Aperiodic loading to lower qt

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Melo theory

Melo theory

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Joined 2009
mtheory.us/
zead said:
So Melo, that vent itself, for non-phase effects/etc also needs to vent a sealed/treated cavity no?
Click to expand...

I'm not sure what this means. Do you mean loading into a closed box like north creek's - George E. Short's white paper (which appears to be unavailable now)
Which talks about this?
Having the vent between two sealed boxes for lowering to q of a driver.

I found a copy of the paper on a forum.

"Abstract: A new driver loading technique in which a driver is loaded by a first small chamber that is aperiodically coupled into a second, larger chamber, is described. The loading technique has the advantages of both simple Acoustic Suspension loading and the Aperiodically Damped loading techniques, without the disadvantage of either. Optimum driver parameters and cabinet volumes are described.

by George E. Short III, President, North Creek Music Systems, March 11, 2001



MAPD loading is an acoustic loading technique I have been thinking about and occasionally working on for over a decade, and have finally succeeding in optimizing in both the North D28 tweeter and North Creek Music Vision Revelator Signature loudspeaker. The new North 19W-6542-S-06 woofer was also designed specifically to take advantage of this technique.


Acoustic Suspension loading, developed by AR (Acoustic Research) in the early 1950's, is a simple sealed box loading in which the box volume is less than or equal to one-third of the driver's suspension equivalent volume, Vas.

Vab = Vas/3

The benefit of Acoustic Suspension loading is that the air "spring" trapped inside the acoustic suspension cabinet is much stronger than the driver suspension "spring" (i.e. the spider and surround). The air "spring" is extremely linear, far more linear than even the best driver suspension "spring". What this means to the driver is that even at very high output levels, the cone motion is very well controlled simply because the air "spring" forces the driver to behave. A side benefit of Acoustic Suspension loading is that at very low frequencies the maximum driver excursion is reduced compared to the same driver in either free air or vented box loading. Also, the high pass roll off is second order below system resonance.

The disadvantages of Acoustic Suspension loading are that it forces the system Qtc to be at least twice the driver Qts, and it forces the system resonance frequency Fc to be at least twice the driver resonance frequency, Fs.

Qtc/Qts = Fc/Fs = 2

While Qtc going up is not necessarily a bad thing, with Qtc between 0.7 and 1.0 generally sounding quite good, a Qtc above 1.1 will ring slightly and tend to very slightly favor the resonance frequency. Although never as overbearing as the "one note bass" found in badly tuned vented systems and most bandpass subwoofers, a Qtc above 1.1 will still tend to lend its "character" to any program the system is trying to reproduce.



Aperiodic Damping of Acoustic Suspension systems with Qtc of above 1.1 is a cure for both the ringing and the "character", as Aperiodic Damping eliminates the ringing and adjusts the effective Q of the system. As the flow resistance of the Aperiodic Damping device can be adjusted, a preferred loading can usually be found rather quickly. What the preferred loading actually turns out to be is purely subjective.

The disadvantages of Aperiodic Damping of Acoustic Suspension designs are 1) that the system high pass roll off becomes third order below system resonance, and 2) the maximum driver excursion is controlled only by the driver suspension, and is actually equal to or greater than the driver excursion of a vented box system, and much greater than the original Acoustic Suspension system from which the Aperiodically Damped system was created.



MAPD (Multichamber Aperiodic Progressive Damping) loading is an extension of the Aperiodically Damped Acoustic Suspension loading, in which the Acoustic Suspension system is loaded into a second, much larger chamber via an aperiodic vent. This loading technique retains or created the following advantages: 1) the driver suspension is still dominated by the linear air "spring" of the first acoustic suspension enclosure; 2) the system high pass roll off remains second order below resonance; 3) the low frequency driver excursion is reduced compared to the vented or standard Aperiodic loading, although still higher than the simple Acoustic Suspension loading; 4) the system's effective Qtc can be "fine tuned" by fine adjustments to the aperiodic vent between the first and second chambers; 5) the MAPD system has superior impulse response compared to an Acoustic Suspension system with Qtc above 1.1. The disadvantage is that the required cabinet volume is several times that required by a simple Acoustic Suspension system.

One could argue that for most drivers, one could achieve superior impulse response by simply using the larger cabinet to begin with. As this results in a lower Qtc and also a lower system resonance frequency. While this is true and the loading technique for drivers with a Qts below 0.55, the larger cabinet does not yield the superior suspension performance of the linear air spring of the small, Acoustic Suspension enclosure.

So what kind of driver is a perfect candidate for MAPD loading? It is an interesting question because the technique was first optimized during the development of the North D28 tweeter rear chamber loading, not with a woofer at all! But, the best driver for MAPD loading is a woofer with a Qtc of about 0.60, and preferably with a small Vas.

