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Re: Q about DIY tube trap

For hollow bass traps, there is an optimum density, too much will no longer absorb, but start reflecting even mid bass and the midrange.
Many of the pre-formed high density pipe insulation sleeves are this dense, and will not work as well as my basic recipe.

For solid chunks of sound absorbing material, higher densities work better up to a point, then start to get worse again. The Super Quick & Dirty bass traps are just rolls of building insulation still in the bags, wrapped in polyester batting, and covered in cloth. (see: the original post where I reveal the latest Quick & Dirty super easy bass traps recipe:
http://www.audioasylum.com/audio/tweaks/messages/15737.html
and petew's post where he goes into detail on what he did.
http://www.AudioAsylum.com/audio/general/messages/70817.html )

These work on the brute force method of just having enough sheer absorbing amterial all in the corners.

As for the internal volume, there is a very real practical limit on how big one can make such a device, and not have it fail to function due to air leaks, and a lack of proper compression of the materials.
I do not recommend trying to build a bass trap bigger than approx. 16" in diameter, it is just too hard to do, and get good results. A 16" diameter by 4 or 5 foot trap will absorb quite a bit of bass modesin the corner, and a set of four of these can work wonders for a room.

As for first reflection point absorbers, these need to be large enough to absorb a bass wave down to the capability of the thickness level. As an example, for a panel using 6" thick fiberglass, and an outer layer of polyester fiberfill, the bass absorption might be good down to about 200 Hz, at which point it falls off. In order to be truly effective, the size of the wall panel must be on the order of 1/2 wavelength wide and/or tall, or the wave will not be absorbed as much as the ASTM specifications would seem to indicate. This is going to be a minimum of about 30 inches or so, hence my recommendation of this as a minimum width for a side wall absorber, and with a height of about 4 feet, the overall size is adequate to absorb down to 200 Hz.

There is still some bass absorption down below 200 Hz, but this is where it begins to fall off significantly.

Place a small 1 foot by one foot patch that is 6" thick (plus poly) directly at the exact 1st reflection point, and it will just NOT absorb all the way down to 200 Hz like the larger piece will.

Jon Risch

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