jbl home entretainment system

8/9/2019 JBL Home Entretainment System

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The Basics of Home Theater Design_____________________________________________________________________________By Floyd E. Toole, Vice President, Acoustical EngineeringHarman International Industries, Incorporated, Northridge, CA

It would be nice if we could custom-design every room that we equip for home

entertainment, but reality is not so kind to us. We need to be able to make do with many sizesand shapes of rooms, and with customers who have large and small budgets, and widely differingtastes in interior dcor and tolerance for things technical.

Gone is the concept of the TV and the stereo set. Video and audio now function together,and the layout of an entertainment system begins with the rules on where things need to go inorder to work as intended. After about 50 years with two-channel stereo, we now are dealing with

5 to 7 channels of audio, subwoofers and video displays of fundamentally different new types high definition, and on and on. It is not simple, but neither is it mysterious. So, lets have a look atwhat the pieces are and how they fit together to make a really good home theater.

1. MULTICHANNEL AUDIO/SURROUND SOUNDAlthough it is through movies that it came into our homes, multichannel audio is

becoming the new standard for music as well. Of course, picture and sound need to be

coordinated, with the three front loudspeakers arranged, left, center and right, with respect to thevideo display. Added to these are two surround loudspeakers, which should be located besideand slightly behind the listeners, making it a 5-channel system. In movie theaters, powerful low-frequency sound effects (LFE) are delivered by a separate channel. This is the 0.1 channel,which explains why we call these systems 5.1-channel surround. In home theaters, the low bassfrom all five channels is usually combined, added to the LFE, and the total signal is sent to thesubwoofer output. This is called bass management, and to properly make use of it you mustremember to go into the receiver or processor setup routine and set the left (L), center (C),right (R) and surround/side left (SL) and right (SR) loudspeakers to small, and then turn thesubwoofer output on by telling it that there is one. Small is simply a way of saying that no lowbass will be sent to these loudspeakers, and that all of the bass (including the LFE) will be sentto the subwoofer. If the customer has two large, full-range loudspeakers, these can be used aslarge front L and R units, and the others can be set to small. The bass manager will direct all of

the bass to the large loudspeakers. A subwoofer then becomes an option if, for example, morebass is needed to fill a very large room. There are other setup possibilities, and given thatmanufacturers do not all follow the same set of rules, it is wise to consult the manual or techsupport for your particular receiver or surround processor to determine what settings areappropriate for a specific installation.

Most movies and increasingly more music is available in multichannel audio formats,but it is important to know that there are algorithms/modes in receivers and surround processorsthat convert two-channel sources, such as CDs, FM and TV audio, into a multichannel signal.

Harmans Logic 7 is an excellent example, as is Dolby Pro Logic II. Nowadays, multichannelsystems have expanded to include one rear surround loudspeaker (6.1 channels) or two rearloudspeakers (7.1 channels). There are a few movies that specifically make use of thesechannels, but receivers and surround processors create the signals necessary to drive them,even if the signal source is a 5.1, or two-channel signal. The advantages of the additional twosurround channels can be dramatic, making the sense of perceived acoustical ambience inmovies and music much more realistic, especially if there are several listeners in the room.

1.1. Locating the Front LoudspeakersThe center loudspeaker does more than 80% of the work in movies and TV sound, and

therefore it needs to be a good one identical to or from the same family as the left and rightloudspeakers. It can be located above or below the picture centered on the picture but makesure that the tweeter is more than about 24 inches above the floor. The picture tends to dominateour impressions of direction to the point that we tend to localize the sounds to the action on thescreen. But there is a limit, and if the loudspeaker is too far away from the picture, as in the

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ceiling above it, dont be surprised if a customer complains about detached voices. So, ifpossible, arrange for the center channel to be on or at least close to the video display.

