Internal Damping


2007-09-08 1:43 pm
Hi, I was just wondering what the advantages of different materials were for internal damping.

I have some old carpet which can be cut up - how good would this be for the purpose?

Also, what is the advantage of damping all internal walls as opposed to just one on each axis?

Can cotton wool be used?

Is it best for the damping to be securely mounted to the panel, or slightly loosely to allow for limited movement?


2007-06-29 8:49 pm
So is Fiber Glass (dangerous), but it is cheap and it is very easy to find. I can find Rock Wool if I search the internet but I've never seen it in a store.

The lowest health hazard would be from polyester seat cushion padding. This is relatively stiff and dense, but is not really cheap. Of course if you are spending a fortune (relatively speaking) on beautiful veneered cabinets and high dollar speakers, a few bucks more for Poly isn't going to matter. Also, the stuff goes on sale frequently at the local fabric store.

Next to the dense Poly Cushion material, is Poly batting. This is sheets of poly that are used to make quilts and so forth. Put several layers on the wall of your cabinet.

Next, is loose fluffy Poly that you would typically find in a pillow. This can be used if you intend to fill your cabinet rather than line the walls.

You can also use convoluted acoustical foam panels. You can see these at Part Express, but they are available from many source. Again, these aren't cheap but they do the job very nicely.
Notice there is a guide to various damping material on the bottom right of this page.

And yes, Cotton batting, fill, or cushion material can be used.

Recently while at the home supply store, my brother mentioned he had seen a home renovation TV show that was using a version of flame retardant COTTON house insulation. This stuff is so safe and non-toxic that one of the people extolling its virtue tore off a piece and put it in their mouth. I don't know about availability or cost though.

Now if you want to get really technical, each of these materials has a dampening coefficient that should be considered, though I don't know were you find this information or how many people really take that into consideration in their design.

Personally, I've always used Fiberglass home insulation on the walls because it is cheap and readily available. You need to use reasonable common sense precautions when using it; wear gloves, probably a dust mask, and eye protection.

One thing I would like to find is fiberglass ceiling tiles. These are very stiff and dense and therefore easy to handle. Though only about 3/4" thick. I'm not sure if they even make them any more, but the ones I remember seeing looked nice. Like I said, it is a 3/4" thick panel of dense glass fiber, backed (or fronted) with a thin white vinyl layer. The thin vinyl layer could easily be pealed off and the remaining fiber stapled to the inside of the cabinet.

As to the details of how and why you would pick one method or one material over another, that is a little beyond me. I leave that to the experts.



2007-11-14 6:02 am
I have had success with simple layering of materials in my projects. On the back wall I use a layer of carpet padding first, followed by the unreasonably expensive convoluted foam. On the sides, carpet padding and fiberglass insulation. On the baffle, carpet padding only. The rest of the cabinet volume I loosely fill will polyfill. It stands to reason IMO that each material tries to address certain ranges of sound- the dense carpet padding for lower frequencies, 'glass for higher frequencies, convoluted foam for diffusing mid frequencies, and polyfill for good measure. The formula that I have arrived at comes from experimenting to get the best sound, which is, of course, a matter of opinion and will not meet everyone's needs or tastes. I have also used the even more unreasonably priced layered foams from PE with good results, but the cheap and easily available materials in the same implementation have a slight advantage to my ear. Ceiling tile could be a good material if it was follwed by polyfill or 'glass.

BlueWizard, a fellow DIY from MN! Whereabouts?
Because of health issues, I have not used fibre glass or rock wool.

Polyester based materials like polyfill, etc, make your speakers think that your box is larger. This property is desirable in many box designs. However, they are not very effective in damping / absorbing (turning the sound energy into heat) the sound, and are pretty hopeless in low frequencies.

Natural sheep wool also makes your speakers think your box is larger and has some ability to absorb the sound, even in lower frequencies (of course, less effective at low frequencies than higher frequencies).

I have used and compared polyester, purposely designed open ceil foam, and sheep wool batts (80% wool and 20% polyester to hold the wool in place), and found wool to be the best (by quite a large margin).
Keep in mind, as Fwater points out, there is no reason why you can't use a combination. Convoluted foam on the back, and thick poly on the sides, and add poly fill to taste.

I think most people use acoustic convoluted (wavey - eggcarton-like) foam, but you can go to the fabic store and get high density convoluted foam for seat cushions or bed padding. I don't see any reason why it wouldn't work.

Preferably the black colored foam.

Also, keep in mind that some foam deteriorate substantially over time, and after a decade or two, it will start to crumble. That is typically the yellow foam.

Again, I simply use fiberglass house insulation because it is cheap and easy to find. You only need 2" to 3", so I take 6" peel the paper off, and split it roughly in half.

