Closed Cell Vs. Open Cell Foam-Which For Damping Loudspeakers?

[kelticwizard]

I am interested in investigating the idea of using spongelike foam for lining the interor walls of Transmission Line loudspeakers. It would not be used a fill to lie in the interior spaces-just used a liner on the walls. I might investigate compressing it somewhat if that helps to give the necessary characteristic.

The characteristic I seek is to have the higher frequencies suppressed quite a bit, then comes a frequency beneath which lower frequencies are damped little. For instance, if the key frequency is 200 Hz, all frequencies above 200 Hz would be suppressed by 12 dB, while 100 Hz would be suppressed by only 3 dB and 50 dB suppressed hardly at all.

Manufacturers make rubber, neoprene and other foams in closed cell and open cell models. Which would be the better one to investigate for suppressing high frequencies while leaving lower frequencies relatively unaffected?

[GM]

Open cell, but you'll have to layer different densities to get your desired performance.

[myhrrhleine]

Originally posted by kelticwizard
I am interested in investigating the idea of using spongelike foam for lining the interor walls of Transmission Line loudspeakers. It would not be used a fill to lie in the interior spaces-just used a liner on the walls. I might investigate compressing it somewhat if that helps to give the necessary characteristic.

The characteristic I seek is to have the higher frequencies suppressed quite a bit, then comes a frequency beneath which lower frequencies are damped little. For instance, if the key frequency is 200 Hz, all frequencies above 200 Hz would be suppressed by 12 dB, while 100 Hz would be suppressed by only 3 dB and 50 dB suppressed hardly at all.

Manufacturers make rubber, neoprene and other foams in closed cell and open cell models. Which would be the better one to investigate for suppressing high frequencies while leaving lower frequencies relatively unaffected?

200 is a very low frequency.
if this application is inside a box, then closed cell.
outside, at those frequencies, either closed or open, depending on foam thickness.
see mfg. specs.

[GM]

Hmm, this pretty much contradicts everything I've read or experienced WRT damping of pressure, standing and reflection waves inside a closed box or horn, so please list some manufacturer's closed cell specs to support your assertions as such a product would make tuning a TL, BLH or other bandpass alignment a simple procedure, one that all manufacturers of such devices I'm aware of apparently haven't discovered yet.

[myhrrhleine]

everything passes sound, even concrete and lead.
It's just a matter of how much is the attenuation?
and at what frequencies?
1" of closed cell foam may do better than 10" of open celled foam.

That's why one should consider the specs. and the specific application.

[marcelnl]

felt, just try natural felt in different thicknesses sizes and shapes, together with Dr Bailys longhair......it's a TL after all

[noah katz]

"1" of closed cell foam may do better than 10" of open celled foam."

On what do you base ythis statement?

Only speculation, but I wouldn't think even 1" of closed cell would absorb as much as 1" open cell.

Open cell allows air movement and viscous air friction loss, whereas closed does not.

[myhrrhleine]

Originally posted by noah katz
"1" of closed cell foam may do better than 10" of open celled foam."

On what do you base ythis statement?

Only speculation, but I wouldn't think even 1" of closed cell would absorb as much as 1" open cell.

Open cell allows air movement and viscous air friction loss, whereas closed does not.

Believe what you want.
read the specs!

[noah katz]

I'd be happy to. Where are they?

Thanks

[myhrrhleine]

Usually the manufacturer's catalogs