Ideally, the left and right loudspeakers should be positioned with their tweeters nearseated ear height, about 40 inches above the floor, although there is a generous tolerance onthis. Again, ceiling locations, while attractive from the visual point of view, are least satisfactoryfrom a sound perspective. Use your charm and expertise to persuade customers to accept in-wall, on-wall or freestanding loudspeaker alternatives. Incidentally, aiming the tweeter of a ceilingspeaker at the listener does not change where the sound is perceived to come from, although thehigh frequencies will probably be louder. Horizontally, try to get the left and right loudspeakers at 20 to 30, relative to the center channel when viewed from the money seat (this is a left-rightspacing of about 0.8 to 1.2 times the distance from the seat to the screen). Putting them close tothe sides of the video display is acceptable for large screens, but they should be spaced awayfrom smaller screens; otherwise, some of the spatial and directional effects will be lost, especiallyin music recordings.

1.2. Locating the Surround LoudspeakersIt may be a hangover from quadraphonics or it may be because we tend to call them

rear channels, but one of the most common mistakes is to put the two surround speakers in a5.1 channel system on the rear wall, behind the listeners. This is wrong. If there are only twosurround loudspeakers, they should be positioned to the sides of the listeners. Again, there is a

tolerance and, in practice, they will be found to work acceptably if they are forced to be slightlyahead, and even better if they are slightly behind. The rear wall is reserved for the additionalsurround loudspeakers in 6.1 and 7.1 systems. We recommend 7.1 channels where possible.

Two rooms arranged correctly formultichannel reproduction of moviesand music. Both work superbly. Thewhite speakers at the back of eachroom are for systems capable ofseven-channel operation, arecommended option. In thearrangement at the right, the spacebehind the TV is perfect for asubwoofer and/or a remote-

controlled rack of equipment. Theseating is also much more sociablethan the formal theater seating.

Surround loudspeakers should be located about halfway between ear level and theceiling or, if the ceiling is very high, about 56 feet above the floor. If the ceiling is of normalheight (about 8 feet) in-ceiling loudspeakers are an option if it is not possible to use in- or on-wallunits. Locate them approximately as shown in these diagrams, not above the listeners heads.

A common troublesome situation is when the back wall is immediately behind the seats.There is no place to put the 6

thand 7

thchannel loudspeakers, so dont even try. Set the system

up as 5.1-channel surround, with the side loudspeakers located just slightly forward of thelisteners, not stuck in the corners. In addition, to tame the powerful sound reflection from theback wall, try to find a way to get some sound-absorbing material fiberglass, acoustic foam,cushions, a fabric wall hanging, etc. immediately behind the heads of the listeners. Two to fourinches of fiberglass, covered with a loose-weave fabric (so sound can get through it) would beideal. High-backed, fabric-upholstered chairs also work.

2. SELECTING AND MOUNTING THE LOUDSPEAKERSThe quality of sound is dependent on two factors: the loudspeakers themselves, and

where they are put. A good loudspeaker in a bad location makes bad sound. Most conventionalbox loudspeakers are designed to sound best when they are freestanding, away from

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everything. But that is not how they always are used. Lets start by trying to understand what it isthat makes a good-sounding loudspeaker.

2.1. The Loudspeaker/Room System

Sounds radiate in all directions from a loudspeaker. Thefirst sound to arrive at the ears travels in a straight linedirect from the loudspeaker, but soon afterwards, thereare numerous other sounds that have been reflected fromthe walls, floor, ceiling and furnishings of the room. Allof these sounds together are combined in our minds toform the perceptions of what we hear. Loudspeakers,therefore, must be designed so that all of these soundsare good sounds, otherwise the timbre, or sound quality,of recorded voices and musical instruments will bedegraded. In fact, since most of the sound that we hearin a room is reflected sound, the off-axis behavior ofloudspeakers is exceptionally important.

