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How to:
Damp a Speaker Cabinet

the "How to" index
the "DIY Cabinets" page

An article about bracing

There are two goals here, and they are separate.

One is to have the cabinet itself, the "wood" box, be as inert as possible. Hence the bracing. You could use more than one, but that first one between the woofer and tweeter is the biggest deal (I like to cut a "wide" piece of material to that precise length, then chop it up into a half dozen pieces that all share that arduously-achieved dimension).

People also add various forms of damping material to the inside - tar paper, rubber sheets, etc. Anything that says "audiophile" will likely cost ten times what it should and include some made-up physics in its promotional material. I see no harm in doing a bit of this, by the way, the only issue are materials being sold as if they were magic. A bit of scrap "Ice and Water" from a construction site or builder would be excellent for this, I think.

You can also add damping to the panels on the outside of the box. That is usually going to be ugly, but if, say, you add a half inch of veneered MDF all around, with appropriate edge-banding, etc. - now you've improved the cabinet substantially from a physical standpoint and from an aesthetic one.

The other goal is to convert the energy going into the box from the rear of the woofer back and forth into heat. This is essentially the same as "reducing standing waves." Standing waves are vaguely like a jumprope - all the motion is in the middle and there's none at the ends. In this case the "motion" is sound, it is alternating compressions and rarefactions. Densely packing the box with material that will be forced to move microscopic amounts by these vibrations is the strategy. And with a sealed box, there is no reason to leave open space, as there might be (I am not convinced) with a vented system.

(That internal energy results in two potential effects on the sound. One is causing the cabinet panels to flex, and then to reradiate that flexing frequency into the room. The other is some of the sound passing "through" the woofer cone back into the room. Absorbing that backwave - and minimizing reflections of it - are the key to reducing both.)

The original speakers usually had fiberglass in them. The same thing we put in our walls and attics to insulate them. At Genesis we bought it from the now-defunct Grossman's Building Supply. They delivered (from half a mile away), which was nice.

A simple "loose fill" of chunks of fiberglass works fine.

Fiberglass is pretty nasty stuff to work with. It's not really an allergen, and it is not a carcinogen like asbestos, but it is one heck of an irritant at best. I usually remove it, throw the bits in the attic, and replace it with the below-described polyfill as I rebuild old speakers.

Polyester fiberfill, as used in toys and comforters, also works very well. It is slightly less effective, so we pack it in more densely.

There are people selling audiophile polyester, as there have always been people selling audiophile sheep's wool (think about that for a minute), etc. These people's credibility is weak, for me, because of the language they use, often hugely exaggerating minuscule effects, and making up their own physics as they go along. Their marketing is often somewhat "creative."

Foam works too, but I don't trust it to hold up properly, even sealed in a box, for decades. Also, getting the "best" kind and using the right amount are harder to address.

You can get polyester fiberfill at places that cater to crafters, quilters, pillow-makers, etc. Places like Michael's, Joann Fabric, and Walmart , or, of course, independent fabric and craft supply sellers.

This link might very well go to a three-pound box of it at Walmart, $15 at the time of this writing (it seems to have gone up recently - quite a bit - my last purchase was under $20 for five pounds). That is probably "more than enough" for a pair of EPI 100-type (roughly 3/4 of a cubic foot) speakers. But more is fine, all it does is make the box "seem" a tiny bit larger, and the 100 and its brethren are smaller, punchier boxes than the optimally-sized 150 (in terms of flat LF extension - one can go even larger for yet a slightly different "sound" again).

It's probably cheaper than two one-pound bags. Check other suppliers, too.

I think that about covers it.

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