Reproduced and translated with permission of Ozawa Takahisa and MJ Magazine. Original material publication ©MJ Magazine. All rights reserved.

Design and Building of Small Speakers No. 23Author - OZAWA TakahisaTranslation - Yasushi Matsumoto

Introduction6.5” (16 cm) full-range drivers are popular among DIY builders, since many of them deliver well balanced high-and-low ends with relatively high SPLs. They can be built easily without any network circuitries and driven well enough with small tube amps. In this serial article, building of a new full-range speaker system will be introduced, which will implement a brand-new 16 cm driver launched at the end of last year.

Features of Markaudio Alpair 12I saw Markaudio products first time in October of last year at the booth of Roppongi Institute of Technology in the Tube Audio Fair. Markaudio, a manufacturer based at Hong Kong, founded by Mr. Mark Fenlon in 2000, is producing several speaker units including mainly metal cone drivers, according to the company’s web site. I heard Mr. Fenlon had been involved in development of an E.J.Jordan driver starting in 2003. The Markaudio’s brand-new model Alpair 12 is the largest full-range driver in the manufacturer’s product line.

The picture beside the titles show everything packaged in a neat carton box including the driver unit and attachments consisting of a seal (this is unusual among imported speakers) and black coated special screws. The unit looks well elaborated showing high level production control.

Markaudio is announcing that their products use only dedicated parts, that are specially designed and manufactured for each product without using any common (off the shelf) parts. Hence Alpair 12 (photo. 1) also equips such dedicated parts specially elaborated for the sound quality.

Its metal cone is made of a very thin (< 0.11 mm) aluminum & magnesium based alloy sheet formed in a curved cone profile. The surface color can be selected from Gold and Gray. Its rubber-base / recentrant-shaped edge (front suspension) is very soft and thin comparing to other drivers and manufactured within a tolerance of +-5 micrometer. Its center cap is directly glued to the coil bobbin to significantly strengthen the adjacent area which suppresses local vibrations and consequently delivers an excellent high frequency response to 20 kHz.

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Figure 1 (above left): Markaudio Alpair 12 in grey or gold: (45,000 YEN/pair, reference only)Markaudio website: http://www.markaudio.com)

Figure 2 Above centre and logo right): Alpair 12 is distributed to Japan via Fidelitatem Sound Ltd. and available from several Japanese distributors such as Roppongi Institute of Technology.

Figure 3 (above left): The frame designed to minimise effects from back pressure and reflectionFigure 4 (above right): The damper designed by a Japanese audio speaker designer.

The frame (basket) is made from resin because of a better resonant property comparing to metal, which is more similar to wood. It looks robust enough due to reinforcement ribs. For the magnet a non-shielded ferrite core is used. The gold plated terminals are fixed securely to the frame via color-coded resin bases (red:+, blue -).

Let’s see the back side of the frame. Each column is structured with three thin plates, which seems intended to minimise sound reflections and resistances (photo 3). It is announced that the damper (spider) is designed by Mr. Matsubara who used to be an engineer in a famous speaker manufacture in Japan and succeeded to significantly reduce a piston motion resistance of the coil while improving its linearity (Photo 4).

Performances of Alpair 12Table 1 and Figure 1 - 3 are official data from Markaudio. The design achieves quite low “Fo” of 46 Hz while maintaining relatively high “SPL” (nearly 90 dB). Since Xmax (9 mm) is large enough to be comparable with some low power woofers, a good sound quality can be expected in the low frequency range. This driver could also be suitably driven by tube amps because of its impedance (8 ohm). Please be aware that the charts in Fig. 1 and Fig. 2 are plotted with a frequency range up to 30 kHz when comparing them with those charts of other units because they are normally plotted in a range of 20 Hz – 20 kHz.

Although the gross outer diameter (207 mm) of Alpair 12 is comparable with those of 18 cm class drivers, its effective radius derived from the effective diaphragm area is only 6.75 cm that is in the range of typical 16 cm drivers. Therefore this time Alpair 12 was compared with other 16 cm units.

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Table 1 Markaudioʼs published specifications.