Consequently, when we measure a loudspeaker, we measure everything that radiatesfrom it, in all directions. Then we calculate, from this data, what a listener in an average room

would hear. The measurements must be done in an echo-free environment outdoors or in anexpensive anechoic chamber so that they are accurate.

dBS

20 100 1K 10K 20K Hz

10 dB5 dB

0 dB

DI

ON AXISLISTENINGEARLYSOUND

dBS

20 100 1K 10K 20K Hz

ON AXISLISTENINGEARLYSOUND

10 dB5 dB

0 dB

DI10 dB

DI5 dB

0 dB

The figure above shows a set of measurements that give us a very good prediction of how thisloudspeaker will sound in a room. The on-axis curve describes the direct sound, thefirst sound to arrive at the ears of somebody seated in the sweet spot. The listening windowdescribes the average direct sound for listeners seated or standing within a 10 vertical, 30horizontal region directly in front of the loudspeaker the entire audience in a home theater, forexample. The next curve describes the sound of the average strong early reflections from theroom boundaries, and the sound power is a measure of the total sound output of the loudspeakerwithout regard to direction. The bottom two curves describe the directivity (DI is Directivity Index)of the loudspeaker: how uniformly it radiates its sound into the room at all frequencies. In a trulygood-sounding loudspeaker, the on-axis and listening window curves will be smooth and flat, and

the other curves will be smoothly and gradually changing. This is an example of an exceptionallygood loudspeaker.

Clearly, a single curve is not sufficient information to describe how a loudspeaker willsound in a room. When we do double-blind listening tests, it is routine for loudspeakers withmeasurements that tend toward the ones shown here to sound good, and get high scores fromlisteners. When they show irregularities, even quite small ones, the scores go down. So whenwe install loudspeakers into our homes, we need to be sure that we are not changing how theloudspeaker behaves. As it turns out, each mounting option has its consequences in respectto how a loudspeaker sounds. Let us look at some common installation techniques.

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First, a 6-inch two-way loudspeaker is put on a stand, three or more feet away from thewalls, as serious audiophiles might do (a). Then it is hung on a wall (b). Then it is placed intoa cavity, such as in the now-popular entertainment centers (c), and then the space around it isfilled with fiberglass (d). Finally, it is flush-mounted into a wall or custom cabinet (e). The dashedcurves in (e) show the result of turning the bass down to compensate for the gain provided by thewall. The top curve of each set is the on-axis measurement and the lower curve is total soundpower. The vertical axis is in dB, and the horizontal axis is frequency, from 20Hz to 20kHz, as inthe previous example.

(a)

(b)

(c)

(d)

(e)

(a)

(b)

(c)

(d)

(e)

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Inspecting these curves, it can be seen that placing a loudspeaker near or in a wallboosts the low-frequency output. This is because all conventional box loudspeakers areomnidirectional at very low frequencies, and the sound that would have traveled backwards, awayfrom the loudspeaker, is now reflected by the wall and adds to the forward radiated sound. Theeffect diminishes with increasing frequency. This simple effect, if it is the only issue, as in (e),can be easily corrected with a bass tone control or a simple equalizer. Without correction, theloudspeaker is likely to sound fat and boomy. If a loudspeaker is designed for in-wall or on-wallmounting, then the correction will have been incorporated in the original design, and no furthercorrection is necessary. This is why we build different loudspeakers for different kinds ofmounting.

The other mounting options shown here have additional problems caused by anacoustical cancellation due to the woofer being the thickness of the loudspeaker away from thewall (b), or a combination of cavity resonances and diffraction effects (c) ugh!! Filling thecavities with sound-absorbing material damps the resonances (d), but some of the problemremains. If this is done, cover the entire opening with grille cloth, such as polyester double-knit, orsome other open-weave fabric. Mounting (c) sounds particularly bad. Sadly, it is a very commonexperience, encouraged by the misleading name that we give to these small loudspeakers:bookshelf loudspeakers. Ironically, in practice we find that the worst place for a bookshelfloudspeaker is on a bookshelf. If the spaces around them are filled with books, the situation isimproved, but this is not always done.

