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G 25th March 2003 09:36 PM

Damping materials for speaker cabinets?
Hi all,

I'm about to embark on my first speaker building project and I have a couple of questions about damping materials. The material I'm using for the cabinets is 1" Birch furniture grade plywood. I've looked at using this material for internal damping:

or this:

or I guess I could use polyfill. Speaking of polyfill. They say that you should use 1 pound of fill per cubic foot of cabinet volume. Wouldn't doing that change the internal volume of the cabinet? Any advice is appreciated.


RobWells 25th March 2003 09:52 PM

Hi Gavin,

Is the box sealed or vented?

The first link you gave is fine to damp wall resonances, and the second stuck over it wouldn't hurt (IMO), :)

But you also asked about polyfill - isn't this stuffing (rather than damping?)

Good luck!


G 25th March 2003 10:01 PM

Hi Rob,

The cabinets are ported. I'm kind of partial to the first option (the sound dampening sheet) but I thought that maybe there was something better that is just as inexpensive. I'm a little confused about the polyfill. I think that it is for dampening also. I've also read that it will effectively "increase" the internal volume but I don't see how.


RobWells 25th March 2003 10:13 PM

I'm pretty sure that with a vented enclosure, most people use a 1 or 2" layer of felt/wadding etc on the panels, and leave the box empty as such..

Ive just stuck a load of closed cell foam on a bitumen backing in my bass cabs, and if you stick your head inside you cant hear any high stuff from outside music.....

If I had the choice of materials I'd use the bitumen(damping) sheets with the foam over the top...

I'd ignore the polyfill...



TheoM 26th March 2003 12:08 AM

Damping in sealed vs ported
My understanding is that in ported cabs you want to leave the basic air mass alone but prevent reflections off the walls. They do lots of bad things including come out the port. Ergo you want to absorb/damp the walls in a ported design. I've seen this time and time again while repairing ported boxes. I would assume that a sheet material plus a 1" acoustic foam would be a great combination (The sheets are better with low end, and foam with high end).

Polyfill is stuffing used is sealed boxes. It impedes airflow - slowing down the air and changing the resonant frequency of the air mass inside the cab while absorbing energy. In a seal cab you want to kill the backwave entirely so stuff the heck out of it with polyfill. In a ported cab if you do this you will change the dynamics of the cab - de-tuning it.

I have seen a lot of materials used to line ported cabs - fiberglass batting, carpet pad - like material. I use dense acoustic foam because its built for the task. Its more expensive which is why industrial concerns don't use it, I guess - but in a diy project its only 20 bucks more so why not?

faustian bargin 26th March 2003 06:02 PM

i just posted this link in another thread, but in case you don't see it, mcmaster-carr has some ideas:

also on the next pages: 3236 through 3241.


tg3 27th March 2003 02:12 AM

Re: Damping in sealed vs ported

Originally posted by TheoM
I use dense acoustic foam because its built for the task. Its more expensive which is why industrial concerns don't use it, I guess - but in a diy project its only 20 bucks more so why not?
Can you post a link to your acoustic foam?


TheoM 27th March 2003 02:48 AM

Absorbers - link per request

I use this stuff - I've tried others and this works better. Generally, Auralex is an acoustic treatment company and the site has a lot of stuff that might be of interest. Specs are available on the site - the actual frequency response of different absorbers - this makes it possible to tune a room or the inside of a box - which I suppose is just a teeny little room. :). tm.

Kanga 27th March 2003 04:13 AM

Vance Dickason in his "cookbook" recommends 4-6 layers of 30 lb roofing felt stapled to the enclosure walls as a cheap and effective material for panel damping.
Roofing felt seems to be used under slate tiles, and is basically a bituminised felt.
I haven't had any luck finding this in Australia, but maybe its easier in the US.
A slate roofing supplier may be able to help.


navin 27th March 2003 05:09 AM

2" of open cell foam is what i use works fine. easy to cut and install.

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