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Old 26th May 2011, 02:04 PM   #11
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Quote:
Originally Posted by markus76 View Post
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.
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Old 26th May 2011, 02:31 PM   #12
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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:

Click the image to open in full size.

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)
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Old 26th May 2011, 02:54 PM   #13
el`Ol is offline el`Ol  Germany
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Originally Posted by T101 View Post
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.
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Old 26th May 2011, 03:26 PM   #14
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Originally Posted by markus76 View Post

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.

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Old 26th May 2011, 03:44 PM   #15
T101 is offline T101  Bulgaria
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Originally Posted by el`Ol View Post
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 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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Old 26th May 2011, 03:54 PM   #16
el`Ol is offline el`Ol  Germany
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Originally Posted by T101 View Post
No half wavelength exist It must be quarter wave resonance or it isn't a resonance.
MJK told me in a mail that a tube open at both ends is a half-wave resonator. But you are right, by looking at it one can hardly believe it is significant. But as I already tried to make clear the short circuit around the wings is MUCH more, not just 50%.
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Old 26th May 2011, 04:01 PM   #17
el`Ol is offline el`Ol  Germany
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Originally Posted by T101 View Post

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.
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Old 26th May 2011, 04:10 PM   #18
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Originally Posted by speaker dave View Post
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.
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Old 26th May 2011, 04:20 PM   #19
T101 is offline T101  Bulgaria
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Originally Posted by el`Ol View Post
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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Old 26th May 2011, 04:31 PM   #20
el`Ol is offline el`Ol  Germany
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I have got rid well enough of it with the tube for my taste (second graph), but it would be nice to know how that works and whether one can transfer that to other speaker principles (boxes).
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