The conclusion to be drawn from this set of data is that there are really only two locationswhere a loudspeaker can be expected to perform truly well: freestanding, or flush-mounted in awall. If a loudspeaker is mounted flush in a wall or cabinet, and it is not an on-wall or in-wallloudspeaker, then some equalization is necessary to compensate for excessive bass.

2.2. In-Wall/In-Ceiling LoudspeakersLoudspeakers designed for installation in walls and ceilings are not new, but ones that

sound really good are. There have been exceptions, of course, but, until recently, most of theseproducts were used in commercial distributed sound applications, where sound quality was not aprime concern. A lot of them evolved from speakers conceived for car audio and, just as in thatapplication, the lack of high expectations and the absence of rigid enclosures led to anacceptance of less than stellar sound quality. Now, of course, they are considered as viableoptions to conventional freestanding loudspeakers, whether the sound quality is there or not.

The Harman brands have worked hard to elevate the sound quality standards of allin-wall models. Some of the high-end in-walls actually compare favorably with very goodfreestanding models, but such quality is not inexpensive. Careful installation of good in-wallloudspeakers can result in absolutely first-class sound. Some high-end models even havevibration isolation, to minimize coupling of the loudspeaker vibrations to the drywall.

At the other end of the quality spectrum are careless and carefree installations, where itseems that no thought has been given to integrating sound and picture the video display can bewell away from the center channel and all of the loudspeakers are in the ceiling because that isthe path of least resistance for those selling the systems or the houses. There is sound, but it isnot good sound. If plumbing systems were designed according to this philosophy, we would findtoilets in closets and sinks in the garage hide them. Instead, it is accepted that they performimportant functions in our lives, and they are placed where they work best, even though itchallenges decorators to find ways to integrate them visually.

All too often we find the fireplace where the video display should be. Well, the fireplace isno longer the hearth of family life. Television, movies, and music are, so let us arrange our livingspaces accordingly. Architects need to be dragged into the current century. I personally have justspent thousands of dollars to move a fireplace located by horse-and-buggy-era thinking, to alocation that matches a 21

st-century lifestyle. It is a relatively new house, so there is no excuse.

In-wall/in-ceiling loudspeakers radiate their sound primarily in a direction perpendicular tothe surface in which they are mounted. If this is a ceiling, the aiming point is the floor. Althoughsound does radiate sideways, it is not the best sound and, even with tweeters that can beaimed, there is still a compromise. The loss can be more than sound quality; it can be speechintelligibility, something rather important in television viewing. The ultimate compromise is that

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sounds that were intended to be localized down at the level of the listeners and the video displayare instead coming from above. As said earlier, this can be tolerated for the surround channels,but it is something to be resisted for the front channels.

For distributed sound applications, i.e., background music for kitchens, bedrooms, baths,recreation rooms, ceilings are excellent locations. In some areas the fact that the sound firesdownward can be used to advantage, directing it at the locations where the action is, by the sinkand stove in the kitchen, for example. If the customer wants to hear sound everywhere, then theissue is the density of the units required to achieve uniform coverage. Low ceilings require moreunits than high ceilings, acoustically dead rooms require more units than more reverberantspaces, and so on. Wide-dispersion loudspeakers cover more area than directional loudspeakers.You may need to do some experimenting.

An important factor in in-wall/in-ceiling loudspeakers that is often overlooked is that thesound easily can leak into other parts of the house. In-wall loudspeakers obviously can talk to anadjacent room. Ceiling loudspeakers, especially those without back boxes, can fill an attic withsound that then can penetrate into other rooms. Loudspeakers close to HVAC ducts have readymade pathways to other parts of a house. All of these cautions need to be carefully consideredand explained to customers who are dreaming of invisible loudspeakers.

Some loudspeakers are designed to work best with back boxes of a specific volume,and some are quite tolerant of the effective volume of the wall space. It is an engineering designoption as far as the bass response is concerned. A back box will help to constrain the sound

within the wall cavity, reducing its tendency to travel throughout the wall, especially if the box isconstructed with perforated steel studs. In any case, it is recommended to loosely fill the wallcavity or back box with glass or polyester or cellulose fiber.

