Closed Cell Vs. Open Cell Foam-Which For Damping Loudspeakers? - Page 3 - diyAudio
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Old 25th October 2007, 06:50 PM   #21
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then what would the T-S parameters of your speaker chassis look like?

It sounds to me as you if are using a more or less typical B-R chassis in a TL like enclosure, and try to smother anything from the lower midrange coming due to improper parameters?

Why not use (my favourite), a passive radiator to have the acoustic coupled mass as the speaker probably likes without having the oompha pipe character.

A TL with only the sides dampened will never be able to sound clean as the frequencies you hope to quench are typically difficult to dampen with foam (if not impossible as that would apply for the 200 400 and 700 Hz range anyway )

Your last question could be answered by plug the whole lot with something solid if you hope to attanuate these frequencies to serious levels and see whether the closed volume has benefits ;-)


High freq are easy to dampen, the lower you get the more difficult it is...

my two cents, haven't ventured into TL lately but I am not aware that acoustics has advanced THAT far beyond my 4th edition of the loudspeaker coockbook ;-)
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Old 8th July 2013, 05:02 PM   #22
rowuk is offline rowuk  Germany
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Well, I have used closed cell foam and have VERY good results.

I think that we have to remember that sound propagation has little to do with "airflow" (electrical equivalent DC) rather with the AC component. Mass and surface area damps, not the ability to pass air. Especially in a transmission line, where the standing wave is "very" powerful, the denser material does provide a different type of damping. In my case, it does a fantastic job between 130 and 500 Hz - exactly where I need it.

Don't believe me? Here is an excellent link a bit more scientific:

http://www.eletel.p.lodz.pl/docs/akt..._Krzysztof.pdf


Primitive test: make a speaker grill for your woofer from open and closed foam. What lets more energy through? What makes anyone think that inside a speaker is different? At very low frequencies, the foam is transparent to the sound wavelengths. In between, the material "resonates" and absorbs. Above the frequencies that are damped, the box does in fact get smaller. At those frequencies, that is insignificant. So, a speaker cabinet wall will even reflect energy that it does not pass back through the foam. You get double the damping.

To be fair, in the first 1/2 of the cabinet I also have some synthetic damping "wool" loosly spread to finish everything that the foam didn't get - 500 - 1000Hz artifacts.
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Old 9th July 2013, 06:20 AM   #23
badman is offline badman  United States
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Closed cell will change the box tuning more than absorb unwanted highs, where open cell will slightly change the box tuning but have a low pass filter characteristic via absorbtion. Open cell is preferred for a TL, though as mentioned, some other alternatives are also good, loose stuffing along the center of the line may be best.
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Old 9th July 2013, 06:08 PM   #24
4Torr is offline 4Torr  United States
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The danger here is that low frequency response will be adversely affected whatever damping material is used. The line must be reactive to be efficient. Ideally, all the resistance should be at the mouth of the line where the sound energy is transmitted to the air. The best solution is to optimize the taper to filter out highs and use stuffing judiciously to fine-tune the line.
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Old 9th July 2013, 07:04 PM   #25
Colin is offline Colin  United Kingdom
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This company sells open-cell foam of various types for use in professional studios. They also have stuff for use in loudspeakers. Their website has a table of absorption characteristics at different frequencies.

Advanced Acoustics - Acoustic Studio Foam Treatment Tiles and Panels, Isolation Booths, Bass Traps, Soundproofing

They have a good range of articles here

Articles and Downloads



On their UK eBay site, they mention that several high end manufacturers use their material.

Acoustic Foam Room Treatment Egg Crate 10" Single Tile | eBay


I had a sample to test and it appeared to work OK - a similar-looking thing from a mattress manufacturer didn't feel similar at all, so I didn't go to the trouble of dismantling the speakers a second time to test it.

I used the foam in a small, stand-mounted reflex, with the foam lining the walls.
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Old 10th July 2013, 04:55 AM   #26
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Talk of this had me thinking about layered control. If a thin viscoelastic foam say 3-6mm layered between a perforated material with high mass like bitumen/dynamat like products should work best for these frequencies.
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