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Author Topic: well, efficiency of multiple boxes raises up to....???  (Read 7573 times)

Tamas Tako

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #20 on: January 26, 2006, 08:39:35 am »

Thanks Tom!

It is good to know there is alway people who can answer questions which are unusual... Very Happy

This all means, that a big array of Vented subs could has less efficiency (25% max vs, 50% max) and less directivity as well than Horn arrays.
This means less and again less sensitivity...

Great knowledge base here!

Just come a further question into my mind...
What are SPL levels at low frequency (20...100Hz) where the air will compress in front of the array so that the pressure wave will not travel as usual, cousing larger SPL attenuation than the 6dB/ distance doubling??

Thanks

Tamas


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Tom Danley

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #21 on: January 27, 2006, 02:50:25 pm »

Hi Guys,

Ryan, maybe it would be more correct to day that unless the sources are about 1 / 4 wl apart, they do not combine in a way that is uniform in all directions or fully “feel” the radiation pressure of the other unit.
For example, two sources a half wavelength apart radiate in a figure 8 pattern with nulls in line with the sources and forward / rear lobe. No gain in efficiency is attained but there is  some forward gain from directivity.
The greater the spacing, the larger the number of lobes and nulls, conversely, once the distance is ¼ wl or less, the radiation pattern is a circle and fully mutually additive.
So, yes the 3.12 feet is how close you need the sources at 90 Hz for them to fully add in all directions.

Tamas, well so far as I’m aware, the phenomena that causes excess absorption is only present at “high” sound levels.  It was called “shocking up” by my old boss Roy Whymark, an old time English acoustician.
Here is what happens (same thing as throat distortion also), sound travels at, well the speed of sound.
The speed of sound is temperature dependant also.
When one has a really loud sound, say 155dB or more, one finds the pressure side of the wave is actually a little bit warmer than the vacuum side. The result is, the pressure side, travels a little bit faster than the vacuum side.  If you apply this to a sine wave, over a number of wavelengths, the top of the sine creeps forward until a saw tooth wave is created.
Here is why the reference to Shock.  A shock wave is created by a pressure which is high enough to heat the air via pressure.  The shock wave travels outward at a new speed governed by how hot it is.
At the tail side, one finds the rarified recovery part of the wave which makes those cool visible expanding clouds of condensed moisture. Loud Sound can be dangerous.
Anyway, the same thing which cause the shock wave to be a saw tooth cause an intense sine wave to become one.   If you can get a 155dB sine wave and a reflector, one can levitate Styrofoam balls up to about  1 /4 wl in diameter. Get to 165dB and you can levitate iron, glass, live cockroaches (in out 21KHz acoustic levitation back at Intersonics).  At 173 dB, one can light a cigarette with acoustic friction.
One can put your finger in a sound this loud (at 21KHz) but any gap between your fingers will be burned immediately.    For reggae or dance floors,  there might be a happy medium, perhaps some non-lethal sound level that would also cause clothing to rapidly fatigue, fail and fall off without the unpleasantness of catching on fire haha.
Kids, I’m just kidding
Best,

Tom
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RYAN LOUDMUSIC JENKINS

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #22 on: January 27, 2006, 04:12:13 pm »

Thanks for the reply Tom, I really appreciate that. Smile
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Ryan Jenkins
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Tim Duffin

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #23 on: February 02, 2006, 01:42:46 am »

Tom, would it then be possible to create a confining wavefront wherein the direct radiators are limited in their dispersion-- imagine a direct radiator that has flares that are opposite the shape of a horn--the sides go inward like a pyramid with the top chopped off.  Theoretically, that shape would create a pressure wave similar to what the turkey feathers of a jet engine do-- concentrating the gases into an ever smaller cross sectional area.    I think that with a woofer with suitable force factor, one may be able to get the total radiating volume of an 18" woofer into a 10"x10" space.  I was thinking that one could array many devices of this type to get much better acoustic coupling AND higher sound pressure levels.  This theory has been done in competition car audio where all the woofers are aimed at a flat solid board with a 2" hole cut in it where the microphone was placed to increase SPL.  If the same theory could be applied to Pro Audio then tremendous SPL's could be with much less space.

By the way, I have sat in a competition car audio truck that hits 166Db at 80hz-- it is very uncomfortable even with earplugs and sounds unlistenable at any freq. above 38Hz.  Presumably this is because of the sawtooth wave phenomenon which you mentioned.

T  

Joe Jones

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #24 on: February 03, 2006, 05:16:35 pm »

Quote:

I think that with a woofer with suitable force factor, one may be able to get the total radiating volume of an 18" woofer into a 10"x10" space.

I think all you'd get is a new way to blow out candles. The effect would be pretty much the same as using too small a diameter port on a reflex box, and the result would be just a lot of chuffing noise.
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Tim Duffin

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #25 on: February 04, 2006, 04:30:52 am »

Well I wasn't really asking for your opinion, but you are free to reply.  I know that your argument that more "chuffing" will be produced is not factual in that you would have to calculate the approximate mach number for a given port diameter and correlate that with the displaced volume from the woofers output that you are trying to concentrate and make sure that it did not exceed 1.  For instance, why is it that the compression section of a bass horn does not create audible chuffing ? The Horn section serves as a load to the cone, much the same way as this "reverse horn" or waveguide that I am thinking of concentrates the waves into a smaller volume.  

By the way, if it was possible to get enough power from a dual 18 to blow out candles 10feet away, wouldn't you want that?

T  

Joe Jones

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #26 on: February 04, 2006, 10:22:41 am »

The candle snuffer analogy might have been facetious, so consider what happens with the duct output of a tapered transmission line, which pretty much approximates your scenario,versus the same driver in a rear-loaded horn. The horn gives high gain over a fairly wide bandwidth, while the TL delivers moderate gain over a fairly narrow bandwidth. If your goal is higher output at lower frequencies you get there by enlarging the radiating plane, not making it smaller, and not by increasing the velocity of the wavefront. A compression device might give very high SPL but it would be over a very narrow bandwidth. If all you care about is the peak reading on an SPL meter with no regard to sounding good then I guess it's a good a way as any of accomplishing it.    
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Chris Davis

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #27 on: February 04, 2006, 05:04:22 pm »

Thanks for the engaging thoughts, Tom!  Nice to see you still around.
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Antone Atmarama Bajor

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #28 on: February 05, 2006, 01:38:52 am »

     Maybe you need a Hyperbolic Contraction.

     I think that a horn is an acoustical transformer that transforms High pressure Low Velocity Energy into High Velocity Low Pressure energy so You would create the opposite.  I think it will still follow the same rules of horn Bandpass behavior.  But you would have to do it right, and what would the application be?  Blowing up skirts and knocking hats off with Bass?  The only problem is the origin of the Horn is normally a reactive radiating component.  I don't think its as simple as swapping the position of the Radiator.  

    Even though it does seem to be what people do with Transmission line subwoofers  But they have to stuff the chamber with all sorts of Damping materials to cut down on Resonance.

     I may have some of that backwards but I have wondered about a reverse horn or contraction.

Antone.

 
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