In-wall subwoofers exist, but if space is available, any subwoofer can be mounted in acavity in a wall or cabinet. If there is a port, be careful not to block it, and allow for a generousacoustical path for the low bass frequencies to escape into the room. If the sub is powered,allow for ventilation, otherwise it will overheat and lapse into an expensive silence. Since lowfrequencies radiate omnidirectionally, it doesnt matter which way the driver faces.

2.3. On-Wall LoudspeakersThese are loudspeakers that have been designed to sound good when they are hanging

on a wall. The bass balance is correct for this kind of mounting, and the shallow depth minimizesthe problematic acoustical cancellation dip seen in (b) of the previous figure. Many multi-

directional surround loudspeakers fall into this category, and there are a few conventionalforward-firing loudspeakers that can be used either as front or surround channels, or fordistributed sound installations where in-wall mounting is not possible.

2.4. Floorstanding LoudspeakersFloorstanding speakers, whether they are towers or bookshelf speakers on stands, are

affected by where they are put relative to the adjacent walls. Avoid placing them in corners. Thebass will tend to become boomy, voices chesty, and the midrange frequencies will be colored.If possible keep them two feet or more from the sidewalls. The correct distance from the wallbehind the speakers depends on the speaker itself. Conventional front-firing speakers arerelatively tolerant, and some can work well when quite close to the back wall. Others wont, sotry them and listen. Again, pay attention to the lower frequencies. Multidirectional speakers, likebipolar and dipolar designs, need space, and some need to be quite far out into the room before

they work really well.If you experiment with the positions of full-bandwidth floorstanding speakers you maynotice that several things are changing at the same time, and they may not be agreeing with eachother. For example, as the speaker is moved out into the room, it may sound spatially more openand natural sounding. However, the bass may be less powerful, or less even. Bass usuallybecomes more potent as a speaker is moved close to a wall, and even more so in a corner.However, the corner is the worstlocation for all other frequencies. This is the sort of thing thathas led to the popularity of subwoofers, which can be located where they produce the best-sounding bass, while smaller speakers can be located where they deliver the best sound andimaging for the bulk of the audible spectrum.

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2.5. Subwoofers

Because they operate only at very low frequencies, typically below 80Hz, subwoofers arevery tolerant about how they are mounted. At these frequencies the wavelengths of sound arevery long compared to the size of the subwoofer, so the sound radiates omnidirectionally equally in all directions. It doesnt matter which way they are pointed, toward the listener, up,down or away. The sound will be the same, as long as the driver is free to radiate its sound andthe port, if there is one, is not blocked. Keeping a 4- to 6-inch clearance between the driver orport and a wall or cabinet should be enough to let the sound radiate freely. If the sub is powered,be sure to ventilate it. In terms of where in the room the sub should be located, and how manysubs are needed, there are other very important considerations that will be discussed later.

Since most of the acoustical power is needed for bass frequencies, the use ofsubwoofers substantially reduces the size requirements for the five or seven main-channelloudspeakers, allowing us the option of using moderately sized in-wall, on-wall or bookshelf units.

3. ROOM ACOUSTICSThe room is the final audio component and, as such, it can make or break a truly

satisfying listening experience. The shape and size of the room, and how things are arranged init, are major influences on the quality of bass we hear. The amount of absorbing material and

furnishings influence how live the room sounds, and this affects imaging and space. All roomsare different, so a guaranteed recipe for success doesnt exist. However, it is relatively easy toavoid the worst mistakes.

Let us separate the issues.

1. Room size and volume. These determine whether you need large or small loud-speakers to satisfactorily fill the space. Obviously, the needs for background music arefundamentally different from those for a home theater. It is the main determinant of howmuch subwoofer power is needed, and here it is important to note that, if the hometheater room is open to other parts of the house, the subwoofer requirements will beinfluenced by the entire coupled space, not just the one where the audience is. Ingeneral, larger rooms will have better-sounding bass, but they will need bigger, morepowerful subwoofers to fill the volume.