Figure 1 Markaudioʼs published SPL vs Frequency graph

Figure 2 Markaudioʼs published Impedance vs Frequency graph

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Figure 3 Markaudioʼs published installation diagram

The enclosure used for the comparison measurements is shown in Photo 5. The box is built using lauan plywood (t = 12 mm) with the outer dimension of W300 x H440 x D 324 mm. Its front and back faces are opened and exchangeable so that a 20 cm class driver can be installed as well as several box types such as open-back, sealed and bass reflex can be tried easily. This time, a sealed box stuffed with moderate amount of sound absorption material was used for a series of measurements (Photo 6).

Figure 5 (above left image) and Figure 6 (above right image).

For benchmarking, DIATONE (Mitsubishi) P-610DB (Photo 7) and DAITO VOICE DS-16F (Photo 8) were selected. P-610DB was not in mint but in good enough condition with no damages on the edge. DS-16F was unused one picked up from my own stock. Outer shapes of these three drivers look different, however the diaphragm areas are almost identical each other.

Figure 7 (above pair left) and Figure 8 (above pair right).

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Impedance CharacteristicsEach impedance was measured with input voltages of 50 mV, 500 mV and 1 V. For these measurements, an open baffle (300 x 440 mm) was used (free space measurement). Ideally the impedance should be constant over different input voltages, however in many cases the minimum resonant frequency shifts toward lower frequency with higher input voltage and its peak value also decreases, consequently the resonance curve moves toward left lower (on the other hand, impedance in high frequency range will increase in this case). This kind of tendency can be clearly seen with P-610DB (Fig. 4) and DS-16F (Fig. 6) while Alpair 12 (Fig. 5) indicates only a slight difference between the resonant frequencies with 50 mV and 500 mV. The resonant curve of Alpair 12 is much different from others, which can be attributed to its low Q value, i.e. 0.913 for P-610D, 0.775 for DS-16, 0.377 for Alpair 12 (all measured with 500 mV).

All drivers have a small resonant peak around 1 - 2 kHz due to edge resonance. In the higher frequency range, Alpair 12 is quite smooth while other two indicate very small resonant peaks. Minimum impedance in middle frequency range of each driver is 8 ohm with P-610DB and Alpair 12, 6 ohm with DS-16F.

Fig. 4 (above): Measured impedance of P-610DB, open baffle, 50 mV/500mV/1V inputs.

Fig. 5 (above): Measured impedance of Alpair 12, open baffle, 50 mV/500mV/1V inputs.

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Fig. 6 (above): Measured impedance of DS-16F, open baffle, 50 mV/500mV/1V inputs.

Characteristics with a Sealed EnclosureThe inner volume of the enclosure used for the measurements is approx. 35 litters. Sound pressure responses of each driver were measured at the axial center with a distance of 250 mm. Please minus 3 dB from values on the charts if you want to evaluate them as values with 2.83 V (1 W) input / 1 m. P-610DB is delivering a quite excellent response (Fig. 7) successfully fulfilling the Broadcasters Technical Standard (BTS) that requires a sound pressure within 10 dB tolerance from 100 Hz to 10 kHz. A dip in middle frequency range (1.3 kHz) is caused due to a negative resonance. Its SPL reaches to approx. 94 dB, which can be one of reasons why many low-power tube-amp users love it.

With Alpair 12 (Fig. 8), the level in high frequency range (> 5 kHz) is approx. 5 dB higher than that in middle range while maintaining the response well up to 20 kHz. Its roll-off is starting from 200 Hz with a moderate gradient because the Q in this enclosure only reaches to 0.442. The SPL is approx. 91 dB.

The result from DS-16F is shown in Fig. 9. Please notice that the vertical scale of this chart is 5 dB different from others. Its SPL of 95 dB is the heights among three. However the response in high range is not so smooth comparing others. This could be said reasonable considering its price (less than ¥2,000).

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Fig. 7 (above): Measured Sound Pressure of P-610DB, 3.5 L sealed box, equivalent to 500mV/1V inputs, Impedance with 500 mV input.

Fig. 8 (above): Measured Sound Pressure of Alpair 12, 3.5 L sealed box, equivalent to 500mV/1V inputs, Impedance with 500 mV input.

Fig. 9 (above): Measured Sound Pressure of DS-16F, 3.5 L sealed box, equivalent to 500mV/1V inputs, Impedance with 500 mV input.

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Off-Axis CharacteristicsThe measurements were performed by horizontally swinging the enclosure mounted on a speaker stand. Since the measuring distance was 250 mm, the results might be worse than those measured with 1 m distance (Fig. 10 – 12).

