom sources that combine with the direct sound from theactual driver. These secondary phantom images create significant ripples inthe midrange response (up to 6 dB) and create delayed sounds whichinterfere with the timing cues necessary to perceive stereo images. Thesedispersion problems are audible as room-dependent colorations, harsh or dimmidrange and treble, fatiguing stereo, and a "detenting" effect that pullsimages in towards the loudspeaker cabinets.

* This list only covers some of the problems of contemporary loudspeakers.There are other problems, not as severe, but still quite audible to askilled listener. These problems occur in all loudspeaker types - dynamicdirect radiator, horns, ribbons, electromagnetic planar, electrostaticplanar, you name it. They all have lots of THD and IM distortionconcentrated at certain frequencies, they all store and release significantamounts of resonant energy, and they all have frequency-dependentdispersion further degraded by diffractive re-radiation.

This is why I treat claims of "perfection", or of a "major breakthrough",with a big grain of salt. Does it address even one of the serious flawscited above? Not too often. By contrast, what we're really seeing is asteady, progressive improvement in materials technology and big steps inmeasurement technology and computer modelling. (I, for one, am grateful,since it's been a major improvement compared to what I had to work with atAudionics in the mid-Seventies!)

* The Past

Modern speakers are far better than the speakers of the Fifties (which Istill remember). Very few people had full-blown Altec "Voice of theTheatre" A-5 systems, 3-way Bozak B-305's, or Klipschorns. Most "hi-finuts" had to put up with University, Jensen, or Electro-Voice 12" coaxialdrivers in big plywood boxes with a single layer of fiberglass on the rearwall. A large cutout served as the vent, resulting in boomy, resonant boxestuned much too high, with 6 to 12 dB peaks in the 80 to 150 Hz region.(Have you ever heard a restored jukebox?)

The coax, or worse, triax drivers went into paper cone breakup at 200 Hzand above, cavity resonances (due to the horn element mounted in the conedriver) at 800 Hz and above, horn breakup throughout the working range ofthe short horn, and phenolic diaphragm breakup at 8 kHz and above. A "good"

driver of this type usually had a plus/minus tolerance of 4 to 8 dB, andit took a lot of judicious pen damping to get it to measure that well.

It wasn't for nothing that early hi-fi acquired a "boom-and-tweet"reputation. The sound quality was closer to modern-day autosound, or anun-renovated neighborhood theatre, than a modern system. The tubeelectronics helped smooth out much of the shrillness, but they couldn'trescue the truly mediocre phono cartridges and loudspeakers of the day.(Even so, the first-generation Quad, the RCA LC-1A, the Tannoy, and theLowther compare very well with modern systems ... but these were quite rareat the time.)

I find it interesting that second-generation transistor amplifiers like theDyna 120, Crown DC-300, and Phase Linear 400 were favorably compared to theclassic vacuum-tube Dyna Stereo 70 and Marantz 9 in the early Seventies.(Even by J. Gordon Holt's "Stereophile" magazine!) That tells us a lotabout the resolving power of the best speakers of the early Seventies.Progressive improvements in speaker design now reveal the actual sonicquality of these second-generation transistor amplifiers as quiterepellent, while the "freshened-up" tube amplifiers sound as good or betterthan many expensive transistor amplifiers made today.

It does make you wonder about the resale value of the current crop ofmulti-thousand dollar transistor amps and D/A converters - what will theysound like on speakers ten years hence? By then we'll be listening tospeakers with evaporated diamond diaphragms and other materials with verylow distortion and stored energy, a long-overdue high-fidelity digitalstorage medium with 24-bit depth and 96 kHz sampling, and the possibilityof replacing conventional 2-speaker stereo with something more natural. Youcan be confident the sound will be far more transparent and realistic thanwhat we're accustomed to now.

* Where Do You Start Your Design?

What kind of sound do you like? People really do hear in quite differentways, and different people assign importance to different qualities ofsound. Some audiophiles value timbral (or "tonal") qualities above allelse, treasuring the sound of their favorite instruments or voices; somelike a sense of immediacy, directness, and emotional impact; some like thesensation of an immense 3D space; and others like a see-through

transparency, a palpable "you are there" quality.

It helps a lot if you know what's important to YOU, what you tend todismiss, and what you don't hear at all. We all tend to unconsciouslydefine the bounds of "reality" by our own personal perceptions andthoughts, but this just isn't so. Reality is far, far larger than anypersonal set of boundaries. That's why getting a second and third opinionis so important.

Since all speakers have serious flaws in the absolute sense, it's up to youto select the qualities that are most important, and most believable, toyou personally. "Perfect Sound Forever" is an arrogant marketing slogan,not a realistic goal for an artist. For one thing, the materials to buildanything of the sort just don't exist. (Unless you've found a way togenerate a controllable room-temperature plasma. If you have, you'd bettertalk to the Department of Energy first.)

* Major Schools of Speaker Design

Since all designers are forced to choose on a subjective basis, there is nosingle "right" or "wrong" way to design a speaker. If anyone tells youthat, it might be amusing to investigate their personal beliefs a littlefurther and see if they worship at a church of religious fundamentalism orthe invisible church of "scientific" fundamentalism.

Where do I stand personally? Well, I'm certainly not a fundamentalist! Oh,you meant speaker design!

I care about to spectral flatness, very low energy storage, minimizing IMand FM distortion, and low diffraction, in that order. Subjectively, I seekout an elusive quality I call "the bloom of life"... that rare"You-Are-There" sensation of being in the presence of musicians creatingbeauty.

In the section that follows, I'll describe the various paths that designersmust choose as they make their way to sonic perfection.

PART 2

This is a continuation of an article published in the Vol. 3, Issue 6 of

Positive Feedback magazine. It is my *personal* view of the art ofdesigning high-end loudspeakers, and is not intended as any kind of "bible"for folks new to high-end audio. Since I've gotten a lot of questions aboutthe kind of thing that appears in Positive Feedback, it's probably simplestto demonstrate by reposting a complete article in this forum.

------------------snip-----------------

* Pulse Coherent (3D-Imaging) Dynamics

Duntech, Thiele, Spica, and Vandersteen systems fall in this group. Thedesigner takes expensive steps to control diffraction, offset the driversfor a coherent arrival pattern, and usually employs a first-order (6dB/Oct) crossover. Some, such as Spica, may use 3rd (18 dB/Oct) or 4thorder (24 dB/Oct) Gaussian or Bessel crossovers.

This is the only type of dynamic to offer accurate pulse reproduction,sometimes even bettering exotic electrostats or ribbon loudspeakers.However, the audibility of phase, and pulse distortion, is quitecontroversial in the engineering community, with the more conservativeengineers feeling it is a waste of time and money to ensure accurate pulsereproduction.

In a typical pulse-coherent design, the drivers are asked to be wellcontrolled 2 or more octaves out of their normal operating ranges, sopower-handling and IM distortion are compromised. Expensive drivers arerequired to partly overcome this problem, along with accurate resonancecorrection in the crossover. Controlling the radiation pattern withfirst-order crossovers and offset drivers is difficult; as a result,speakers of this type may sound quite different sitting and standing up.The yardstick for this type of design is the spatial quality of the stereoimage; if it isn't signicantly better than any other type described below,the design is not a success, since that is the primary potential benefit ofthis design approach.

* Flat Response (Objective-Design) Dynamics

Most British and Canadian speakers fall in this group. They have very flatspectral responses, with the British paying more importance to the 1 or 2meter on-axis response curve, and the NRC-influenced Canadians paying more

importance to the frequency response averaged over a forward-facinghemisphere. These design priorities have been arrived at by BBC broadcastprofessionals and NRC listening panels respectively, using statisticallyverifiable double-blind listening tests.

This school of design is most closely identified with an "objective"engineering-oriented philosophy. Not by accident, the engineers with themost advanced academic credentials tend to design speakers using thisphilosophy. These folks are not going to be sympathetic to exotic wires,resistors, capacitors, the directly-heated triode mystique, or anything notaudible to a double-blind listening panel.

To its great credit, the BBC is known as the first group to accuratelymeasure and identify sources of driver and cabinet resonances in the earlySixties, and many British speakers still excel in this area. Since audibleresonances may be as far as 20 dB below a conventional sine-wave responsecurve, the BBC was the first organization to identify and measurecolorations that were completely missed by the conventional sine-wave or3rd-octive pink-noise measurements. These types of measurements are nowconsidered a standard part of FFT, TDS, or MLSSA systems.

Objective-design crossovers are usually 3rd-order Butterworth or 4th-orderLinkwitz-Riley, which offer the flattest, most accurate response curve andthe best control of out-of-band IM distortion (at the expense of pulsedistortion and overshoot).

Laurie Fincham of KEF deserves credit for designing the firstcomputer-optimized crossover using acoustically-accurate rolloff curvescombined with driver resonance correction (in the early Seventies). Thiscrossover optimizing technique is now available at far less cost by using a 386, 486, or 586 PC with programs such as XOPT or LEAP. Competentdesigners are now expected to be knowledgeable in the design ofacoustically-accurate crossovers, regardless of their design philosophy.

Recent British designs also focus on a high-quality mechanical ground pathto the floor (exotic stands). Objective-school designers usually ignorepulse response, diffraction control, and subjective areas such ascapacitor, inductor, and wire quality. In contrast, research is focussed onsteadily improving driver quality, cabinet resonance control, and accuratepair-matching in production.

* Minimalists (Subjective-Design) Dynamics

Some Italian, Scandinavian, English, and American speakers fall in thisgroup. The crossover is extremely simple, sometimes consisting of onecapacitor to protect the tweeter (no, I don't know how Sonus Faber protectstheir tweeter without a capacitor). Drivers and crossover components are ofthe very highest quality, along with exotic wire and cabinet materials.

Measurements usually play a minor role in the development of this type ofspeaker. Since this design philosophy leaves driver resonances uncorrectedand accepts the resulting frequency and pulse response aberrations producedby the minimal crossover, compatibility may depend strongly on the sonicflavor of the rest of the audio chain. Personally, I find these types ofspeakers somewhat colored, but with an exciting, dynamic, and involvingsound.

* Horns and High Efficiency Dynamics

Most Japanese high-end speakers fall in this group, as well as a handful ofFrench, Italian, English and American systems. They use design philosophiesderived from Western Electric theater speakers, Paul Voigt's Tractrix horn,or a combination of both. Nearly all horn systems are at their best whenused with low-powered, sometimes single-ended, Class A directly-heatedtriode amplifiers, with transistor amps, even Class A types, frequentlygiving very poor results.

Horns typically have extremely low THD, IM, and FM distortion, reasonablyflat response, and sharp cutoff characteristics at both ends of thefrequency range, which may be fairly narrow. Historically, horns wereusually beset by intractable problems with impulse response, diffraction,and smooth dispersion, which is why most high-end designers (in the West)avoided horn systems, leaving them to the studio monitor and PA markets.

