10/09/2011

Introduction of ZAP System with Alpair 6M


I’ll introduce the latest status of my new desktop system, named "ZAP" using Mark Audio Alpair 6M drivers.


Fig.1: ZAP System

Data:
Main speakers: Mark Audio Alpair 6M 3” metal-cone full-ranger; sealed enclosure with 2.5L volume
Sub-woofer: Dayton Sound DA135-8 5.5” metal-cone woofer; sealed enclosure with 2.5L volume

1. Introduction
This system is placed just above the display screen in front of me. The distance from the drivers to my ears is approx. 65cm. Since I don’t like too much stereophonic effects, the two Alpair 6M are placed considerably closer each other (the center-to-center distance = 230mm). This L/R distance is just same as KERO. I’ve found this short-span layout sounds most comfortably for listening music. Generally I’m not at all interested in so called “perspective” for enjoying music. I have never been aware of "stereophonic effects" so much. Even monaural could be acceptable. I’m not sure whether stereophonic is really necessary for most of music listeners.

2. Comparison of Alpair 5 and Alpair 6

Fig.2: Alpair 5, 6M, 6P measured at 10cm distance (Black: A5, Red: A6M, Green: A6P)

All are measured with the same 2.5L sealed box. Alpair 6M produces approx. 7.5dB higher response than Alpair 5 at 40Hz. This has improved Low-end capability significantly, thus now I can enjoy even Madonna’s heavy low beats (40 – 45Hz) with more than enough sound volume level (I call it WRC limit; Wife’s Red Card Limit) without perceptible distortions, which was not possible with the Alpair 5 system.

3. Conventional Sub Woofer Mode
I added the woofer mainly for enjoying Internet Radio, DVD, CD, whatever those sources that cannot be played back via Frieve Audio equalizer. The woofer works as a conventional sub-woofer. This was easily done because I had already studied this kind of system with Alpair 5 as described in the previous article.


Fig.3: Conventional Sub-Woofer Mode Operation

So, it is not necessary to show details of this configuration. However, I’ve found some very important aspects for LEANAUDIO concepts through this work as described below.

4. Important Aspects for LEANAUDIO Concept
4-1. Air Tightness
LEANAUDIO approach is based on "small sealed box" and "big excursion in low-end" driven by digital signal boosting or an additional amplifier. In such system, air tightness of the entire system (including box and driver) is very important. If air leaks to some extent, then you will lose low-end response somehow (because the air pushed by the cone will be sucked from the clearance to some extent), and you will hear harsh air-flow noises on big excursions such as heavy bass drums for instance. Due to this, drivers with so-called “Phase plug” (e.g. B&W drivers) or Co-Axial drivers are not suitable because of their poor air tightness.
Fig.4: Driver’s Cross Section

As you can see, in some special types of drivers (e.g. with phase plug, or co-axial type) air can flow through clearances at the coil. On the other hand center-caped drivers like as Alpairs can ensure the air-tightness regardless of other structures. I had not been aware of this issue, and bought a nice looking Japanese-made woofer with a beautiful phase plug, then had to throw it away due to the air leakage problem. After that I bought the Dayton woofer that had certainly a CENTER CAP. BUT, the CENTER CAP was NOT air-tight..... It was made of some kind of fabric. WTF ! To make it air-tight, I had to coat it with woodwork glue (very popular glue among Japanese DIYers, white one).


Fig.5: Coated Center Cap


Fig.6 Response of Dayton Woofer after the Modification

Red line indicates the response measured after the modification. Fortunately, it’s not so bad or even better than original because of less peak/dip.

4-2. Xmax
The low-end capability of this woofer is almost equivalent to that of Alapir 6M x 2. This was assessed by playing 40Hz sinusoidal signal and gradually increasing the amplifiers volume with observing the sound wave-form distortions.


Fig. 7 40Hz Sinusoidal Signal Distortion, significant 3rd order condition for comparison (red: Alpair 6Mx2, blue: Dayton woofer)

These sound waves are measured with a microphone located at the listening point. As increasing the amplifiers volume, the 3rd order distortion becomes more significant than the 2nd order and the wave form becomes triangular. Absolute sound magnitude (sound wave amplitude) at this transition point (i.e. FFT 2nd peak < 3rd peak point) is almost same with both drivers (i.e. Alpair 6M x2 = Dayton Woofer x1). I expected more capability with 13cm woofer, however this result seems reasonable if consider the parameters as below:

Alpair 6M x2: Sd: 36.3 x 2 = 72.6 cm2, Xmax = 3.4mm, Sd x Xmax = 247cm3
Dayton Woofer x 1: Sd: 75.4 cm2, Xmax = 3 mm, Sd x Xmax = 226cm3

Sd: effective diaphragm area
Xmax: maximum diaphragm excursion (one way)

"Sd x Xmax" represents the "maximum swept volume" of the diaphragm. Although I’m not sure how directly this index can represent actual low-end capability, it might be useful for making very simple estimations. For example;

CHR70 x 1: Sd(50cm2) x Xmax(4.5mm) = 225cm3 (equivalent to Dayton woofer even with smaller diameter)
Alpair 10 x1: Sd(88cm2) x Xmax(7.5mm) = 660cm3 (seems have plenty of capability, try it!)