With all drivers, the roll-off of high frequency response starts from 2 kHz, with deteriorating more with larger off-axle angle. Only one exception is Alpair 12 with 30 degree off-set, which is increasing over 10 kHz.

Fig. 10 (above): Off-Axis response of P-610DB, equivalent to 500mV/1V inputs, smoothed data.

Fig. 11 (above): Off-Axis response of Alpair 12, equivalent to 500mV/1V inputs, smoothed data.

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Fig. 12 (above): Off-Axis response of DS-16F, equivalent to 500mV/1V inputs, smoothed data.

Cumulative SpectrumsThe cumulative spectrums were measured in a frequency range from 1 kHz to 20 kHz with 5 dB step width in order to eliminate room effects (reflections). Each scale of the chart is as follows: horizontal = frequency (1k – 20kHz), vertical = sound pressure (5dB step), depth = time (100 micro-sec step). The time scale starts from the farthest side (0 sec) and then propagates toward the nearest side (2 milli-sec) with 100 micro-sec step (Fig. 13 – 15). Ideally, only the farthest profile should be visible on the cart (other nearer ones should be hidden below the horizontal plane).

Comparing to P-610DB, Alpair 12 is indicating stronger resonances at 15 kHz and 20 kHz, however considerably excellent in the range below 5 kHz. DS-16F has most significant resonances and reflections among three drivers.

(Figs 13, 14 and 15 on next page)

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Fig. 13 (above): Cumulative spectrum of Alpair 12, 500mV input, frequency 1 – 20 kHz, time 0 – 2 milli-sec with 100 micro-sec step.

Fig. 14 (above): Cumulative spectrum of DS-16F, 500mV input, frequency 1 – 20 kHz, time 0 – 2 milli-sec with 100 micro-sec step.

Fig. 15 (above): Cumulative spectrum of P-610DB, 500mV input, frequency 1 – 20 kHz, time 0 – 2 milli-sec with 100 micro-sec step.

Step ResponsesFor full-range speakers, the ideal response should draw a rectangular triangle. The oblique line becomes more jagged if resonances or reflections are stronger. Due to the high Q value, Alpair 12 is indicating a well damped response with a less minus amplitude.

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Fig. 16 (above): Step response of P-610DB, 500mV input.

Fig. 17 (above): Step response of Alpair 12, 500mV input.

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Fig. 18 (above): Step response of DS-16F, 500mV input.

Harmonic DistortionsThe harmonic distortions were measured using a pure tone with 1/12 octave intervals with a distance of 100 mm. Each measurement was performed 6 times and averaged in order to eliminate environmental noises. Fig. 19 is showing a measure result on Alpair 12 with using a pink noise signal (1/12oct RTA mode, 100 mm distance). From this chart you can see a resolution of 1/12 octave. Fig. 20 – 22 are showing total and 2nd order harmonic distortions, and Fig. 23 – 25 are showing total and 3rd order harmonic distortions. The vertical scales are displayed in %, where 3.162% corresponds to -30 dB and 1% corresponds to -40dB. The total harmonic distortions are compared between P-610DB and Alpair 12 in Fig. 26 as well as between P-610DB and DS-16F in Fig. 27. The Alpair 12 demonstrates significantly less harmonic distortions and the distortion is strongly dominated by 2nd order component through the whole frequency range.

Fig 19 (above): Alpair 12 pink noise signal (1/12oct RTA mode, 100 mm distance).

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Fig 20 (above): P-610DB, 2nd order harmonic distortion at 650mV.

Fig 21 (above): Alpair 12, 2nd order harmonic distortion at 650mV.

Fig 22 (above): DS-16F, 2nd order harmonic distortion at 650mV.

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Fig 23 (above): P-610DB, 3rd order harmonic distortion at 650mV.

Fig 24 (above): Alpair 12, 3rd order harmonic distortion at 650mV.

Fig 25 (above): DS-16F, 3rd order harmonic distortion at 650mV.

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Fig 26 (above): Distortion compared between P-610DB (upper line) and Alpair 12 (lower line).

Fig 27 (above): Distortion compared between P-610DB (lower line) and DS-16F (upper line).

ConclusionThe performance of the state-of-the-art Alpair 12 is quite excellent. A new enclosure optimised for this driver will be introduced next time in a new issue of MJ magazine.

Measurement equipment: praxis (available from Roppongi Institute of Technology, Japan) .

March 2010.

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