In the past decade, though, Dr. Bruce Edgar in the US, and others in Japan,have made very significant improvements in horn design for high-end andultra-fi audio, which are just beginning to be recognized by magazines likeSound Practices. According to the movers and shakers in the Americanultra-fi renaissance of the 90's(Joe Roberts, Herb Reichert, and MikeLeFevre), these new horns, and the Edgarhorn in particular, are in a class

of their own, superior to ribbons, electrostats, planars, exotic dynamics,etc.

My own opinion? Well, I feel that the trio I mentioned above have prettygood taste, so it's entirely possible I'll also like the new horns. Rightnow, I can't say anything until my own set of Edgar midrange horns arriveand I design them into a full-range speaker system. Stay tuned.

* Electrostatic Planars

A few English, American, and Japanese companies make this class of speaker,which I have to admit are old-time favorites of mine. A well-designedelectrostatic offers the most linear and completely uniform diaphragmmotion of any class of loudspeaker (and low IM distortion), as well as thepotential for the best pulse response. The first Quad electrostatic is themost famous example of a speaker decades ahead of its time.

There is a downside, of course, and that is very low efficiency, anextremely reactive amplifier load, restricted dynamic range, fragility,limited bass, and a tricky room-sensitive dipolar radiation pattern thatbecomes quite directive at high frequencies. These problems are not easy tosolve, particularly the large-panel dispersion, which is not an asset, buta serious problem for stereo imaging.

The old Quad pioneered the most widely used solution, a side-by-side 3-waysystem, using progressively narrower panels for the higher frequencies. Thenew Quad uses a complex phased array system which approximates a sphericalradiator. The Martin-Logan uses an unusual cylindrical panel relying oncurved damping pads stretched across the perforated high-voltage stators.Problems still remain, though. All of the electrostats I have measured showmoderate resonances below 200 Hz (primary room-diaphragm resonance) andmultiple sharp resonances above 8 kHz (non-homogenous diaphragm motion andstanding waves in the HV stators or metal grill-frame assembly).

In short, wonderful midrange and depth perspective, and good-but-not-greatat the frequency extremes, reasonable-to-fair stereo imaging, and somewhatlimited dynamic range.

* Electromagnetic Ribbons and Planars

A few are made in the USA, being represented by Apogee, Magnepan, EminentTechnology, and others. These fall in two classes: the true ribbon, whichis a very thin corrugated aluminum "voice coil" hanging freely like astreamer in a side-by-side magnetic field, and the magnetic-planars, whichare sheets of stretched Mylar or Kapton film with the "voice coil" eitherprinted or glued on the film.

Magnetic-planars use arrays of magnets on the back side of the film (notgood for distortion) or on both sides (operating in push-pull, but alsocreating a cavity between front and rear magnet pairs). The arrays ofmagnets provide a somewhat uneven drive field, so the uniformity ofdiaphragm motion is not in the same class as an electrostat. Then again, HVarcing is not problem, so the magnetic-planars can play a lot louder thantheir electrostatic cousins.

The magnetic-planars have weak magnetic coupling, which is much lower thana dynamic driver due to the large magnet spacing and shorter length of wirein the gap. In a dynamic driver, a high "BL-product" means a strongmagnetic field in the gap (the "B") and a long helical voice coil immersedin the field (the "L"). Dynamics with a high BL-product provide thetightest amplifier-speaker coupling, which is why they are sensitive toamplifier damping factor and wire resistance. By contrast, in aplanar-magnetic, damping is mostly provided by the stretched film and theair load, and little by the amplifier. Both impedance and efficiency arelower than dynamics, and attempts to raise both by increasing the amount ofaluminum wire on the film usually degrades the transient response.

The true ribbon is free of the stretched film resonances and obstructingmagnets of the planar-magnetic, so it offers outstanding pulse response,uniform drive, and a good approximation of a line source, but theefficiency and impedance are both phenomenally low and it is not usable asa woofer due to the small area. Most practical ribbons either use astepdown transformer or ask the amplifier to drive a 1/2 ohm load (not ajoke, unfortunately).

Magnetic-planar sound quality is usually midway between a good dynamic andan electrostatic, with a significant freedom from cabinet colorations atmid and high frequencies. Radiation pattern is similar to an electrostatic,which means problems with imaging and bass, with the minimal amplifier

damping making some rooms unusable. This type of speaker is probably themost room-sensitive of any type, requiring the listener to choose betweensmooth bass and accurate imaging in some rooms.

These kinds of speakers aren't my cup of tea, but I know many people whoreally enjoy the neutral, relaxed type of sound they can offer. Inaddition, a true ribbon offers some of the best treble around, superior todynamics or electrostats, exceeded only by the "massless" exotics.

* Hybrids

Each type has a quite distinctive sound, and some, naturally, are hybrids.This gets pretty tricky when the dispersion patterns are different, alongwith different IM distortion spectra. It helps if the original designer isthe one making the decisions, because a consistent philosophy is then beingused for the entire system (we hope). Even so, if the designer isunfamiliar with the strengths and weaknesses of each type, the result canbe the typical disjointed "hybrid sound", with the crossover region quiteobvious.

The most common blunder seems to be high-performance electrostats unhappilymated to low-performance woofers in thin-walled closed-box cabinets. Atleast there's a precedent for this, I guess, going right back to the AR1-Wand the JansZen 130 tweeter.

* "Massless" Exotics

One day, I'd like to design one of these myself. The "massless" speakersfall into this category ... Ionovac, Magnat, and Plasmatronics (what aname!) They DO sound exotic, and measure the same. No resonances at all,and accurate pulse and frequency response up to 100kHz or more. Lowdistortion too ... like an amplifier. Actually, the "diaphragms" do havemass. But it's not much. It's the same as air, so the acoustic coupling is1:1. Efficiency is a little difficult to state, though, since the outputtube plates of the power amplifier are providing a high voltage thatdirectly modulates a conductive gas.

I remember hearing the Plasmatronics at the 1979 Winter CES, and I must sayI've never heard a tweeter that even came close to that one. They darkenedthe room, and you could see this weird purple glow through the grill cloth

that looked for all the world like a gassy triode ... but it was thetweeter! It glowed and pulsed with the music!

The rest of the speaker, though, was a pretty mundane paper-cone setup in ahuge cabinet ... oh well. Even so, the Plasmatronics was a wild thing, aglimpse of the future, a SR-71 Blackbird next to a bunch of Cessna 172's.Not too surprisingly, the designer was a plasma physicist at Los AlamosLabs. Talk about being ahead of your time! This was 12 years before anyonewas talking about turning swords to plowshares, or in this case, Dr.Teller's atomic toy into the next breakthrough in audio!

The real-world problems? Well, if you ionize air (by using RF heating) youstrip apart oxygen-2 and get oxygen-3 (ozone gas). Not very healthy, andillegal in the USA. The Plasmatronics ionized helium gas, which got aroundthat problem, but it required a fresh tank of helium every month (I'mserious!). I remember seeing a full-size helium tank, gauges and all, in aspecial compartment inside the subwoofer enclosure.

* Summary

In Part 1, I covered the basics of the art of speaker design from apersonal perspective, briefly illuminating the past, the future, and thepresent, and went on to describe the many camps that now exist in the ranksof speaker designers and manufacturers.

In Part 2, I'll discuss the different qualities of dynamic drivers, the waythey affect the sound of the total loudspeaker, and the effect they have onthe sound quality of the listener's high-fidelity system.

PART 3

The second in a series called "The Soul of Sound" where I wear my professorhat. These articles are my *personal* view of designing high-end speakers.I expect this article to be thoroughly out of date in two years or less ...so enjoy it while it's fresh!

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Looking Over My Shoulder, Part II

by Lynn Olson

* Drivers

The speaker driver determines the ultimate potential of the entireloudspeaker, and plays a dominant role in the sound of the entirehigh-fidelity system. As mentioned in the first part of the series, thereis no perfect driver at the present state of the art, and that goal is manydecades away, since it requires a driver with a density equal to air,completely uniform motion at all frequencies, and no distortion of anykind.

We have a long road ahead of us ... but take heart! Major advances inmaterials sciences are happening right now, with many improvements due tooccur in this decade. I confidently expect we'll be seeing breakthroughsevery 2 or 3 years at the present rate of progress.

This is thanks to major advances in computer modelling of mechanicalbehaviour and the research conducted in the aerospace, automotive, andsports/recreation industries for lightweight, high-performance materials toreplace costly and heavy traditional materials. We now have Kevlar, carbonfiber composites, and forged aluminum drivers; we can expect syntheticdiamond, ultralow density aerogel-silica glasses, new types of metallic andcarbon monocrystals, and new classes of composite drivers before the turnof the Millenium. * Why Drivers Sound Like That

The major challenge facing the driver designer is combining uniformity ofmotion (rigidity) with freedom from resonance at mid and high frequencies(self-damping). This is the major tradeoff in speaker systems of all types(except the "massless" group discussed in the first part of this series).There are additional problems introduced by cavity resonances and magneticnon-linearities, which are discussed later.

* Uniform Motion

Rigidity means accelerations from the voice coil are accurately translatedinto cone or dome acceleration over the entire driver surface; thistranslates to ruler-flat frequency response, fast pulse risetime, low IM

distortion and a transparent, "see-through" quality to the sound.

Audiophiles usually describe this type of sound as "fast", much to thedismay of measurement-oriented objectivist engineers. "How can can a mid orwoofer possibly be fast, since the crossover limits the pulse risetime to afifth, or tenth, of what any tweeter can do?" This quickly leads to whatdiplomats call a "full and frank exchange of views", in other words, amutual exchange of misunderstandings.

As usual, both sides are right, and both sides are wrong. They're justspeaking about different things. What the audiophile is actually hearing isuniform cone motion; this phenomenon can be measured by the absence of IMdistortion, a flat frequency response in the working range, and good pulseresponse with a clean and quick decay signature.

Well, that's great, you might think, just make the cone, or dome, orwhatever as rigid as possible. How about a metal, like bronze, perhaps?That's nice and strong, and it can be formed into nearly any shape.

You can see the direction this is taking. Bells are made out of bronze.Another problem raises its head .... resonance! After all, why does a bell,or any other rigid metal, ring so long, for many thousands of cycles?

The answer has two parts, one obvious, one not so obvious. First, the metalis rigid, and formed in a shape that increases the rigidity even further.Second, the only path for the bell to release mechanical energy is to theair itself, which takes a long, long time, since the density of air andbronze are quite different, resulting in very weak coupling, and verylittle damping by the air load. This leads us to another desirable propertyfor the speaker driver, which is ...

* Self-Damping

We also want the voice coil to stop the cone or dome, not have the cone ordome play a tune all by themselves. Unfortunately, the most rigid materials(traditionally metals) have very little self-damping, resulting invibrations of very long duration (high Q). One way to control the problemis to extend a heavy rubber surround partway down the cone, and pay a lotof attention to the damping behaviour of the spider and surround materials.

