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.

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.



2002-08-15 6:21 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...




2003-03-25 4:19 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?


2003-03-25 4:19 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.
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.


first of all, I apologize because I'm a newbe in english forums, so sometimes my technical terms may not be correct (maybe literally translated from the german expressions).

What has been said is in some way true but you always need to have a closer look at the actual application. Is it a Subwoofer, a huge 3 oder 4 way speaker? Maybe 2 ways in a tall narrow cab.

In a subwoofer standing waves (is that correct?) a no problem, because the dimensions of the cab are small compared to the wavelength, so you don't need to worry about them. Just build the cabinet as stiff as possible. Even in a ported box, you don't need stuffing. No mids will escape the port, when they simpley are not reproduced by the driver.

When you have to stuff the cab, IMHO there is no need to fill the hole box up. This overstuffing sounds quite whipy and boring to me. Just put it in the place, where the air velocity haas its max: right in the middle. Trying to damp the max of the pressure is much more ineffective. Build to horizontal dividers and drill lots of big circles (dia>50mm) or do this with a jigsaw and place them up und below the middle of the box. Put lots of polyfill in between.
Here are some links for bitumen roofing products that might be cheaper than buying smaller quantities from speaker suppliers.

Johns Manville used to make a product called Horsey-Set that was a co-polymer rubber coating, that was easy to use. I later heard that Karnak had the recipe, and was marketing it under the name Karnak TDI. I would try talking to local wholesale roofing suppliers to find these products, and builder/roofers for extras and scraps. Self-adhesive rolled roofing doesn't seem to have a long shelf life, when it comes to initial adhesion.

Karnak coatings




diyAudio Moderator Emeritus
2002-10-24 10:19 pm
My understanding is that in ported cabs you want to leave the basic air mass alone but prevent reflections off the walls.

This is exactly correct. The normal T-S calculation make assumptions about box-loss Q. In most tables, it's set to 7. When you stuff a ported cabinet, you change that Q dramatically (downward, and often non-linearly) and your alignment won't work.

Oversizing cabinets a bit will more than make up for the loss of volume from wall dampening or stiffening treatments. One should oversize anyway, since it rarely happens that everything is in spec; when it comes time to do the tuning, box volume may have to be adjusted and it's a lot easier to make it smaller (can you say "brick?") than to make it larger.

You ARE planning to do measurements to verify tuning, aren't you? If not, well, you've got a 1 in 10 chance at best of having the system perform the way you think it will. If I were Dictator of the World, I wouldn't allow anyone to do a ported cabinet for a first DIY project.
I'm building the bass reflex cabinet at the bottom right of the PDF in the link below. I'm assuming that the engineers have made the correct calculations. I'm building them out of 1" birch plywood. Before everybody jumps on me with the normal "that speaker won't work in that enclosure!" let me just say that I have heard the speakers I'm building. I went to Dave Dicks house a listened to them being powered by my 5 watt SE EL84 amp and they souded great. The cabinets that he sells are built out of MDF. I chose Birch because I don't want to deal with veneering. Once they are built I will use a router to "bullnose all of the edges with a 3/4" radius curve. They should turn out nice. Besides. The enclosure is desogned by Fostex. Though Dave's enclosures don't use exactly the same dimensions as Fostex's do.