Generally, once in place, fiberglass is reasonably stable and not generally a health hazard. But when you are handling it, little flakes of glass can get embedded in you skin and they will itch like mad. Now imagine what the absolutely inorganic glass fiber does when it gets into your lungs (or eyes). That simply can't be good. So once again, exercise reasonable common sense precautions. And try not to install it while on the living room carpet.

The fiberglass ceiling tile are not as stiff as normal ceiling tiles. The glass is still pretty floppy, but it is not as loose and fluffy as house fiberglass. The fiberglass ceiling batting is relatively dense, holds its shape nicely, and can be easily stapled to the wall of the speaker. This about like taking 3" of fiberglass house insulation and compressing it down to 1" and you will have a rough idea.

And, by the way, I am in sunny southern Minnesota at the crossroads of America.



2006-04-04 11:24 am
Those interested go to this site:

Initially my speaker was very lightly damped, one layer of acrylic damping behind the speaker. Then I add in mat(similar to bitumen pad) on the behind main driver and tweeter and on top of the speaker and then cover it with a 1" thin of acrylic damping, also on one side of the cabinet.


better imaging, midrange, however the perceived sound becomes slow and some bass is missing from original.

Any suggestion, I think I need to remove some acrylic damping, what do you think?


2007-11-14 6:02 am
I should have clarified that using the 'glass, foam, and polyfill has been in sealed enclosures, in instances where the woofer would be tolerant of cabinet volume swings. For bass reflex, I have used the prefab layered foam from PE and small amounts of 'glass. I should also mention that I have overdone damping to the point of slow and/or diminished bass output... but experimenting is a big part of DIY, no?

Ridged pink foam might be good for thermal insulation, but not for acoustic insulation. You need something soft and porous to absorb sound and prevent it from being reflected.

The soft 'egg-crate' cushion foam, absorbs and diffuses the sound. That which is not absorbed by the little holes, is reflected back as small weak scattered waves and therefore helps prevent a big standing wave inside the cabinet.

So, whatever you use needs to have a combination of absorption and diffusion of sound.

David and eStatic,

As I said, reasonable precautions need to be taken. Any fiberglass dust you get in your lung is really little splinters of glass, and there is no way that can be good for your lungs.

However, a common dust mask is probably sufficient for the amount you will be handling the product.

Once in place, unless you are putting your lips on the port every day and sucking in internal cabinet air (or using your speakers as a really big bong), and really why would anyone do that, you are probably safe. I really don't see a lot, or any, leakage of glass dust from the cabinet through the reflex port. Even considering the amount of vibration in the cabinet.

However, polyfill and convoluted foam are readily available anywhere, even in the smallest towns, and if not, then by mail order. If I were to build a new speaker cabinets today, I would probably go with some combination of foam and poly, simply because it is easy to handle and has virtually NO health hazards.

In this day and age when modern options are available, we might as well use them. However, I don't regret or worry that my existing speakers have fiberglass, and I wouldn't discount using fiberglass in the future.



2002-01-07 6:02 pm
HiFiNutNut said:
Natural sheep wool also makes your speakers think your box is larger and has some ability to absorb the sound, even in lower frequencies (of course, less effective at low frequencies than higher frequencies).

I have used and compared polyester, purposely designed open ceil foam, and sheep wool batts (80% wool and 20% polyester to hold the wool in place), and found wool to be the best (by quite a large margin).
Got a local source for me please? My new speaker design will be *ahem* large and sealed.
Davide82 said:
rockwool and fiberglass are hazardous through the vents not only handling.

I would like to question that. Ok, that when you insulate a whole attic with fibreglass you should use a breathing protection, but for the small amount that is used inside a speaker I'd say there is no risk at all, not even during the construction.

...or do you know of any scientific work that would support that the small amounts of fibreglass leaking out through a vent would be hazardous?
Hi Svante,

I think there is always a risk. Also like with asbestos or other materials.
But from what I know rock wool don't pose such health risks of that magnitude, only fiberglass. Meaning I would use it in place of the former. They have identical sonic properties. If something is dangerous you don't need a study to be careful. I would use materials that are integral after 10+ years. What you don't want is to open a speaker and to find dust inside because of various reasons.

ps. I have been reading your software documentation (The Edge, Xdir and Basta!) and (I remember that) didn't you use to have one software that would resolve the vector summing output and phase shifts of two speakers, and print the x-over frequency curves as a result. Where can I read about this topic, do you know?:att'n:


Disabled Account
2007-03-10 1:20 am
Davide82 said:
rockwool and fiberglass are hazardous through the vents not only handling.

Svante said:

I would like to question that.

I would have been dead long ago...:dead:

Insulated quite a number of houses, including my own, using both pink fiberglass (for thermal) and rockwool (for sound deadening).
I'm still alive and healthy.

BTW, I like rockwool.


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