At the present, though, even the best Kevlar, carbon-fiber, or aluminumdesigns show at least one high-Q peak at the top of the working range,requiring a sharp crossover, a notch filter, or preferably both to controlthe peak. Unfortunately, this peak usually falls in a region between 3 and5 kHz, right where the ear is most sensitive to resonant coloration.

Self-damping results in an absence of coloration, as well as contributingto a relaxed, natural, and unfatiguing quality. Interestingly, manyaudiophiles (and reviewers, too!) are unaware of the particular sound ofdriver-material resonance, calling it "amplifier sensitivity", "roomsensitivity", or similar term that seems to point away from the loudspeakeritself.

There are highly-reviewed (by the large-circulation "underground"magazines) 2-way speakers that use 7" Kevlar drivers crossed over tometal-dome tweeters. Technically, these loudspeakers operate with uniformmotion over the range of both drivers; in practice, though, the crossoversare hard pressed to remove all of the energy from the Kevlar breakup regionbetween 3 and 5 kHz.

The reviews of these particular 2-way speakers go on at considerable, andamusing, length about the trials in finding an amplifier that "revealed"the full quality of the loudspeaker. In reality, the reviewer was forced touse an amplifier that was particularly free of coloration in the regionwhere the Kevlar driver was breaking up. Since most audiophiles andreviewers are unfamilar with the direct sound (and measurements) ofcommonly-used raw drivers, they can't evaluate how much "Kevlar sound", or"aluminum sound", remains as a residue in the finished design.

This is a problem, by the way, that plagues all current 2-way Kevlar,metal, or carbon-fiber loudspeakers ... at the current state of the art,the 6.5" or 7" drivers are forced to operate right up to the edge of theirworking ranges in order to meet the tweeter in a moderate-distortionfrequency range.

If you lower the crossover frequency, tweeter IM distortion skyrockets,resulting in raspy, distorted high frequencies at mid-to-high listeninglevels; if you raise the crossover frequency, the Kevlar breakup creeps in,resulting in a forward, aggressive sound at moderate listening levels, andcomplete breakup at high levels (unlike paper cones, Kevlar, metal, and

carbon fibers do not go into gradual breakup).

This presents the designer with a tough choice: rough sound in the entiretreble region, or the characteristic Kevlar forwardness, which can at timesactually give a snarly sound to the speaker system. At the present, thebest choice is a fourth-order (24dB/Oct.) crossover with a sharp notchtuned to the Kevlar resonance.

I should add, by the way, that I like Kevlar and carbon-fiber drivers verymuch ... but they are difficult drivers to work with, with strong resonantsignatures that must be controlled acoustically and electrically.

As mentioned above, rigid cones have advantages, but are difficult to dampcompletely. A different approach is to use a cone material that is madefrom a highly lossy material (traditionally, this was plastic-doped paper,but this has been supplanted by polypropylene in most modern loudspeakers).The cone then damps itself, progressively losing energy as the impulse fromthe voice coil spreads outwards across the cone surface. The choice ofspider and surround are then much less critical.

This type of material typically measures quite flat and also allows asimple 6dB/Octave crossover; personally, though, I don't care for the soundof most polypropylene drivers, finding them rather vague andblurry-sounding at low-to-medium listening levels. Without access to a B&Kswept IM distortion analyzer, I have to resort to guesswork, but I stronglysuspect that this type of cone has fairly high IM distortion since it isquite soft. In addition, it is quite difficult to make a material that hasperfectly linear mechanical attenuation; in practice, distortion creeps inwhen you actually want a progressive attenuation of energy over the surfaceof the cone.

I think what is really happening is similar to many soft-dome tweeters; thecone is actually breaking up throughout the entire frequency range, but theheavy damping hides this from the instrumentation (but not the ear). Toovercome this subjective effect, the best drivers of this type (Dynaudio,Scan-Speak, and Vifa) are actually composites, adding silica, talc, ormetal dust to the plastic, which significantly improve rigidity withoutlosing the characteristic polypropylene smoothness.

* Cavity Resonances

Even though the dust cap in a mid/woofer (or the dome in a tweeter) lookspretty harmless, the space between dustcap and the polepiece of the magnetcreates a small resonant cavity. One example of this was the (in)famous KEFB110 Bextrene midbass driver dating from the early Seventies (as used inthe BBC LS 3/5a).

Although this driver was probably the one of the first high-qualitymidranges available, it also had a number of problems, such as lowefficiency, limited power-handling, a broad one-octave peak centered at 1.5kHz (corrected by the crossover), and group of 3 very high-Q peaks centeredaround 4.5 kHz (only slightly attentuated by the BBC third-ordercrossover). These upper peaks, which reviewers mistakenly ascribed to thetweeter, were also very directional, which is typical of dustcapresonances.

The popular tweeters of the 1970's, including the Audax and Peerless 1"soft-domes, also had similar resonances between 9 and 16 kHz, which werepartially damped by a felt pad nearly filling the space between the domeand the polepiece. Since the soft-domes were much more lossy than the stiffB110 dustcap, the resonances were much broader and only 1 to 3 dB inmagnitude ... but they were still there, and they were responsible for someof the fatiguing quality noticed by attentive listeners.

Not surprisingly, the problems were much worse in the phenolic, fiberglass,and hard paper domes used in the more mundane speakers of the day. (Ah yes... who can remember such paragons of excellence as the BIC Venturis? TheCerwin-Vegas? The Rectilinears? The JBL L100's? In a prior life, I actuallyhad to sell these awful things! "Wait'll you hear 'Dark Side of the Moon'on these babies!")

Returning to the present, the best midbass and tweeter drivers now sidestepthis problem in two ways: a vented polepiece assembly, used by theScandinavian manufacturers Dynaudio, Scan-Speak, Vifa, and Seas; and abullet-like extension of the polepiece, which replaces the midbass dustcapentirely, used by the French manufacturers Audax and Focal.

The Dynaudio Esotec D-260, Esotar T-330D, and Scan-Speak D2905/9000tweeters are the most notable examples of using a vented polepiece loadinginto a tiny transmission-line to damp the backwave from the tweeter dome;

their use in the Sonus Faber Extrema and ProAc Response Threes is a commenton how successful this technique can be.

By contrast, the Focal T120 and T120K, which use a rigid fiberglass orKevlar inverted dome directly above an undamped polepiece cavity, show aseries of high-Q resonant peaks at the top of their operating range, whichare caused by the resonant cavity coupling to the first breakup region ofthe rigid dome.

I must admit I was rather puzzled by the public acclaim for these driverswhen they first came out; I didn't like the way they sounded, and I wasn'ttoo impressed by their measurements.

>From all accounts, though, the new Focal titanium-dome T120Ti andtitanium-dioxide T122Ti-O2 are excellent, and I liked what I heard when Iauditioned a speaker that used the Focal T120Ti at a recent Triode Societymeeting.

Magnetic Non-linearities

Most audiophiles are aware that loudspeaker drivers are inductive; afterall, the voice coil is wound around a ferrous polepiece, and that's how youmake an iron-core inductor (or "choke"). Not as many audiophiles know aboutthe myriad of problems this creates.

If the inductance were constant, like an air-core inductor, there would beno problem; just adjust the crossover design to allow for it (using asimple R-C network) and off you go. Unfortunately, this is an iron-coreinductor, and much worse, the inductance varies with the position of thevoice coil.

The varying inductance has profound consequences, since the inductance isactually a important factor in determing the upper rolloff frequency of thedriver, and its resulting acoustic delay (relative to the tweeter). Varythis inductance, and the rolloff frequency and acoustic delay move alongwith it. When does this happen? Whenever the driver moves a significantproportion of the linear region of voice-coil travel, which is less thanyou'd think. In the excellent 8" Vifa P21W0-12-08, this linear region isonly 8 mm (plus/minus 4 mm either way). A more typical figure for lineartravel would be 6 mm for most 8" drivers, and 1 to 3 mm for most midranges.

Play some deep bass, and the effects of inductance modulation begin toshow, creating IM and FM distortion over the entire frequency spectrum.This is a genuine problem for 2-way systems and 3-way systems using a lowmidrange crossover; it means that any time you can actually see the driversmove, there are quite significant amounts of IM and FM distortion. Whatdoes this sound like? You can expect a loss of resolution that is dependson the bass content of the program material, which may be masked byproblems in the power amplifier (such as output transformer saturation orinadequate power supplies).

Are there solutions? Yes. The best drivers from Scan-Speak (SD System) andDynaudio (DTL-System) plate the polepiece with copper to short out eddycurrents induced within the magnet structure by the voice coil. Thespecification that gives this away is the voice coil inductance.

The 8" Scan-Speak 21W/8554, probably one of the best 8" drivers in theworld, has an inductance of 0.1mH, which is far lower than the 8" VifaP21W0-20-08, which has in inductance of 0.9mH. Both are excellent drivers;the Scan-Speak, though, is almost certainly going to have more transparentsound when asked to carry bass and midrange at the same time.

The inductance figure also has a another hidden meaning; remember, theupper rolloff frequency of the driver is the combined function of themechanical rolloff and self-inductance of the voice coil. If you calculatethe electrical rolloff frequency by using the VC inductance and the DCresistance, a few drivers have an electrical rolloff well above themeasured acoustical rolloff. This is desirable; it means that theinteraction between the two rolloff mechanisms is going to be small.

Other drivers (and this is true of most drivers) are going to have anelectrical rolloff well below the measured acoustical rolloff. How is thispossible? The mechanical system actually has a broad peak which is maskedby the self-inductance of the voice coil. This is not good; any change ineither the mechanical system or the electrical system is going to stronglymodulate the frequency and transient response.

This, by the way, is the same kind of problem found in the oldmoving-magnet phono cartridges. Most moving-magnets (typically Shure andStanton) were mechanically peaked, then rolled-off electrically by the

combination of cable capacitance and cartridge inductance. Notsurprisingly, this type of cartridge usually sounded much less transparentthan its high-end moving-coil brethren, which had less than one-tenth theinductance and a much flatter, more accurate mechanical system.

In the section that follows, I'll show you how you can make your owndecisions about which drivers you like (and second-guess the manufacturer,reviewer, and your audio friends).

* Selecting A Driver

I use a method that's so crude it might sound kind of dumb; I put thedriver on large, IEC-sized baffle (135cm by 85 cm) and listen to it. Nocrossover, no enclosure, and if it's a tweeter, not very loud at all. Ilisten to pink noise (to assess the severity of the peaks that may appearin the sine-wave and FFT waterfall measurements) and music (to get a senseof how much potential resolution the driver posseses).

This does take an educated ear, though, since you have to listen around thepeaks that the crossover might notch out, and not hold the restrictedbandwidth against it. However, this listening process tells you a lot abouthow complex the crossover has to be, particularly if you remember that thecrossover can never totally remove a resonance ... it can just make it alot more tolerable.