The design example is the Scan Speak 15S-8530K-01, which North Creek has tested over the last two years and averaged the following Theile-Small numbers:

Fs = 44 Hz

Qts = 0.610

Qes = 0.65

Qms = 9.8

Vas = 12.50 liters

Here the loading we use is that featured in the new North Creek Vision Revelator Signature loudspeaker, which is an MTM. The box volumes per woofer is 4.17 liters for the first enclosure, 12.5 liters for the second, yielding an overall net volume of 33.3 liters for the pair. The aperiodic damping for the woofers is provided by a single Scan Speak Scan-Vent adjoining the first, 8.34 liter section to the second, 25 liter section. The volume of the second enclosure is exactly equal to the drivers' equivalent suspension volume, Vas, to provide symmetry around the small Acoustic Suspension enclosure.

Vab1 = Vas/3

Vab2 = Vas

Whether other volume ratios offers advantages is yet to be determined, but selecting this volume ratio greatly simplifies the mathematical model, which usually directly translates to better results in the real world.

The systems overall characteristics are: the Acoustic Suspension loading of the first chamber raises the system Qtc to 1.22. Acoustic damping (stuffing) in the first volume drops the Qmc from 19.6 to about 12.0, so the effective Qtc is actually closer to 1.21. Aperiodically coupling into the second chamber drops the Qm much lower, to about 5.0 (hence the term "Progressive Damping"), and also lowers the system resonance frequency slightly. Overall, the system performs as one with an effective Qtc just under 1.0, and is -3dB at 68 Hz. For a resonance frequency in the 80 Hz range, this is rich, robust, and detailed yet satisfying tuning.

For a loudspeaker using 5" woofers to cleanly reproduce mid-bass at respectable volumes, suspension linearity of even a Scan Speak driver is insufficient. MAPD loading provides the optimum environment for this driver.


A Caveat for Technical Information on this Subject:

Invariably when we release a publication of this nature, we receive a very large number of requests for more detailed technical information, copies of the derivation and measurements, and occasionally requests for a more detailed explanation. So I have to say at the beginning that North Creek Music does not have the facilities or the personnel to provide more in-depth technical support on this subject other than this brief report and the cabinet drawings for the North Creek Vision Signature loudspeaker. The best free advise I can give on MAPD is this: with a driver with a Qts of 0.6 or above, this is the best way I have found to tune its low end, and with generally spectacular results. Try it!"
 
Melo theory

Melo theory

Member
Joined 2009
mtheory.us/
I don't believe this. I think it would just act as a large sealed box. I think the results he got would be duplicated by a large box.

We do aperiodics to avoid large enclosures.
And as far as phase issues, they are of no consequence, the frequencies that exit the vent contain wavelengths that are usually over at least 10ft long. All that happens is cancellation at frequencies below the Q of the system, similar to a dipole.
 
IG81

IG81

Member
Joined 2008
Melo theory said:
http://www.diyaudio.com/forums/full...e-execution-aperiodic-vent-2.html#post2330229
These look nice. Did the use of real wood give you any problems?
Nice vent :)
Click to expand...

They were more of a test-box than anything and they were dismantled. Pine was not too expensive and used its nominal dimensions to keep cutting to a minimum. I learned a lot by doing this and the vent.

I forgot the focused arrays were aperiodic, those are nice!

I suppose these were somewhat aperiodic too. Pioneer A11 in a small chamber "vented" by a ~115Hz small TL stuffed all the way:

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


IG
 
Z

zead

Member
Joined 2013
melo, don't you get any port noises from these openings... and, how big a driver have you used... i think your technique does solve the pressure issue, but i'm not convinced that two parallel walls is the optimum arrangement for midrange/midbass clarity... additionally, i think you loose one of the real improvements in using that smaller cabinet geometry (pistonic control of the woofer... that, it is said, is what is accomplished with Aperiodic usage
 
Melo theory

Melo theory

Member
Joined 2009
mtheory.us/
zead said:
melo, don't you get any port noises from these openings... and, how big a driver have you used... i think your technique does solve the pressure issue, but i'm not convinced that two parallel walls is the optimum arrangement for midrange/midbass clarity... additionally, i think you loose one of the real improvements in using that smaller cabinet geometry (pistonic control of the woofer... that, it is said, is what is accomplished with Aperiodic usage
Click to expand...

which speaker are you reffering to? The focussed array?
 
Moondog55

Moondog55

diyAudio Member
Joined 2007
There have been many "Acoustic Vents" over time.
I have just remembered a big Plessey 3-Way my son paid too much money for and asked me to fix. 12inch woofer in a small box.
Bottom of the box had 2 thin plywood layers separated by about 50mm and the space filled with fiberglass, in the middle of this panel ( both the inside layer and the outside of the box ) was a small hole; 5 or 6mm in diameter apart from that the only damping material in the box was a layer of cheap felt on the back and sides held in place by staples
 
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