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el`Ol 26th May 2011 10:05 AM

remove standing waves without damping
 
3 Attachment(s)
Hello all!

I had a U-frame with a nasty 200 Hz peak and first searched for a heavy enough damping material for the side wings that is not detrimental to my lungs, but didn't find anything I could get in small quantities. I said to myself why not reflect instead absorb and strangely the first attempt worked. I wonder: Could such a tube also be an alternative to damping for large sealed enclosures?
And what other means do you know? Threadjacking allowed.

croat47 26th May 2011 10:22 AM

Why would the rest of the range shift from 90-95dB to 95-100dB? Could position have changed and resulted in shift in the 100-300 range as well?

speaker dave 26th May 2011 10:50 AM

I see quotes about this regularly on these threads. Without absorption the energy will still be there, although you can break up the reflections to the point were the frequency response effects are well distributed. Your lower resolution curves may not show it distinctly but the energy is still there. I guess this is analogous to adding diffusion to listening rooms rather than absorption. Each reflection is spread in angle and lowered in magnitude but not in total power.

Also not possible: rooms with shapes devoid of standing waves or cabinet panels with shapes without resonances.

By the way, fiberglass is itchy but it is not known to be carcinogenic. If you don't like it though, you can try different foam absorber material or felt or reprocessed wool.

David S.

markus76 26th May 2011 11:09 AM

Quote:

Originally Posted by speaker dave (Post 2584093)
you can try different foam absorber material or felt or reprocessed wool.

... or polyester batting. CARUSO GmbH in Germany even cuts it to customer specifications.

speaker dave 26th May 2011 11:25 AM

Except polyester batting doesn't absorb much sound. Certainly not the pillow stuffing material. I don't know why people keep filling speaker cabinets with it, or BAF wadding or Dacron fluff.

Nothing beats fiberglass for absorption vs. thickness.

David S.

jameshillj 26th May 2011 12:16 PM

Hi el 'Ol,

Did you take a measurement/test with the tube around the other way, with the curve facing the back of the driver?

markus76 26th May 2011 12:30 PM

Quote:

Originally Posted by speaker dave (Post 2584131)
Except polyester batting doesn't absorb much sound. Certainly not the pillow stuffing material. I don't know why people keep filling speaker cabinets with it, or BAF wadding or Dacron fluff.

Nothing beats fiberglass for absorption vs. thickness.

David S.

I was talking about room treatment not speaker stuffing.

Polyester batting works just like any other porous absorber. It's not important what the porous absorber is made of, important is the flow resistivity.

To make a porous absorber work at low frequencies it has to be thick (porous absorption works only at sound velocity points). If the flow resistivity is too high then sound waves can't penetrate deep enough - the absorber becomes reflective and not much energy is dissipated.
A low flow resistivity allows a deeper penetration. Unfortunately absorption effectiveness is decreases at the same time.
So it's always a trade off between thickness, density and effectiveness at lower frequencies.

The pyramids commonly known from anechoic chambers are filled with very fluffy stuff.

el`Ol 26th May 2011 12:38 PM

Quote:

Originally Posted by croat47 (Post 2584073)
Why would the rest of the range shift from 90-95dB to 95-100dB? Could position have changed and resulted in shift in the 100-300 range as well?

Measured on two days with two uncalibrated amps.

el`Ol 26th May 2011 12:39 PM

1 Attachment(s)
Quote:

Originally Posted by jameshillj (Post 2584172)
Hi el 'Ol,

Did you take a measurement/test with the tube around the other way, with the curve facing the back of the driver?


T101 26th May 2011 01:36 PM

My knowledge is that U and H frames do not experience standing waves. They do have a quarter wave resonance peak though. It is governed by the shortest way around the frame - multiply that distance by 4 and you get the resonance wavelength. 200 Hz is about 1.7 m wavelength, a quarter of that is 42.5 cm - does that lenght has something to do with your H frame? I think it is very probable. If you read MJK's site "quarter wave" you'll find many answers. The way I see things is that you somehow terminated the quarter wave resonance by putting a reflecting lens focused at the membrane. Another obvious circumstance is that you run your U frame at frequencies that are too high. As it can be learned from MJK's researches, the quarter wavelength resonance must be around an octave above the low pass point, thus combined raise from resonance and crossover slope result in a flat output and thus resulting in a acoustical crossover point that is about an octave higher than the electrical one. Either way the resonance is normal and is expected to be there and it is not a standing wave driven one. And again it should be outside the operating range. Best regards!


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