2. Room shape. Rectangular rooms are fine. Splayed walls, or other exotic shapes are notnecessary for good sound. L-shaped rooms and three-sided rooms force us to be morecreative, but can also work well. The shape of a room has a lot to do with roomresonances at low frequencies. In perfectly rectangular rooms, the resonances can becalculated with good accuracy so that, if there is a problem, we have a chance ofidentifying what is causing it, and fixing it. In nonrectangular rooms, it is not possible toeasily predict what will happen, requiring higher math and a big, fast computer. Notions ofcertain room dimensions or proportions that are somehow ideal are fanciful, since thequality of bass that is delivered to our ears is determined by where the loudspeakers arelocated within the room, how many there are, and precisely where the seats are located.

3. Acoustical treatment. At some time in our lives, we have all been in a totally empty

room, an experience in untamed acoustics. Sounds are extremely live

they reflectand reverberate. Clap your hands and the impact is repeated hundreds of times, as thesound is reflected among the hard, flat room boundaries. Talk, and voices take on anartificial richness in the lower frequencies, and intelligibility is reduced because eachutterance is prolonged by overactive reverberation. Put some carpet down, and thingsimprove dramatically. Add drapes, some furniture, and the room is transformed into amuch more pleasant space in which to live, converse, and listen to music and movies.Fill the room with too much stuff, including lots of upholstery, cushions, heavy drapes,

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etc., and the place can become overly dead and stuffy. In a normally well-furnished room,adjusting room acoustics is mostly a matter of tweaking the furnishings.

If you really get serious about optimizing the room acoustics, be prepared tomodify the dcor. The goal is to make the room neither too live nor too dead. A happymedium is the objective.

Makes a room more live Makes a room more deadHardwood or tile floor with small area rugs. Wall-to-wall carpet, large thick area rugs.Thin low-pile or looped-pile carpet. Thick, clipped-pile carpet with a felt or porous

foam underlay.

Light Scandinavian furnishings, leatherupholstered chairs and sofas.

Bulky, fabric-upholstered chairs and sofas, withcushions.

Lightweight curtains and drapes. You can seebetween the fibers, and blow through the fabriceasily.

Heavy, dense-weave, velour or velvet drapes.Fabrics that are more opaque, and that offermoderate resistance to blowing.

Flat, unobstructed walls that act like bigacoustic mirrors.

Walls that are broken up with irregularities,such as fireplaces, bookcases (excellent!),paintings, display cases, etc., to add diffusion.

If you are partial to hardwood, stone or tile floors, more attention must be paid todeadening the room with drapes and the type of furnishing. If you are partial to the stylishwood and leather Scandinavian style of furniture, then youd better think about heavycarpeting or rugs. A combination of hardwood flooring with the light furnishings yields anarchitecturally fashionable, but acoustically hostile, environment for good sound.

Thanks to the built-in ambience of multichannel sound, the error that is easiest tolive with is for the room to be too dead. Some styles, cultures and climates encouragehard, lively rooms, with wood or tile floors, and even plaster or masonry walls tocompound the situation. Others encourage the cozy surroundings of heavy carpets,drapes and soft furnishings. It can be a difficult balancing act, if decorating tastes donot match the needs for good acoustics.

Of course, there are dedicated acoustical devices that can be purchased to addabsorption or diffusion to a room. These work, but they tend to stand out in a listeningroom that is also a normal living space. Be creative, and try to find some furnishings,

artistic wall hangings, window treatments, etc., that maintain the homey atmosphere.In a dedicated theater room, the rules are very different, and it is much more likely thatoptimum acoustics can be achieved.

