Damping material.

Alright, I pretty much understand that with sealed boxes stuffing with damping material is a good thing. But I've heard people say not to do it with vented, and others say to do it. Is it good or isn't it? I know lining the walls of a vented is good, but is filling with fiberglass or wool good? My dad hast an old pair of Sansui SP-5500X speakers which are ported and do appear to be stuffed with wool, so I'm not sure. Just hoping someone can set me straight o nthis one, thanks.


2001-08-17 6:00 pm
Joe, I'm not pretty sure about this either.But it's a good idea to have the internal surfaces damped. As for the stuffings I have a pair of Mirage full of them ( feels like cotton wool? and i wonder why :) ) But if you should experiment, I'll say go easy on the stuffings.
JoeBob, I've repaired many ported speakers, some fully stuffed some with only dampening. Going form the theory though stuffing should reduce the ports output,(decouple the port from the driver). It would also further reduce internal reflections which often improves the mid range.
I usually stuff to taste! So try it with and without, see what you prefer, it has a dramatic effect. Realise that stuffing speakers that weren't designed to be stuffed will reduce the low frequency response. Also not stuffing speakers that were designed to be stuffed will give you a booming bass.
I think that covers it, I'm stuffed:)
Regards WALKER

[Edited by walker on 11-23-2001 at 03:21 AM]
For subwoofer applications, I only line the cabinet walls with damping materials, particularly a very high density egg-crate foam that is available only available to other companies commercially. Stuffing can not only raise the cutoff frequency, but it can restrict airflow and alter other resonances.

However, you can lightly stuff a vented box in some cases, such as smaller midrange applications. Here, you can be less concerned with the monumentous amount of airflow associated with some of todays bass drivers, while still cleaning up some midrange "muck". However, it is still advisable to leave an unblocked channel admist the stuffing from the back of the driver to the port tube.

(By the way, I own a pair of Mirage speakers which are indeed stuffed, but fairly lightly aside from the damping on the sidewalls. Also, the stuffing material seemed to be a "long haired" material, which is arguably better sonically. Long haired wool is rumored to be one of the best materials to stuff with, though its scarce availability and price don't allow too many people to test it.)
You can use polyfill, known also as polyester wadding for damping. Note that if the damping material is compressed enough while doing the stuffing, in a closed box, you can artificially raise the internal volume of the box. The system operates in constant temperature (I forget the technical term for this). That is why you would normally find very tight/compressed stuffing in a small box.

Lining the walls of a box calculated for Qts of around 0.7 will reduce standing waves within the box. Boxes which have a higher Qts will tend to boom and you can carefully use stuffing to produce warm bass although transients will suffer.

In general the port proper and the opening to the port is not lined. A little space behind the magnet of the speaker should also be free of stuffing material.

Dacron is a material used by some manufacturers and is available at dedicated audio outlets.
Here's my big chance to sound intellectual. The term Samuel was after is ANADAIBATIC, (non-adiabatic) or ISOTHERMAL. Meaning that pressure is converted and lost as temperature making the volume seem larger, (thats a simplified explanation).

As I understand the acoustic volume can be increased by as much as 15% for Dacron and 18% for fibreglass. I have never tested this, maybe soon with my next project, but I have factored it into other projects.

Well it's all downhill from there.

Regards WALKER