In another thread, someone brought up the topic of a "dual aperiodic" enclosure. I know nothing about them, but I believe the idea is that the box has two chambers, separated by an aperiodic vent.
Here's the idea. I built my new Jordan JX92S monitors using the .3ft^3 (.25ft^3 nominal) prefinished boxes from Parts Express. I also bought a pair of ScanSpeak aperiodic vents, which I decided not to use. The boxes have a vertical shelf brace running down the center of the box with two circular cutouts. The hole is just the right size so you could glue the ScanSpeak vent to it using Goop brand goop. You could seal off the other hole.
I don't know what if anything that would gain, and I won't try it unless I can figure out a way to make it temporary, because, well, I can't imagine they could sound any better. But it's an idea I wanted to throw out there.
[Mr. Moderator, I posted this on the wrong BB. It should be in loudspeakers/loudspeakers. Sorry about that.]
Here's the idea. I built my new Jordan JX92S monitors using the .3ft^3 (.25ft^3 nominal) prefinished boxes from Parts Express. I also bought a pair of ScanSpeak aperiodic vents, which I decided not to use. The boxes have a vertical shelf brace running down the center of the box with two circular cutouts. The hole is just the right size so you could glue the ScanSpeak vent to it using Goop brand goop. You could seal off the other hole.
I don't know what if anything that would gain, and I won't try it unless I can figure out a way to make it temporary, because, well, I can't imagine they could sound any better. But it's an idea I wanted to throw out there.
[Mr. Moderator, I posted this on the wrong BB. It should be in loudspeakers/loudspeakers. Sorry about that.]
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Interesting idea,
I have often pondered a similar idea..although a bit more complex.
Take a cabinet similar to yours except maybe a MTM w/ 2 6.5 inchers on the front baffle. In the baffle separating the front and rear chambers mount one of the same drivers. Use a high current pot to either:
1.run the coil on the 'inner' woofer variably open <--to--> shorted. (I have no idea if that would do anything.
2. Run the inner speak actively through a pot. Sort of like a 'loose' isobarik with adjustability ??!
I wonder what would happen. could we adjust Q on the fly?? HMMM
I have often pondered a similar idea..although a bit more complex.
Take a cabinet similar to yours except maybe a MTM w/ 2 6.5 inchers on the front baffle. In the baffle separating the front and rear chambers mount one of the same drivers. Use a high current pot to either:
1.run the coil on the 'inner' woofer variably open <--to--> shorted. (I have no idea if that would do anything.
2. Run the inner speak actively through a pot. Sort of like a 'loose' isobarik with adjustability ??!
I wonder what would happen. could we adjust Q on the fly?? HMMM
Un-hijacking now...
I found this, googling around: http://www.northcreekmusic.com/MAPD1.htm
They recommend that the front chamber be 1/4 the size of the rear chamber, which should be about equal to Vas. For a speaker with a Vas of about 6.4 liters, you could leave both circular holes open and run a horizontal partition in the box, so the "front" chamber would be the top/front quarter of the volume.
I found this, googling around: http://www.northcreekmusic.com/MAPD1.htm
They recommend that the front chamber be 1/4 the size of the rear chamber, which should be about equal to Vas. For a speaker with a Vas of about 6.4 liters, you could leave both circular holes open and run a horizontal partition in the box, so the "front" chamber would be the top/front quarter of the volume.
The classic dual-chamber aperiodic box
Web page: http://www.t-linespeakers.org/classics/dynaco.html
Also i believe NAIM is doing something similar with some of their speakers.
At higher frequencies the driver mostly sees the little box... as frequencies decrease the driver will start to see the larger box thru the resistive port....
dave
Web page: http://www.t-linespeakers.org/classics/dynaco.html
Also i believe NAIM is doing something similar with some of their speakers.
At higher frequencies the driver mostly sees the little box... as frequencies decrease the driver will start to see the larger box thru the resistive port....
dave
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Dave,
I believe Dynaco was the first company to make this design and they used a tuned slot for the aperiodic loading instead of a variovent. The speaker was very sucessful in it's day.
Here is information on the design from one of North Creeks white papers: Sorry, I didn't see the link above until after I posted this but here is the information from the white paper in case the link gets lost.
North Creek Music's new MAPD driver loading
MAPD = Multichamber Aperiodic Progressive Damping
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!
Hezz
I believe Dynaco was the first company to make this design and they used a tuned slot for the aperiodic loading instead of a variovent. The speaker was very sucessful in it's day.
Here is information on the design from one of North Creeks white papers: Sorry, I didn't see the link above until after I posted this but here is the information from the white paper in case the link gets lost.
North Creek Music's new MAPD driver loading
MAPD = Multichamber Aperiodic Progressive Damping
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!
Hezz
Dave Jones said:
I recall posting something about a kit from Dynaudio called the TWYNN that is an MTM with 17 W-75 drivers each having a separate chamber and an aperiodic vent. The vent dampens the impedance peak and the whole system with a D-28AF tweeter sounded beutifiul to me.
Northcreek does something similar but they fill the normal type vent with straws, or you can simply drill small holes in the cabinet I'm told and achieve the same effect.
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