4. GETTING GOOD BASSTo most people, good bass is a big deal. While novices are easily impressed with the

quantity of bass, more experienced audio folks want more. They want deep, tight, clean, non-resonant (i.e., not boomy) bass and, ideally, they want it at all seats in the room. Two-channelstereo, which we have enjoyed for the past 50 years or so, is optimized for a single listener inthe sweet spot it is fundamentally antisocial, even though most of us violate the position rule,and simply enjoy the music without enjoying the stereo imaging that is also part of the totalexperience. However, multichannel audio is fundamentally different it is social, an experience tobe shared by multiple listeners, seated at different locations in a room. After all, it originated in

cinemas, where hundreds of customers pay to share an audio/visual event.In very large rooms, like concert halls and cinemas, bass can sound good almost

everywhere. However, in rooms the size of home theaters, resonances cause some bassfrequencies to be too loud and others to be too soft depending on where the subwoofer islocated, and depending on where the listener is seated. So, the dimensions and shape of theroom determine the resonances. The location of the subwoofer determines which resonancesare energized. The locations of the listeners determine which of these will actually be heard.Complicated, eh?

It is a situation that can be improved by adding more subwoofers. So, there are threevariables that we can play with: the number of subwoofers, subwoofer location and listener

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location. Simple rectangular rooms allow us to make some basic assumptions, and topredict from calculations what might happen. For these rooms we have some standardrecommendations, which we will talk about. But rooms with complicated shapes, or that havelarge openings to other rooms, are mysteries until they are built, and we can get into them foracoustical measurements. The solutions here are more complicated. Although most rooms androom arrangements exhibit problems, others dont. If you get a good one, thank your lucky stars.If you get a bad one, dont despair; it can be fixed if you have the time and the customer has thebudget.

4.1. The Single Subwoofer SolutionIn general it can be said that a single subwoofer is certainly better than none. However, it

is not a complete solution because it normally does not yield similar or good bass at all seats inthe room. A good location for a single sub is in a front corner or somewhere along the front wall.Start with it in the corner and go to the prime seat and listen to a selection of music with goodbass content kick drums and bass viols or guitars are excellent test signals. Have someonemove the sub from the corner along the wall and observe how the sound changes. With luck youcan pick a good spot. Finding a good recording is a task in itself, because the problem we areaddressing also exists in recording control rooms. Consequently, not all recordings have well-recorded bass sounds!

If you are able to do frequency-response measurements you are in good shape, because

what you are seeing is not a matter of personal opinion, and the quality of the recording is not afactor. For this purpose, an analyzer with at least 1/10-octave resolution is best because it allowsyou to see resonant peaks, which can be very annoying. However, even a simple 1/3-octave real-time analyzer (RTA) will give useful overall indications of what is happening.

Because of acoustical reciprocity, you can do this little experiment alone. Put thesubwoofer at the prime seat location and take your ears or measuring microphone to all possiblelocations for the subwoofer. Dont be afraid to experiment, because the best location may not beon the front wall. When you have decided on a location for the sub, put it there and do some morelistening. If the chosen location is not on the front wall, pay special attention to whether you canlocalize the bass. It is not a good idea if the bass is heard to come from a direction different fromthe front loudspeakers. Sometimes it happens because the sub is too close to a seat. Sometimesour ears are attracted by a fault in the sub, such as vent noise, distortion or buzzes or by rattles innearby windows, or furnishings. If everything is correct, it will be very difficult or impossible to

identify the location of the subwoofer if it is crossed over at 80Hz (this is part of the receiver orsurround processor setup). Very small satellite speakers cannot handle low frequencies and,therefore, need to be crossed over at higher frequencies. This often is the case in HTIB (hometheater in a box) systems or smaller packaged systems. In these cases, the subwoofer needs tobe placed among the front loudspeakers to prevent it from being localized.

Once the best location for the sub is identified, there is still more that can be done equalization. With a single subwoofer, the sound at the prime listening location can be equalizedto minimize any residual room resonance effects. It may also work at other seats, but this will beimpossible to predict. Chances are, they will be less satisfactory. Equalization will be discussed ina later section. If you really want good bass at several seats, you will need more subwoofers.