In addition, I very carefully assess the results of the MLSSA PC-basedmeasurement system (using the same IEC baffle), looking at the:

1) Impulse Response. (How fast does it settle to zero? Is there chaotichash in the decay region or is it a single, smooth resonance? Are there twoor more resonances?)

2) The Group Delay vs. Frequency Response. (How ragged is the frequencyrange above the first breakup? Can it be fixed in the crossover?)

3) The Waterfall Cumulative Decay Spectrum. (Can I accept the resonancesthat can't be fixed in the crossover? If crossover correction is required,how complex is it going to be?)

4) The flatness of Frequency Response in the working band. (Can I accept

the broad, low-level colorations that may appear here?)

Listening and measurements are equally essential. Both give only a partialpicture of the actual driver. Even the finest modern audiophile system willhave very serious sonic deficits 5 years from now; measurements provide areality-check on colorations that present-day equipment may not reveal. Inturn, the MLSSA system can point out troublesome colorations to listen for;some are much more audible than others.

The thoughtful designer is obliged to be as careful as the craftsman (orcraftswoman) who lavishes a full measure of care and attention on even thehidden parts of their creation.

------------------snap-------------------This is a revision of an article published in Vol. 4-2 of Positive Feedbackmagazine, a publication of the nonprofit Oregon Triode Society located at:4106 NE GlisanPortland, OR, 97232USA

For reprints, back issues, T-shirts, or subscription information, call JohnPearsall at: (503) 234-4155-- >From Lynn T. Olson

PART 4

The second in a series called "The Soul of Sound" where I wear my professorhat. These articles are my *personal* view of designing high-end speakers.I expect this article to be thoroughly out of date in two years or less ...so enjoy it while it's fresh!

-------------------snip-------------------------

* Types of Drivers

It helps when you start listening and comparing to have a good grasp on thebasic characteristics of the driver, so you can determine if it is a goodexample of its type. By listening carefully and examining all of therelevant specifications, you can find out just how well the driver

designers solved the problems of making a good driver.

* Paper Cone Drivers

This dates back to the original Rice & Kellogg patent in the late Twenties.Paper ranges in quality from the worst TopTone clock/radio speakers to thesuperb Scan-Speak 5" cone/dome midrange used in the Theil line of speakersand the SEAS 6.5" midbass used in the Wilson Audio WATT. This oldest ofmaterials is actually a composite structure, and changes propertiessignificantly when it doped with an appropriate plastic (the choice ofdoping is invariably a trade secret of the driver vendor). The doping isquite important, since paper undergoes significant alterations with changesin humidity and time if it is left undoped; the doping stabilizes thematerial and typically improves the self-damping.

Strengths are: Good-to-excellent self-damping, potentially excellentresolution and detail, very flat response potential, and a gradual onset ofcone breakup. It can be used with low slope linear-phase crossovers withoutmuch trouble. Paper is a material that sounds better than it usuallymeasures ... this is an asset, not a disadvantage.

Weaknesses are: Not as rigid as the Kevlars, carbon fibers, and metals, soit lacks the last measure of electrostatic-like inner detail. Doesn't go asloud as the materials above, but the onset of breakup is much more gradual.Paper-cone drivers usually require modest shelving equalization in thecrossover for best results.

Paper is not as consistent as synthetics, so pair-matching isn't quite asexact, which may affect imaging, depending on the precision and quality ofmanufacture. Properties may change with time, even with doping.

Best Examples are: Scan-Speak 8640 5" cone/dome midrange, with linearresponse out to 13kHz, very low distortion, excellent pulse response, andexcellent inner detail.

SEAS 6.5" midbass (as used in the Wilson Audio WATT, but possiblymodified).

Audax PR170M0 6.5" high-efficiency midrange. (100 dB at 1 meter!)

I have heard from several sources that Kurt Mueller makes the best papercones and surrounds, which are used by Scan-Speak, Seas, Vifa, and others,as well as many high-quality proprietary drivers made in Britain and theUnited States.

* Bextene Cone Drivers

This is an acetate plastic derived from wood pulp, not petrochemicals, andis always damped by a layer of doping material to control the strong firstresonance it displays around 1.5 kHz. It was originally developed by theBBC in 1967 to replace paper with a more consistent and predictablematerial for monitoring purposes. It came into widespread use in the earlySeventies, with the typical audiophile speaker using a 8" KEF or AudaxBextrene midbass driver with an Audax 1" soft-dome tweeter.

The BBC-derived designs always employed equalization to flatten theBextrene driver in the midband; the most famous (or infamous, depending onwhether you were the listener or the designer) driver was the KEF B110 usedin the BBC LS 3/5a minimonitor.

Bextrene has been replaced by BBC-developed polypropylene, which gives amuch flatter response, does not require a layer of doping material, andprovides a 3-4 dB increase in efficiency due to the decrease in cone mass.Bextrene is now considered an obsolete material by nearly all speakerdesigners.

Strengths are: Consistent batch-to-batch, excellent potential imaging (bymid-Seventies standards). Inner resolution higher than most paper cones.

Weaknesses are: Very low efficiency (85 dB at 1 meter), requires a strongnotch filter in the midband, "quacky" coloration by modern standards,sudden, unpleasant onset of breakup at not-so-high levels, and numerousresonances at the top of the working band.

Best Examples are: None. Modern designers are not willing to tolerate thecomplex notching and shelving equalization required to make these driversacceptable.

* Soft-Dome Tweeters

These came into common use in the early Seventies with the introduction ofthe Peerless 1" soft-dome (remember the tweeters of the original Polkspeakers?), followed by the superior Audax 1" tweeter, which found its wayinto many British and American designs during the Seventies and earlyEighties.

These designs fell into disfavor with the introduction of titanium andaluminum domes and the Focal inverted-fiberglass domes in the mid-Eighties,which swept the Audax-class soft-dome drivers off the audiophile market.

In the last two years, the soft-domes have made a surprising comeback withthe introduction of the Dynaudio Esotec D-260, Esotar T-330D, andScan-Speak D2905/9000 1" tweeters, which compete on even terms with anymetal-dome around. These new designs combine sophisticatedtransmission-line back-loading with new dome profiles and new coatingmaterials. As a result, they have the sonic resolution and detail of thebest metal domes without the characteristic 22 to 27 kHz resonance.

Strengths are: Intrinsic self-damping and potential for extremely flatresponse and first-class impulse response. Potential for natural, opensound without intrusive and fatiguing resonances, a valuable quality whenlistening to many digital recordings.

Weaknesses are: The older class of soft-domes had a dull sound with ahard-to-pin-down fatiguing quality. Many had quite limited power-handlingand required a high-slope 18 dB/Octave crossover to minimize IM distortion.The high-profile dome, required for rigidity, has more restricted HFdispersion than the competing metal-domes, which have flatter profiles.

The new units mentioned above do not have these faults, however, with theexception of dispersion.

Best Examples are: The Dynaudio Esotec D-260, Esotar T-330D, and Scan-SpeakD2905/9000 1" tweeters.

* Soft-Dome Midranges

These things are dogs! I've listened to the AR-3, AR LST, ADS systems,Audax 2", and the Dynaudio D-52 soft dome midranges, and they barked, theysnarled, they chewed the rug, and made a mess on any loudspeaker they

approached! They measure flat, all right, but they sound opaque, fatiguing,strongly colored, and 2-dimensional.

The first problem is that soft-dome midranges have a limited bandwidthresulting from restricted linear excursion (1-2 mm typical) and do notgracefully tolerate even a 500 Hz crossover, operating best over a limited800 Hz to 3200 Hz range.

A second problem is that they are quite prone to side-to-side rockingmodes, since there is no spider combined with the surround to force themovement into a linear back-and-forth motion.

A third problem is that the doped silk dome is just, well, too soft for thejob it has to do in the power band of the midrange.

The newer class of cone-domes, such as the 5" Scan-Speak 13M/8636 and13M/8640, and the 5" Dynaudio 15W-75, are another story. These 3 driversare actually constructed as high-precision cone drivers, not midrangedomes. The only thing they have in common with the soft domes is a largedustcap, which does act as a dome at high frequencies.

This class of driver has much more excursion, much lower distortion, andmuch wider frequency response than the older soft-dome midranges. Thecone-dome drivers are capable of realistic and transparent sound. They aredescribed in more detail in the other sections, since they use Kevlar,paper, and polypropylene respectively.

Another "special case" is the professional-grade ATC 3" dome with anintegral short horn. This driver uses a dual spider to eliminate therocking problem that plagues most soft-domes, reducing the IM distortionvery significantly. Ron Nelson (of Nelson-Reed) recommended this driver asone of the very best midranges around, and I take his recommendationseriously. This is a very expensive driver (around US$300/each) and needsto be hand-selected so the resonant frequencies of the left and rightchannels match.

Strengths are: None. Metal-dome midranges have some potential, but theyrequire sharp crossovers on both ends with an additional sharp notch filterat high frequencies to remove the first (and worst) HF breakup mode. Note:This does not apply to the ATC driver or the cone-domes.

Weaknesses are: High distortion, fatiguing sound, high crossover frequency,limited bandwidth, limited power-handling, and misleading frequencyresponse measurements. It takes a detailed swept IM distortion measurementand laser holography to get the goods on these drivers. Note: This does notapply to the ATC driver or the cone-domes.

Best Examples are: ATC 3" professional-series - a totally different animalthan the usual soft-domes. About 4 times as expensive, though. The DynaudioD-54 in a Edgarhorn is reputed by the "Sound Practices" group to be thefinest midrange in the world ... yours truly has a pair on order, so I'lllet you know.

* Polypropylene Drivers

This material was developed by the BBC in 1976 (my dates may be off) as areplacement for Bextrene. Since it is intrinsically highly self-damping, acorrectly designed polypropylene driver is capable of flat response overits working range without equalization. In addition, it typically attainsefficiencies of 88 to 91 dB at 1 meter, which is a significant improvementover Bextene.

This material has become nearly universal, since it requires a minimum ofhand treatment to assemble a loudspeaker - the only difficult problem wasfinding adhesives that would stick to polypropylene, and that problem wassolved in the beginning of the Eighties.

This material is used in speakers ranging from mundane rack-stereo trash tothe first-rank ProAc Response Threes and Hales System Two Signatures. Thecone profile and additional materials added to the polypropylene mixstrongly determine the ultimate quality of this type of driver.

Strengths are: Very flat response if correctly designed, very lowcoloration, good impulse response, crossover can be as simple as onecapacitor for the tweeter, good efficiency, and gradual onset of conebreakup. The best examples can be as transparent as the best paper-conedrivers, which is an excellent standard.

Weaknesses are: Not quite up to the standard of transparency set by therigid-cone class of drivers and the planar electrostatics. Many poly

midbass units do not mate well with the popular metal-dome tweeters, withdifferences in resolution that can be audible to the skilled listener. Notthe best choice for woofers 10 inches or larger unless the polypropylene isquite thick and reinforced with another, more rigid, material. Largewoofers are better served with stiff paper or carbon fiber.