Typically, Mark Audio drivers have very big Xmax comparing to other drivers of the same class, and they also have good air-tightness due to their center-capped structure. From these two points of view, I can say, Mark Audio drivers are well suited for LEANAUDIO concept.

5. Further Improvement of Low-End Capability
Besides the conventional sub-woofer mode, the woofer can be used in a different way to assist only very low end (<50Hz) of the digital boosting method. When the woofer is working in the conventional sub-woofer mode, only the woofer works effectively in very low end range (<50Hz). Contribution of Alpair 6Ms is quite small or negligible in this case. This is wasteful. If they share the load evenly (i.e. Dayton x1 and Alpair 6M x2 contribute half and half), the low end capability could be simply doubled.


Fig.8: Very Low-end Assist – Equalizer OFF (Red: Woofer ON, Blue: Woofer OFF)


Fig.9: Very Low-End Assist – Equalizer ON (Red: Woofer ON, Blue: Woofer OFF)


Fig.10: The Strongest Bas Drum Signal in “Rite of Spring”

Fig.10 is showing the strongest low-end signal in my collection (Stravinsky/Rite of Spring (the strongest bass drum)). Gray indicates the raw signal in CD, Red indicates 60Hz High-Cut signal, Green indicates -6dB Sinusoidal 40Hz signal. To assess advantages of the new configuration, this 60Hz High-Cut signal is played back with my maximum sound volume level (i.e. WRC limit level).


Fig.11: Only Digital Equalizing to 20Hz
Significant 4th order distortion can be seen.


Fig.12: Very Low-end Assist + Digital Equalizing to 20Hz
Much improved, however still relatively high 3rd distortion can be observed.


Fig.13: Madonna's Beat (Red: Only Digital Boost, Blue: Digital + Assist)
Assist seems not necessary for Madonna's.


Fig.14: Source Signals(<60Hz)(Red:Madonna, Blue: Rite of Spring)
40Hz Sinusoidal signals are also shown (-6dB and -12dB). You can see how big the drum is!


Fig.15: Frequency Analysis of Different Tunes

Spectral profiles of some typical tunes in my collection are shown in Fig.15 ( Red: Stravinsky/Rite of Spring (40Hz the strongest bass drum), Blue: Madonna/Erotica (45Hz low beat), Green: Miles Davis/Madness (low strong part), Gray: Beethoven/Symphony No.5-I (ending)). Most of tunes in my collection including Jazz, Classics, Rock, Pops do not include strong signal in super low range (<50Hz). Green and Black in the figure represent typical profiles of those normal tunes. Alpair 5 could handle most of those tunes with the digital boosting but Madonna’s. Now, Alpair 6M can handle Madonna’s (blue in the figure), but Rite of Spring. Finally with the Very Low-End Assist method, now I can say, Alpair 6M ZAP system can playback all tunes in my collection with full boosting (down to 30Hz) with maximum sound pressure level of 80dBA with considerable margin at my listening position (dist. = 65cm).

Fig.16: Rite of Spring; Comparison of Source Signal(gray) and Actual Sound at the Listening Position(red: boost+assist, blue: no boost), without high-cut filter (i.e. raw signal)

Fig.16 is showing spectra and wave forms of "Rite of Spring" from the source signal(stereo) and the actually recorded sound(monaural) at the listening position(boost + assist, no boost). High-Cut is not applied (raw muusic soure). Volume position was WRC limit. The measured sound pressure level at the listning position was normally varying within 75-85dBA range with the peak sound pressure level of 91dBA (SLOW filter). It's a quite good result, I think, although the 3rd order distortion (128Hz) and the room effect (180Hz sharp dip) can be seen. Such very accurate sound wave reproduction of this super low range/super attack signal could never be obtained from bass-reflex boxes.

video
due to poor camera microphon, sound quality is bad (no low contents)

Possible further improvement could be ;
- Alpair 10 as a woofer to improve low-end capability significantly (it can be used as a monaural full ranger)
- Digital channel divider to improve phase shift problem

RECOMMENDATIONS:
Basically I would recommend limiting the low-end boost down to 50Hz (then the response decays with -12dB/Oct to approx. -6dB/40Hz -10dB/30Hz). With this setting, you will not need any special cares because Alpair 6Ms are quite tough in >50Hz range. The response seems almost sufficient for most of music.

I think 40Hz/-3dB would be the ultimate target in a practical sense. I can not feel any benefit with more than this level. Therefore, I usually apply the equalizer down to 40Hz and a very sharp digital low cut filter from 35Hz toward 20Hz to avoid unnecessary excursions.

In Super Low range (<50Hz), driver’s excursion increases drastically. The world seems completely different. I'm explporing this area as a pure technical challenge. If you challenge 30Hz flat setting, you must know spectrum profiles of critical tunes and you must exactly know the limit of your system. Otherwise you might destroy the drivers.

Further, If you are listening relatively distant from speakers (i.e. if you are not a near-field listener, e.g. if your listening position is more distant than the center of your room ), the very low-end response(<50Hz) at the listening position might increase considerably due to the room effects. In such cases, 30Hz flat driver will produce too much bass and you might feel boomy.

If you want have big sound from a compact system, I would recommend applying a steep low cut filter for safety reason like as KERO (see previous article).