4.2. Multiple Subwoofer SolutionsThis is a case where more are better. Better, in this context, is not more bass in terms of

quantity (which might also be true), but better bass in terms of sound quality and uniformity atdifferent seating positions. As stated earlier, the problem is room resonances, or standing waves,which cause the bass to sound different at different locations in the room. If we could acousticallydamp these resonances, removing energy from them in sound absorbers, the problem would bereduced. The problem is that such absorbers are very expensive to buy, very difficult to build and,in both cases, very large. This solution can work, but is not attractive for many home theaterinstallations.

Using multiple subwoofers, it is possible to position them so that they cancel some of theroom resonances, and they do not excite others. The result is more uniform and better qualitybass over the listening area, not just at one seat.

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Equalization can fix this

Equalization cannot fix this

BEFORE andAFTER one bandof parametric EQ

Equalization can fix this

Equalization cannot fix this

BEFORE andAFTER one bandof parametric EQ

The curves above show measurements of a subwoofer in a room that had a very strong

resonance around 47Hz (red curves). The left-hand figure shows high-resolution frequencyresponses and the right-hand curves show the corresponding time responses. Originally, itsounded very boomy, with strong one note bass effects. A single parametric filter was all thatwas needed to fix the problem in this room, and when the peak had been attenuated (blackcurves) the result was very neutral-sounding bass with excellent transient response. It isimportant to remember that, with a single subwoofer, this equalization works only at the

measurement location one seat. When multiple subwoofers are used to obtain more uniformbass over a listening area, then the equalization will work at several seats. Since multiplesubwoofers actually attenuate the standing waves, less equalization tends to be needed.Sometimes none.

6. WIRESIn spite of a lot of marketing smoke about other real or imagined qualities, the most

important aspect of any loudspeaker wire is its gauge, or size. The wire should deliver the signalfrom the output of the power amplifier to the loudspeaker without changing it. It can do that best ifthe resistance of the wire is small compared to the impedance of the loudspeaker. Obviously, thelonger the run from the amplifier, the larger the wire needs to be.

The table to the left shows estimates of thewire requirements for different loudspeakersat different distances. These values shouldprevent significant changes in soundquality, preserving the performance thatthe manufacturer created. Of course, if theloudspeakers are poor to begin with, noneof this matters much.

35 ft.26 ft.18 ft.16HEATER CORD

56 ft.42 ft.28 ft.14

88 ft.66 ft.44 ft.12

140 ft.105 ft.70 ft.10

8 OHM

SPEAKERS

6 OHM

SPEAKERS

4 OHM

SPEAKERSWIRE GAUGE

35 ft.26 ft.18 ft.16HEATER CORD

56 ft.42 ft.28 ft.14

88 ft.66 ft.44 ft.12

140 ft.105 ft.70 ft.10

8 OHM

SPEAKERS

6 OHM

SPEAKERS

4 OHM

SPEAKERSWIRE GAUGE

7. CALIBRATIONWe are almost done. The speakers should be aimed so that the best sound gets to the

important seats. Most good speakers have wide enough dispersion to cover large horizontal

angles but others dont, and some are like acoustical flashlights. Get someone to angle thespeakers while you listen, or move yourself around. A recording of broadband pink noise is agood test signal. It should sound similar in the important seats.

Using a sound-level meter (the Radio Shackones work fine) adjust the levels of theindividual channels to the level specified in the manual for the receiver or surround processor.This should be done for the prime listening location in the room. From this same location,measure the distance to each of the front and surround channel loudspeakers, and follow themanufacturers instructions for adjusting the delays in each of the channels. If the system is set

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up to play DVD-Audio and/or SACD, these adjustments may need to be done in the DVD multi-player as well, because the receiver or surround processor will be in analog bypass mode.

None of this is very difficult but, like many things in life, a little extra effort at the beginningcan pay off handsomely in long-term satisfaction. And the best thing of all is that most of what wehave discussed costs little or nothing.

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jbl home entretainment system

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