Best Examples are: The Scan-Speak 18W/8543 7" midbass, as used in the ProAcResponse Threes, is probably the finest polypropylene driver in the world.

Another closely-ranked contender is the Dynaudio 17W-75 Ext. 7" midbass, asused in the Hales System Two Signature.

The Vifa P13WH-00-08 5.5" midbass/midrange unit is another excellentperformer, well suited for midrange or minimonitor use. It is unique inhaving a textbook-flat midrange combined with a completely smooth 2nd-orderrolloff.

* Metal-Dome Tweeters

Advances in German metallurgy (at Elac and MB) resulted in thin profiletitanium and aluminum domes in the mid-Eighties, with drivers from severalvendors in Germany, Norway, and France now available. This type of drivercan offer very transparent sound, rivaling the best electrostatics ifcorrectly designed.

The downside is the lack of self-damping, with aluminum coming ahead oftitanium in being better behaved in the ultrasonic region. At the present,all metal-dome drivers have significant ultrasonic peaks, ranging inmagnitude from 3 dB (excellent) to 12 dB (not so good).

There's controversy about the significance of this peak, since theall-wise, all-knowing, all-seeing potentates of Philips and Sony have goneto great lengths to make sure that none of our wonderful new "perfect soundforever" recordings will ever have any musical information above 20kHz.Maybe one day we'll finally have a digital system designed withhigh-fidelity in mind ... something with an upper limit of at least 32 kHzand a true resolution of 20 to 24 bits.

Strengths are: Uniform piston action right up to the HF resonance,providing sound of very high resolution, transparency, and immediacy if

correctly designed. Dispersion is typically excellent, since the metaldomes have flatter profiles than soft-domes.

Weaknesses are: Potential for (I hate to say it) "metallic" colorationcaused by the HF peak intermodulating with the inband sound. Some earlydesigns have restricted power-handling. If significantly overloaded,breakup distortion is obvious over the whole frequency range.

Best Examples are: Vifa D25AG-35-06 1" aluminum dome, which is even betterwith the plastic phase disk removed. This dome has a vented pole piece, sopower handling is quite good, and the ultrasonic peak is only about 3 dBeven with the phase disk removed. Also, the brand- new Focal T122Ti-O2 isreputed to be outstanding.

* Rigid Drivers

Aluminum

The first rigid drivers to find limited use in high-fidelity applicationswere the small Jordan Watts 4" aluminum-cone units. The hand manufacture,high price, and low efficiency limited the market for these drivers, andvery few appeared in the United States (I have heard of them by reputationbut have never auditioned them personally). Some new British speakers arealso using 5" and 7" aluminum-cone midbass drivers, which tend to have verylow efficiency and require a notch filter in the crossover.

Expanded-Foam

The next generation were the expanded-foam bass units, with the KEF B139being the most famous examples. This class of driver offered piston-bandoperation through the midbass, but suffered from very low efficiency,limited power-handling, and severe high-Q resonances in the midband. (Itwas not generally known that the B139 had a 12dB peak at 1100Hz with a veryhigh Q. Many reviewers blamed the midrange for problems that were actuallycaused by the B139 not having a notch filter in the crossover.)

They were quite popular in 3 and 4-way transmission-line systems in Britain(IMF) and the United States (Audionics) in the Seventies.

The Audio Professor Digresses

I remember working with the KEF B139, B110, Richard Allen 7", and front andrear KEF T27's in my first commercial design, the Audionics TLM-200 4-waysystem. My baptism into the mysterious arts of speaker design went asfollows:

Charlie, my boss: "Hey Lynn! You remember what Laurie Fincham was talkingabout when we visited KEF last year? All that stuff about impedancecorrection and frequency response target functions?"

Yours Truly: "Well, I didn't write it down, but I remember some of it."

Charlie: "Great! You can do the crossover for this!" ... pointing at amassive six-foot-tall loudspeaker with the 4 aforementioned drivers. Theprevious designer had left town without warning, leaving Audionics with amonster transmission-line complete with drivers and no crossover.Truestory, folks.

Carbon-Fiber (The Audio Professor Returns to the Topic)

The next generation were the Japanese carbon-fiber units, which made theirfirst appearance in the pro studio monitor (prosound) 12" TAD units withvery high efficiencies and very high prices (around $300 each in 1980).Carbon fiber prices have now dropped, and Vifa and Audax make good examplesof this type of driver. The Japanese make lots more of them, havingpioneered the technology, but they have been difficult to obtain if you area non-Japanese small-run specialist manufacturer.

These drivers have true piston action, outstanding bass and midbassresponse (the best I have ever heard), but also have nasty, chaotic breakupmodes at the top of their range. Removing these breakup modes requires asharp slope and one or two very sharp notch filters (this type of driverand filter is used in the top-of-the-line Linaeum speaker).

Although I very much dislike drivers that require filters as complex asthis (after doing the TLM-200, I vowed I would *never again* design a57-component crossover), I must admit the Vifa 8" and 10" carbon fiberwoofers are the only direct radiators where I've actually felt, not heard,tactile bass.

Kevlar

Kevlar drivers made their appearance in the mid-Eighties with the FrenchFocal and German Eton lines, with the Eton having superior damping due tothe higher-loss Nomex honeycomb structure separating the front and rearKevlar layers. The Eton and much newer Scan-Speak Kevlar drivers now sharethe limelight as the worlds pre-eminent high-tech drivers.

A unique and quite desirable property of the latest Scan-Speak Kevlardrivers is a smooth rolloff region above the usual Kevlar peak. All of theother Kevlar drivers (that I have measured and listened to) have chaoticbreakup regions; the Scan-Speaks are the only ones that appearwell-controlled in this region, which is certain to provide a significantimprovement in smoothness and transparency compared to other types.

Composite

Audax has made a surprise reappearance in the high-end market with anunusual composite technology, called HD-A. This is an acrylic gelcontaining a controlled mix of grain-aligned carbon-fiber and Kevlarfibers. The initial factory measurements show a striking combination ofpiston-band behaviour combined with minimal high-frequency peaking and asmooth rolloff above that point.

The Future

Evaporated diamond coatings are now available at low cost with a recentRussian breakthrough (cheap enough to coat floppy disks!), and I very muchhope that Scan-Speak and others pick up this technology quickly. As I saidearlier, things are changing fast.

* Overall Strengths & Weaknesses of Rigid-Class Drivers

Strengths are: Best available transparency, imaging, and depth presentationof any type, equalling or exceeding electrostats if carefully designed.High efficiency, high peak levels, and very low IM distortion in the bestexamples. This class of drivers is considered at the state of the art bymany designers, and this field is expected to advance quite rapidly asmaterial technology advances.

Weaknesses are: Older designs have severe peaking at the top of the workingband, and nearly all have a uncontrolled chaotic breakup region above thehigh-frequency peak. This would cause fatiguing sound over the long run anda compression of depth perspective and "air".

Any loudspeaker that does not use a correctly designed notch filter with aKevlar or carbon-fiber driver can be considered faulty; since the HF peakdoes not lend itself to correction with a conventional low-pass filter, andwill be quite obvious to any listener familiar with the sound of anunequalized Kevlar or carbon-fiber driver. Unpeaked rigid drivers are notcurrently available ... stay tuned, this will probably change in less thana year.

Although these types play quite loudly, the onset of breakup can be quitesudden and unpleasant, akin to clipping in a amplifier. Some Kevlar andcarbon-fiber drivers require an extremely long break-in period (>100 hours)to soften the fibers in the cone and the spider; this is a fault, since itindicates the materials may not be mechanically stable with extended use.

Best Examples are: The Scan-Speak 13M/8636 5" midrange, 18W/8544 7"midbass, and 21W/8554 8" bass drivers. These are the only Kevlar driversthat have reasonably well-behaved rolloff regions above the high-frequencypeak.

The Scan-Speak drivers also have vented pole-pieces that are copper-coated,reducing inductive types of IM distortion by tenfold or more. These driversare probably at the pinnacle of rigid-driver technology (in Spring of 1993at least).

The Audax HM130Z0 5.25" midrange, HM170Z0 6.5" midbass, and HM210Z0 8" bassdrivers in the HD-A series also look very interesting.

The German Etons also bear close watching, since the manufacturer iscontinuing to work on techniques to maintain the cone rigidity whileimproving the self-damping characteristics.

* More Coming Attractions

The next two articles that follow aren't on the Net, unfortunately, since

they make extensive reference to construction drawings of the cabinet,suggested placement in the room, and lots of MLSSA measurements. The twoarticles (The Soul of Sound, Part III and IV) describe an efficient (92dB @1 meter) 2-way transmission line suitable for use with triode amplifiers ofmodest power (8 to 22 watts work just fine). Look for these articles inVol. 4-4 and 5-1 of Positive Feedback magazine - back issues are stillavailable, as far as I know.

If I can convert the Macintosh PICT artwork to something that will survivethe uuencoding process and figure out an appropriate FTP site, I may postthese in the future. In the meantime, if you good folks on the Net areinterested, I could post my review of the Reichert 300B and the Ongaku inthis forum. (The review articles are entitled "The Outer Limits" and are*not* the Authorized Version from StereoPriest, Absolution Sound, or theAudio Cleric.)

------------------snap-------------------This is a revision of an article published in Vol. 4-2 of Positive Feedbackmagazine, a publication of the nonprofit Oregon Triode Society located at:4106 NE GlisanPortland, OR, 97232USA

For reprints, back issues, T-shirts, or subscription information, call JohnPearsall at: (503) 234-4155-- >From Lynn T. Olson

A series of meditations and thoughts on the differences between tube and transistor amps

Yah sure, you betcha, it's time for another Positive Feedback article ...a preview from the next issue, Vol. 5, Issue 2. This is a continuation ofthe "Soul of Sound" series on the art of high-end audio design.

Readers who are collecting the entire series might notice the absence ofPart III and Part IV ... well, those two parts are the constructionarticles for the Ariel high-efficiency transmission-line loudspeaker, andI'm still working on the politically correct way to transmit the ratherlarge graphics files over the Net (I may set up an FTP site here at

Teleport). If you just can't wait to find out about the Ariels, you'reprobably better off subscribing to the magazine, which is chock-full oftweaks, reviews, construction articles, and off-the-wall pieces of allkinds.

-------------------------The Soul of Sound, Part V

by Lynn Olson

This month's column is a collection of meditations on the art of audiodesign. They arise from a series of questions from fellow OTS members ande-mail that appeared in my Internet mailbox - which is:

lynno@teleport.com

For those who haven't followed these electronic discussions, here they arein print. I also have a bonus feature at the end of the column, anewly-arrived little brother for the Ariel.

*Frequently Asked Questions*

Q: Why do tubes sound different from transistors? Is this due to"euphonic" second harmonic distortion?

I don't think "euphonic distortion" exists. Granted, some sonic flaws area lot more tolerable than others, but I've never found any class ofdistortion or signal bending that makes the sound more transparent, morereal, and more lifelike.

