remove standing waves without damping

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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.
 

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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.
 
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.
 
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!
 
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.

Works "like" just not to the same degree. Any useful acoustical material will have absorption vs. frequency listed in a number of architectural acoustical materials listings (alpha vs. f and NRC). If it is a good material it will, for a given thickness, have an absorption approaching 1 for frequencies down to its 1/4 wave thickness (or even 1/6th wave thickness). I can't find poyester batting listed. A material with good absorption is equally useful inside an enclosure or on the walls of your room (a slightly larger enclosure).

Polyester battting, in the typical densities, has little absorption for mid or low frequencies. Google it and you will see that it is frequently used as a spacer under home theater wall coverings or as a cover layer over fiberglass in hanging absorbers, but never as a primary absorption material.

You might be able to make useful absorbing material out of polyester, but the commonly found pillow, blanket stuffing material isn't it.

David S.
 
Dave,

It's a little bit more complicated than looking into architectural acoustical materials listings. Angle of incidence, flow resistivity, thickness, air gap, location, amount, ...
This Porous Absorber Calculator gives some hints of what to expect.

The stuff I've referenced is basically felt made out of polyester. Here's a picture of the boards I use:

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


Data: CARUSO-ISO-BOND - CARUSO GmbH Vliesstoffwerk: Vlies, Filz, Dämmstoff, Lieferung, Iso, Bond, Absorber, Absorption, Akustik, Baffel, Schall, Lärm, Wärme, Kälte, PES, Polyester (sorry, German only)
 
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.

The baffle is 60x100 cm with the driver (coaxial) in the center. A simple 60x100 cm baffle should give a 350 Hz peak and a 100x100 cm baffle should give a 300 Hz peak according to Edge. The acoustic short circuits over the wings are too long in comparison. The 200 Hz would very well fit to the average heigth of the wings seen as a 86 cm half-wave resonator open at both sides.
 
The stuff I've referenced is basically felt made out of polyester. Here's a picture of the boards I use:

Thats a different story. Your material looks like a type of foam made from polyester. No reason why it shouldn't have good characteristics. I was talking about the typical very open white polyester material that so frequently gets used in speaker cabinets (like you might pick up at a fabric store). Generally worthless as a standing wave absorber.

David S.
 
The baffle is 60x100 cm with the driver (coaxial) in the center. A simple 60x100 cm baffle should give a 350 Hz peak and a 100x100 cm baffle should give a 300 Hz peak according to Edge. The acoustic short circuits over the wings are too long in comparison. The 200 Hz would very well fit to the average heigth of the wings seen as a 86 cm half-wave resonator open at both sides.

No half wavelength exist :eek: It must be quarter wave resonance or it isn't a resonance.

What is the distance between the driver center and the top edge of the baffle?

IMHO you need additional driver to cover the frequencies between the bass driver and the tweeter.

If 60 cm is just width and this gives 300 hz peak in the OB mode of EDGE, then if you add the wings, the peak might just go to ~200 hz...

I really don't see conditions for standing waves, or even if they occur they would be too weak to be significant.
 

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What is the distance between the driver center and the top edge of the baffle?

IMHO you need additional driver to cover the frequencies between the bass driver and the tweeter.

The driver is in the center of the 100 cm high baffle. And why do I need an additional driver? In some 15" coaxials one really misses a midranger, but 10" crossed at 2000 Hz is still OK with so much damping coating.
 
Thats a different story. Your material looks like a type of foam made from polyester. No reason why it shouldn't have good characteristics. I was talking about the typical very open white polyester material that so frequently gets used in speaker cabinets (like you might pick up at a fabric store). Generally worthless as a standing wave absorber.

David S.

If one has the space that fluffy stuff is probably the only way to create an effective low frequency absorber out of porous material. But now we're talking about a thickness that is measured in meters, not centimeters.
For constrained spaces other absorption principles are more effective.
 
The driver is in the center of the 100 cm high baffle. And why do I need an additional driver? In some 15" coaxials one really misses a midranger, but 10" crossed at 2000 Hz is still OK with so much damping coating.

Because, you have the 200 hz resonance in the woofer band and you need to get rid of it somehow.

A good way to do it is by putting the resonance in the middle of the acoustical crossover point between two drivers that have lower and higher electrical crossovers.

And then you can still use the 2000 hz crossover point for the tweeter.

There is no doubt that there are 10 and even 12 inch drivers covering 2000 hz, there are a number of 12 inch woofer that can do 4000+ hz, Visaton has some for instance, our local Bulgarian driver manufacturer does a number of models that have ~25-30 hz resonance and go flat to 4 khz...
 
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