People have been making gizmos like Aphex Aural Exciters, delay circuits,and "T-function shaping" for decades now, and all they really do is makemusic sound more like commercial FM radio. To my ears, at least, when Iremove a known problem (such as a resonant mode in a cabinet, a standingwave on a driver cone, a problem in a power supply, or improve thelinearity of an amplifying element) it pretty much always sounds better... more real, more truthful, more expressive, etc. In short, fixingproblems makes it sound truer to life.

So why do tubes typically, but not always, sound better than transistor

circuits, despite worse overall distortion? The answer doesn't lie incircuit simplicity, since if that were true, first-generation transistorunits like the Dyna PAT-4 and the Marantz 7T preamps would be audioclassics. Well, they're not, and a quick listen on even a half-decentsystem tells the tale.

I don't like measurements that only apply to the entire "black box", be ita speaker, an amp, a CD player, or whatever. They tend to mislead both thereviewer and the buyer. The measurements that tend to be the mostmeaningful are ones you can make on elements of the black box ... anactive device, a single driver, the way the crossover behaves faroff-axis, a single reflection at a cabinet edge, the frequency vs. sourceimpedance of a power supply, etc.

In other words, measurements are primarily useful as a tool for thedesigner to isolate problems, not as a go-nogo signal for the reviewer orbuyer. The reason is pretty simple: when you work at the component level,you can actually isolate cause-and-effect, and deal directly with thecause of the problem. When you get the vector sum of all of the stuff inthe black box, the measurements get a lot less useful, unless the designerreally made a big mistake.

Case in point: I've lived with the Audio Note Ongaku SE211, as well as theKassai PSE 300B and the Reichert SE 300B. I've also had access to mytrusty Audionics CC-2 (not a bad transistor amp), a modern multi-kbuckClass A transistor unit, and a souped-up Dyna Stereo-70. They all soundeddifferent, particularly to non-audiophile friends.

The Ongaku, by far, had the worst THD and power measurements ... 22W at 3%distortion. It also made the Ariel sound better than any electrostat I'veever heard ... in fact, the best sound I'd heard in many years. Itcertainly sounded better than anything I heard at the 1994 Winter CES. Sowhat's going on here? Maybe THD is simply measuring the wrong thing.

I have a hypothesis I want to explore. Any of you who have access toserious measuring equipment are invited to join in. The hypothesis isbased on a very simple premise:

If it sounds much clearer, more natural, more true-to-life, that implies aproblem (or type of distortion) has been removed. (Even if the problem

cannot be measured with present-day equipment.)

I am hypothesizing (bear with me here) that the problem with analogtransistor devices is actually non-linear Miller capacitance. You see,bipolar transistors, MOSFETS, and even diodes exhibit very significantchanges in capacitance with applied voltage (possibly current andtemperature as well). As capacitors go, it is my understanding thattransistors are very poor quality, worse even than electrolyticcapacitors.

To test this, of course, you'd want to bias the device and measure thebase-collector capacitance with a high-quality bridge that measures DF andDA. Even though the capacitance is only in the pF region, it isMiller-amplified by the gain of the transistor. This means any capacitancenon-linearities are multiplied by the forward gain, which is alsonon-linear. So we have two non-linearities multiplied by each other, andworse, one is in the time domain, and constitutes a type of FM modulationof the audio signal.

Now, this effect is probably very small, and hard to measure as well.Nevertheless, Walt Jung successfully identified very small problemsrelated to the DF and DA of capacitors more than a decade ago, and it isgenerally agreed now in the upper reaches of prosound and audiophileengineering that the choice of dielectric is very important in speakersand electronics. I'm just taking the same thing and applying to it toactive devices, and throwing in the implications of Miller-amplificationof DA and DF problems.

Note that tubes are very different in this respect. As capacitors go, thedielectrics are basically a little mica, glass, and a high vacuum ...pretty close to theoretically perfect. In addition, this capacitance isunmodified by voltage, current, temperature, or even if the tube is stonecold and completely inactive.

In a working circuit, of course, the capacitance itself is stillMiller-amplified, and yes, the tube is only somewhat more linear than itssilicon brother, but the non-linear part of the capacitance basicallydrops out of the picture.

Well, I agree, this is all pretty esoteric, but non-linear Miller

capacitance is one area where the tube and the transistor are verydifferent indeed. Of course, none of this shows up in a THD measurement,which is why I feel that the search for the ever-lower THD spec has ledthe audio industry in the wrong direction over many years, even decades.What we're looking for in the 21st Century may have very little to do withTHD ... I suspect that more meaningful measurements are awaitinginvention, and don't yet exist.

Q: What about crossovers and measurement techniques? How can I design myown crossovers?

A: This isn't going to be the answer you want to hear. Using an unmeasured"textbook" crossover is certain to produce poor results ... nowhere closeto the potential of the driver. Why? Drivers are nowhere close to a purelyresistive load and do not have a flat response ... both of which arerequirements for a "textbook" crossover. Using a stock crossover willproduce errors from 3 to 20dB, as well as wildly variable inter-driverphase characteristic, resulting in nulls that sweep through the listeningposition. Results? Grossly non-hifi sound. Don't do it.

Unfortunately, a FFT (or similar) measurement system is pretty much theminimum these days, although some designers still swear by (or at) 1/3octave pink-noise systems. The major benefit of FFT-class systems is thatthey allow examination of frequency, phase, and group delay responses, aswell providing the frequency resolution required for automatic crossoveroptimizers like LEAP, XOPT, CALSOD, etc. Of course, all designers havetheir favorite measurement systems, but I don't know of any well-regardedspeakers that are designed by ear without benefit of instruments ... thedays of Bob Fulton are long gone in the speaker business.

The minimum system is probably the IMP FFT board published in SpeakerBuilder magazine. The IMP and the Mitey Mike are about $400-500 from OldColony. Be aware that pure FFT systems have severe problems withsignal-to-noise ratio, requiring a quiet room and an amp that can throwbig pulses at the speaker. Old Colony Sound Lab is at: (603) 924-6371.

The next step up is the LMS from LinearX, made right here in Portland USA(I have no affiliation with LinearX ... I don't even get a discount). Thisis a chirp-based system with excellent frequency resolution and noiserejection, a very good integral mike, but no directly-measured phase or

group delay info.

The LMS is used by many big-name manufacturers for production-linetesting, as well as design work. If you don't need fancy waterfalls, oraren't into linear-phase designs, the LMS is a great choice, particularlysince it is well-tuned for making measurements for LEAP, XOPT, CALSOD,etc, crossover optimization programs. Glenn Philips, a fellow designer inPortland, knocks out custom designs all day long using mostly LMS andLEAP. (They sound pretty good, too.) The LMS + mike cost $1000 last time Ichecked. LinearX is at: (503) 620-3044.

I use MLSSA (so does Glenn Philips and plenty of big-name manufacturers).The MLSSA has largely displaced the Crown TEF, since it delivers prettymuch the same performance at about 1/3 the price and doesn't force you tobuy a reworked Kaypro as a host computer. (I've heard the new TEF unituses a PC host and has a price comparable to MLSSA ... maybe even cheaper.I haven't kept up on this.)

The MLSSA measures freq. response, phase, group delay, does waterfalls,calculates polar response, sum-and-difference curves, washes the dishes,rents the movies, etc. (Well, maybe the last two items are in the nextupgrade.)

MLSSA uses a pseudo-random noise sequence called a Maximum LengthSequence. The math coprocessor of the PC does a cross-correlation on theknown sequence, deriving a pulse, then does a FFT on the pulse, providingthe freq. response with excellent noise rejection. All of thesemathematical monkeyshines happens in a fraction of a second (on a 486DX).I also use the ACO Pacific 1/2" condenser microphone, since I couldn'thandle the heavy price of the B&K original. The MLSSA is a big step upwardin price: $3500, and possibly $4000 by now. The ACO Pacific mike, FET amp,and charge box are $1050. The MLSSA is made by DRA Labs, but unfortunatelythe manual shows no phone number. I think they are in Pennsylvania andthey do advertise fairly often in the AES Journal.

Yes, there are instruments that cost more, with the Audio Precision comingto mind. Since I remember an all-up price of >$12,000, I haven't checkedit out. I'm not sure if the AP offers true time-domain measurements, butit does have a lot of slick automated test features that lend it to fastproduction checking. The AP is just the thing for amps and CD's, though,

so if my little startup ever shows a profit, I'll buy one (after I payback my wife/sweetie/partner).

Now that I've thoroughly snowed many readers, I should stop and make apoint. Discussions of measurement systems quickly degenerate intoreligious wars for many speaker designers, with distinct overtones ofWindoze/Macintosh, UFO/Phil Klass, Fundie/Materialist/NewAge wrangles.Phooey on all of 'em. I use these machines to find out why the speakersounds strange, and I don't expect the instrument to think for me. Thereare plenty of things that are plain as day and don't show up with anymeasurement technique. Likewise, there are things that are just slightlyaudible, yet look quite nasty on the screen. This is where experiencecomes in.

So, to design a crossover, you must measure. Rock-bottom minimum is 1/3octave pink-noise or the IMP board. After that, you can design using theole cut-n-try (don't go beyond 18dB/octave) with Butterworth as a startingpoint, adding inductance compensation as necessary, notch filters asnecessary, and bumping the Q up and down until it sounds and measureshalfway right. Or you can save yourself a lot of trouble and optimize bycomputer ... but you may very well hand-tune the end result in 5% stepsanyway.

A point about crossover tuning: a lot of people don't realize thatinter-driver phase is far more audible than actual absolute phase (thatis, phase referred to the original source material). Why? The inter-driverphase characteristic controls the polar pattern above, below, and at thecrossover frequency, and I can assure you that a null sweeping across thelistener at a certain frequency is very audible. That's why subtleadjustments of Q (rolloff shape) are a lot more audible than you'd expect.

I use a simple subjective technique to check for coherency and finaltuning: I turn the speaker on its side, play good-quality pink-noise, getabout 1 meter away, and listen very carefully for a smooth spread of soundbetween the mid and tweeter. A good crossover should actually sound likeone driver ... and yes, it can be done, and must be done if you wantcohesive sound with decent image quality.

Q: When you design a system, where do you start?

A: In speaking with designers whose work I respect (and in looking at theways I've done my best work), I've found three main themes, or guidelines.These "guideline" aren't rigid rules, although I may have made them soundthat way; they are just straightforward techniques for designing audioequipment that meets your subjective goals. The essence of these rules isminimizing the number of variables and not making "automatic" assumptionsabout what's audible and what's not. Audio has a way of humbling the"know-it-alls"; this is a field that requires an open mind and aninquiring spirit.

1) Use the appropriate test equipment intelligently.

This requires equipment at a professional standard; fortunately, PC's havereduced the "entry price" very significantly. For loudspeakers, therealistic minimum is the LMS IMP (as seen in Speaker Builder magazine),the ATI LMS system, the DRA Labs MLSSA, or the Crown Techron TDS system(in order of increasing price). All of these systems require ahigh-precision calibrated microphone (which is included in the price ofthe LMS system).

Not good enough: General-purpose FFT systems, 1/3 octave analyzers, andwarble tones used with sound level meters. These systems do not haveadequate resolution to disclose narrowband resonances, are nowhereaccurate enough for crossover optimizing software, and do not provideinformation on time-domain behavior (the FFT does, but it suffers fromserious problems with signal-to-noise ratio).

For amplifiers, a distortion analyzer is required that can resolve out tothe 5th harmonic or higher. The Audio Precision is an obvious but quiteexpensive choice; secondhand Sound Technology, Hewlett-Packard, or Radfordunits work well, provided that the distortion residual is then fed to aspectrum analyzer or oscilloscope. (The systems mentioned above forloudspeaker analysis also work as FFT analyzers, but they must be usedtogether with a distortion analyzer, not as stand-alones.)

Not good enough: Total Harmonic Distortion analyzers with no way todiscern the proportion of the harmonics. THD, by itself, is an utterlymeaningless and potentially misleading number. What is much moreinteresting is the proportions of the harmonics, which requires anadditional FFT unit connected to the analyzer output. Although an

oscilloscope is an essential piece of equipment for examine high-frequencyresponse and overload behavior, it provides no direct information ondistortion behavior unless the distortion is very high (distortion isn'tvisible on a scope until it reaches 3% ... which is an overload conditionon any amplifier).

Used intelligently is far more subtle than it appears at first glance.This means you are well aware of the limitations of the equipmentmentioned above. For example, MLSSA is a superb tool for analyzing theperformance of drivers and complete loudspeakers. However, it won't tellyou beans about the swept IM distortion of a loudspeaker, which is animportant part of the character of any loudspeaker. Not only that, butswept IM measurements require a top-quality anechoic chamber and some veryexpensive B&K equipment to get meaningful results.

In addition, MLSSA only provides a very indirect picture of the radiationpattern of the speaker versus frequency: This requires many tediousmeasurements and a motorized stand to get repeatable results. Also, thereare plenty of things that are quite audible that just flat-out resistmeaningful measurements. The sound of wires and resistors fall in thiscategory, and there are lots of subtle decisions you can make withloudspeakers or amplifiers that just don't show up on any equipment atpresent.

2) All components going into the new device must be of known-goodsubjective quality.

This includes speaker drivers, tubes, transistors, enclosure materials,interconnects, capacitors, resistors, wire, solder, transformers,rectifiers, even dumb things like power cables.

It's all audible. In differing degrees, to be sure, but everything makes adifference. Please don't assume just because you can't hear some subtledifference that no-one else can. Not true! Everyone hears in differentways.

Well, how do you select these "known-good" components? There are two ways:

A) Pick out things you really, really like. (For example, I really, reallylike the Audio Note Ongaku and the Reichert amps. You might like Krell or

Levinson.) Find out what's inside. Don't be a pest, just quietly trackdown the capacitors, resistors, wire, etc. on your own.

Do research. I subscribe to Sound Practices magazine, and I take whatKondo-San and Herb Reichert say quite seriously, no matter how outlandishit may appear at first glance. Why? Because I respect their products, andI know from direct experience that quality never happens by accident.

B) Do your own sonic investigations. Go ahead, tweak away, and see whichparts you like! Just keep good notes, ask friends over and record whatthey say, and tweak something that's already good to start with! Don'tdesign something new and start playing with parts! You'll just go aroundin circles, since you'll be dealing with far too many variables all atonce.

Of course, once you start researching parts quality, you'll soon discoverthat tweaking takes just much time as "real" designing!

That's why I ask around and find out what other designers are alreadyusing, and start from there. Speaking for myself, I just find it easier tostart constructing with "known-good" parts that have worked well in otherdesigns that I respect.

Simple example: I'll probably use a 5AR4/GZ34 or 5R4-GY rectifier tube inmy first amplifier. Why? Amps that I like personally (based on the way itsounds in my system) use these rectifiers. When I ask around, the peoplethat design these favorites are quite definite in their opinions on theimportance of a given type of tube rectifier, even talking about thedifferences between brands and year of manufacture. Frankly, I have no idea why these archaic rectifiers with all theirproblems sound better than solid-state rectifiers. I don't care. I want todesign a good tube amplifier, not become an expert on rectifiers. Now whenI'm happy with the overall sound of the amp, and not before, I mightexplore other types of rectification ... and maybe not.

3) To audition the new device, you need a "standard of reference" for therest of your hi-fi system.

Don't assume, by the way, that anything in the "Recommended Components

List" or the latest hot-dog $50,000 system at your local salon representsa "reference system". If you live in a medium or large metropolis, chancesare excellent that someone has a superb system ... and audio clubs are agreat way to find them.

In my experience, and in the experience of designers I respect, hi-fisystems at this level of performance are not off-the-shelf systems, do notrepresent mainstream AES engineering philosophy, and may actually becomparatively inexpensive in dollar terms.

Audio systems at this level have hundreds of listening hours invested insubtle alterations at every point, and the development process takesnothing for granted. For example, I used to think that tweaking powercables was basically nuts. I don't anymore, because I've heard thedifference for myself, and that ended that question.

If you don't have access to a high-quality system, there is anear-certainty that you'll start compensating for the defects of yourother components. Mass-market CD players can either be shrill or very flatand one-dimensional sounding. Most commercial speakers have unnaturaltreble, a forward midrange, overblown bass, and poor imaging. Mosttransistor amps sound coarse and one-dimensional. The list goes on and on.That's just the way most mass-market stuff sounds, and not all "high-end"equipment lives up to the reputation. You just have make your ownassessments.

The real secret is to track down first-rate systems, hear for yourself,and find out what others have discovered. Tune into pleasure ... the rightequipment will have the extraordinary ability to touch the emotional truthof nearly any recording, regardless of "sound quality." Seek it out!---------------------

This article is continued in "Positive Feedback V (2/2)"

-- Lynn T. Olson ........ lynno@teleport.comSenior Editor, Positive Feedback magazine,a publication of the nonprofit Oregon Triode Society.For subscriptions, call (503) 234-4155 or fax (503) 254-3866.

-----------------------Q: Why did you draw such big distinctions between schools of design inPart 2 of Soul of Sound?

A: Well, I was stretching things just a little. There is a philosophicaltension in high-end audio, though, and I wanted to point it out. There areplenty of well-respected designers who are thoroughgoing materialists; asan example: "If I can't measure it on my AP/MLSSA/pinknoise system thatwas blessed by the National Bureau of Standards, you must be foolingyourself again, since you are a totally unscientific nitwit that couldn'treason your way out of a paper bag."

In response to this kind of name-calling, I can't resist quoting from the"The Science of Beauty" article by Herb Reichert which appeared in Vol. 5,Issue 1 of Positive Feedback:

"In America, we assume that if it were to sound "just like live", allof the emotional content of the performance would be conveyedautomatically.

I submit this is a poor premise to base an entire industry on. There isno law that demonstrates or suggests that a perfect reproduction of theoriginal soundfield would convey any of the original artistic quality ofthe performance. Additionally, there is no parallel in other media. Aphotograph is no substitute for a painting. Motion picture film or videowill not capture a theatrical production or sports event. Bronze castingsof marble sculpture lose most of the original beauty. What makes us thinkthat vibrating transducers will communicate artistic quality just becausethey are almost linear?

What is important to remember here is to remember that the recordplaying system is a media of its own like film, paint, or clay. With anyartistic media, communication is achieved through the creative use ofdynamic contrasts (drama) and profound architecture (form and structure).In this country, the creators have chosen to build audio that emphasizesthe mental picture of the performance. Image, depth, transparency, andgrain are of high importance to American audiophiles. Alternatively, theEuro/Asians have chosen qualities that emphasize the emotional content.

These music lovers design for maximum dynamic contrasts, presence,vividness, and effortlessness."

As you can see, Herb's article is an open challenge to the philosophy of"perfect accuracy" that has dominated the US high-end industry and theallied slick magazines.

As I have hinted in my previous articles, there is real turmoil in the USindustry at this time. Going to the Winter CES was an interestingexperience ... the great-numbers boys were doing THX demos with theirmegawatt amps and high-definition projectors, and the weirdos were playingLPs (in about 1/4 of the suites) with their directly-heated 300B and 211amplifiers. The majority of the exhibitors at the Sahara were using tubeamps ... for the first time since the mid-Sixties!

As my patient readers have noticed, I digressed pretty far from the lastquestion. Well, it covered a lot of territory. It's hard to convey theflavor of this industry until you spend a little time inside of it, whichis why I slide over into a kind of gonzo journalism when I describe thesocial side. The dry, "this-is-what-I-saw-at-the-CES" stuff just doesn'tconvey the zany aspect of the high-end business.

My heart is with the zany side ... I saw the Japanese eat Scott, Fisher,and Marantz whole, watched as Gordon Holt, Bob Fulton and Audio Researchrose from the ashes of the American consumer electronics business tocreate an independent high-end market, then saw the two "underground"magazines mature and become the all-powerful gatekeepers for the high-endindustry, and now ... full circle again ... a new set of magazines and anew philosophy is sweeping through!

Maybe we'll all be listening to Sovtek 10 and 45-powered Tractrix horns inthe next Millenium (the rebels at Sound Practices win). Maybe we'll watchdigitally-compressed HDTV, with sound provided by diamond-cone drivers,powered by German 500-watt PWM digital amps, and Windows software-basedcrossovers (the numbers-guys win). All I can say is that future of audiois wide open.

*Yes, A Bonus Construction Feature!!!*

Folks have been asking me about an easy-to-build version of the Ariels,then David mentioned that this issue might not have any constructionarticles in it. This prompted me to turn on the LEAP program on my PC anddesign a modestly-sized near-Bessel vented enclosure for the Arieldrivers. (I selected the near-Bessel alignment because it has the leastovershoot and time-ripple.)

Since the new arrivals are minimonitors, efficient, and 2-way, and are thelittle brother of the Ariels, Karna suggested christening them the ME2.Why not? So ME2 it is.

The ME2 is 18" high, 8" wide, and 8.5" deep, with an internal volume of521 cubic inches, and a pair of 1" diameter vents that are 4.125" long. Itretains the large diameter radius (1.25" on top, bottom, and one side) ofthe Ariels. Everything else is the same as the transmission-line Ariels... same drivers, crossover, front panel layout, composite MDF/plywoodconstruction, etc.

The LEAP modelling system shows a -3 dB point of 70 Hz, a -6 dB point of53 Hz, and a box frequency of 48 Hz. (Since they are intended for use withtriode amps, the alignment is centered on a Damping Factor of 8 when theyare connected to the 4 ohm tap of the output transformer. They are usablewith Damping Factors between 4 and 100).

Your initial reaction might be: 70Hz! That's not very much bass! Well,actually, it's not too different than plenty of other minimonitors ... alot of them are actually spec'ed at the -6dB point, which gives a bettermatch to the perceived low-frequency limit.

Still, I agree, you're not going to shake the room with these speakers.The ME2 aimed at three groups: folks who like the unadorned minimonitorsound, multichannel systems, and/or systems that use one or twobiamplified subwoofers.

A Multichannel Digression

Part of the reason I designed the ME2 was listening to the Cogent ResearchSPI, which is a modern sum-and-difference 4-channel decoder usingpsychoacoustically optimum decoding coefficients (whew!) This unit offers

high-resolution "frontal quad" without the listening fatigue that mostpeople associate with surround and quadraphonic systems.

Long ago, I invented the Shadow Vector quadraphonic decoder for Audionics,which became one of the first dynamic-matrix decoders ever built. (Thiswas far more advanced than the commercial CBS/Sony "Full-Logic" system,which used gain-riding and a static matrix.) As a result of months offine-tuning the Shadow Vector, I became quite aware of the action of adynamic-matrix decoder on various types of recordings.

Many years later, after quad disappered from the map of hi-fi, the DolbyPro-Logic Surround decoders became popular for theatres and home videosystems. The modern Pro-Logic family of decoders all use a dynamic-matrixtechnique closely related to the Shadow Vector, the CBS Paramatrix, andthe Sansui Variomatrix of the quad era. Since they use the same basicoperating principles as the previous devices, movie soundtracks are mixeddown in the presence of an engineer from Dolby Labs, to ensure that themix doesn't baffle the logic circuits of the decoder used in the theatre.(Yes, it is quite possible to encode a mix that can knock the logiccircuits for a loop. Happens all the time in multimiked classicalrecordings.)

After many years (decades!) of designing and listening to multichannelsystems, I've arrived at two main conclusions:

1) Multichannel systems require *better* electronics and speakers thanstereo ... in the exact same way that stereo requires better equipmentthan mono.

Also, as with stereo, all of the speakers and amps must be *precisely*matched. Unlike stereo, though, quality mismatching doesn't result in aleft/right asymmetry, but instead, a very rapid increase in "phasiness"and fatigue. This is why so many people who use mismatched low-cost rearspeakers eventually give up on quad. Using a "logic" system to force thelocalization into the mismatched speaker simply results in more listeningfatigue, not less, as frequency response, phase, and polar patterns gothrough extremely rapid changes as the sound source is steered around theroom.

2) Any system that requires the user to flip through mode switches for

each type of program (mono sources, movie surround, classical, jazz, rock)is fundamentally flawed. You don't need to do this in stereo, after all. Acorrectly designed multichannel system should provide stable, accurate,and fatigue-free localization of all possible encoding possibilities ...mono, panpot stereo, multimike stereo, Blumlein stereo, Dynaquad, EV-4,Ambisonic, Dolby Surround, etc.

The fundamental principle of all logic-steered systems is that theysharpen the localization of the loudest sound and defocus the quietersounds, with "attack" and "release" times of the logic-steering systemcarefully adjusted to be as unobtrusive as possible. Unobtrusive, though,isn't the same as inaudible, and this kind of fool-the-listenermonkyshines can be detected when you play an ungimmicked stereo recording.Of course, logic-steered systems always provide logic-off or "hall"switches for this condition. This is **bogus**! It requires the user tochoose between sharp localizations and natural sound.

The Cogent, and the Ambisonic/Gerzon family of decoders, take thedifferent tack of very carefully optimizing the fixed decodingcoefficients, and trusting the user to build a fully matched multichannelsystem with correctly located speakers. This an approach I agree with.There's really no point in a multichannel system unless it is better inevery way than a 2-channel system ... after all, stereo can be prettyfatiguing and unnatural if not done right, and multichannel is *much more*fatiguing and unnatural if not done right.

My prelimary listen to the Cogent showed a lot of promise ... very wideand spacious soundstage, stable imaging, with reasonable but not greatsound. BUT we were listening to a quad mid-fi Rotel amp and fourDefinitive Audio speakers at $300 each. Comparing it to the two Ariels andthe Kassai 300B was ludicrously unfair ... the Kassai is a superb $40,000amplifier, and the Ariels would have to sell for $4000/pair in a retailenvironment.

Which brings us to the ME2 speaker. It's more compact, less expensive tobuild, and offers the same sound quality as the Ariels, with 25 Hz lessbass. It's a prime candidate for an audiophile-quality multichannel systemusing the Cogent decoder, particularly with a high-quality pair of stereosubwoofers, which brings us to ...

The ME2 and Powered Subwoofers

I recommend trying a 80 or 100Hz crossover as a starting point, and usingno more than a simple high-pass capacitor for the high-frequencyamplifier. I also recommend a simple, high-quality triode amp for the highfrequencies, and a good transistor or MOSFET amp for the lows. This letseach amp function in its best frequency range ... in particular, thetriode amp will sound far better if it doesn't need to amplify lowfrequencies, which is what the input capacitor accomplishes.

Stereo subwoofers are a good idea, but not for the reasons you mightthink. It is indeed true that localization is very difficult to perceivewhen the wavelengths are longer than 10 feet. However, there is an effectknown as "Spatial Impression", which conveys the impression of size, orspace, and it is quite important at very low frequencies. A monosubwoofer, in other words, won't distort the image, but it won't sound as*spacious* as stereo subwoofers! In addition, it has been shown that a2-speaker stereo image grows narrower as the frequency is lowered ...which means that it is desirable to place the stereo subwoofers muchfurther apart than the normal 60 degree stereo-pair. Good results havebeen reported with 120 or even 180 degree spacing of the stereosubwoofers.

Level-setting shouldn't be a big chore ... select some piano music thatuses the bass register (I use the full-keyboard scale on the ProSonus testCD) and adjust the subwoofer level and phasing for the most natural and"fast" sound. Using piano music as a reference should curb the naturaltendency to let the subwoofers creep up in level.

The ME2 Crossover

The crossover is the same as the Ariel, since the factors that affect themid and upper regions are unchanged (cabinet width, driver layout, edgeradius, etc.) I would stick with North Creek Audio as a supplier,particularly for the resistors ... do not use the typical sand-cast powerresistors found in most crossovers.

I recommend and use Spague 730P polypropylene capacitors, North Creek 12or 14 gauge air-core inductors, Ohmite 10W 1% non-inductive powerresistors, isolated grounds for the low-pass and high-pass sections, and a

bi-wired connection to the amplifier.

Cabinet Construction

The drawings pretty much tell the tale. Use Baltic Fir or Apple-Plyplywood for the internal members, and premium-grade MDF for the outershell. The 1.5" thick front panel plays a significant role in quietingdown the cabinet and is worth the hassle of bonding two 0.75" panelstogether. Be sure to make the speaker in mirror-imaged pairs and try toachieve the 1.25" radius shown on the drawings ... I found out that thereis a pretty big sonic difference between 0.75" and 1.25" radius when Imade the Version 1 and Version 2 Ariels.

Lightly fill the V-shaped rear half of the cabinet with long-fiber wool(best for mids) or crimped Fortrel (like Acousta-Stuf), but don't coverthe inlets of the vent tubes. If you're using the ME2 with subwoofers,though, feel free to stuff the vents themselves (this slightly improvesthe midrange at the expense of bass response).

You'll notice that there really isn't any room inside the cabinet for acrossover ... that's intentional, since vibration of crossover componentscan degrade the clarity of sound. A small external box for the crossoveris the best solution ... but keep it at least 18" away from steel standsand transformers.

Setup

I recommend placing the ME2's at the tips of an equilateral triangle withyour listening position at the bottom of the triangle. Aim them at a pointabout 1 foot in front of you, with the tweeters on the inside of thestereo pair. You should be able to see just a little of the side of thecabinet that has the large radius (consider the speaker in the drawing theRight speaker).

Next, adjust the height of the stand so the centerline of the tweeter isbetween 38" and 40" high. If the tweeter is much below ear level, thestability and overall size of the stereo image will be degraded, so adjustthe height so the tweeter is at or above ear level.

When the ME2's are correctly set up and used with the right electronics,

they will have a very big, spacious sound, a stable stereo panorama welloff-axis, and will sound quite natural and lifelike even from anotherroom. They are "tuned" for the best possible voice reproduction, so userecordings of male and female singers to do the final in-room tuning withcrossover levels, cables, damping material, location from the back wall,etc.

When you are all through with the tune-up process, the ME2's will have allof the sonic quality of the Ariels you've read about in previous issues ofPF ... they're just as well braced, use the same drivers and crossover,and just as efficient at 92 dB/metre. So turn on your favorite 2A3 or 300Btriode amp, get out your favorite sides, and have some fun!

Suppliers:

Cogent Research25145 Manzanita DriveDana Point, CA, 92629(714) 493-0721

There are three models of the Cogent SPI, with identicalsum-and-difference decoding coefficients, but progressively more elegantinternal electronics. For people who enjoy multichannel sound but don'tcare for the subtle fatigue that logic decoders can induce, the Cogentunits are well worth a listen.

North Creek Music SystemsRoute 8, PO Box 500Speculator, NY, 12164(518) 548-3623

Top-flight drivers, capacitors, inductors, resistors, binding posts, glue,etc. as well as good information on subtle points of cabinet construction.I haven't heard the kit systems, but they look very good from what I cansee of the descriptions.

Madisound Speaker Components8608 University GreenBox 44283Madison, WI, 53744-4283

(608) 831-3433

Many, many drivers, all with spec sheets. Also, all of the littlebits-n-pieces needed for a complete loudspeaker.

A&S Speakers3170 23rd StreetSan Francisco, CA, 94110(415) 641-4573

Even more drivers, but no spec sheets unless you ask. Good info on whichvendor is using what drivers in their commercial speakers. VMPS kits aswell as their own.

ORCA1531 Lookout DriveAgoura, CA, 91301(818) 707-1629

They sell the very effective "Black Hole Five" damping material as well asthe interesting Axon speaker cables. All kinds of interesting odds-n-endsfrom these folks.

The Speaker Clinic1 SE 47thPortland, OR(503) 230-0366

Glenn Phillips offers a wide range of consulting and cabinet constructionservices using his MLSSA, LMS, and LEAP computer systems.

Speaker Builder Magazine andGlass AudioPO Box 576Peterborough, NH03458-0576(603) 924-9464

A unique and outstanding resource for information. I've subscribed to bothfor many years now, and the quality has steadily improved the whole time.

About the only source for info on transmission-lines, build-it-yourselfelectrostats, and long-forgotten vacuum-tube innovations.

Standard Disclaimer: All trademarks and registered names mentioned in thisarticle are the property of the respective trademark and registered nameowners.

-- Lynn T. Olson ........ lynno@teleport.comSenior Editor, Positive Feedback magazine,a publication of the nonprofit Oregon Triode Society.For subscriptions, call (503) 234-4155 or fax (503) 254-3866.