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In Reply to: Re: Q about DIY tube trap posted by FAT on October 27, 2001 at 11:07:36:
As with many things audio, it depends. Application and end purpose come into play.For the type of tube trap using an acoustical air space inside, and a layer of compressed fiberglass material, it seems to be around 1.75 to 2 lbs per cu. ft. HOWEVER, if you use fiberglass with this density already present, it does not seem to work as well as when the material is compressed from a lower density.
Rock wool can use a slightly lower density.
For sound absorption inside a speaker cabinet, from 0.5 to 0.7 lbs per cu. ft. density of fiberglass or other good absorbermaterials is best, with the lower density directly behind a woofer or midrange, and the higher density in the corners and on the side wall away from the woofer.
Sound absorbing wall panels with air space behind them like to be in this range or slightly more dense, as there is also the issue of too much upper mid/HF reflectance with increased density.
This is all from personal experience and experimentation, using both instrumentation and listening comparisions.
Jon Risch
Follow Ups:
From what I've read the impedance match must be correct otherwise reflections, but your densities seems to be on the lighter scale from the rec'd 2.5-6.25pcf ?
Just looking at specs in a book, or absorption coefficients in a manufacturer's chart do NOT tell the whole story.Many such charts or data show absoprtion that EXCEEDS 1. This is, of course, patently impossible, but due to the methods used to measure such things, bogus numbers get generated. The one thing you seldom if EVER see is: what is the reflectance of the material? Here, the higher densities do not work as well at higher frequencies. Note that many charts end at 4 kHz or 8 kHz, and that actual HF reflectance does not tend to show up in the absorption coefficient numbers to any degree due to the way they are measured.
I specifically stated that this information stems from my own personal experiences and measurements. I have spent some significant amount of time measuring HF reflectance, and testing many different materials for reflectance.
Of course, most of the higher density materials can be "fixed" by adding a layer of 3/4 to 1" polyester hi-loft type batting.
Jon Risch
Thank you Jon! A higher density mineralfibre tube increases bass absorption at the expence of high freq reflections? (Sounds fair for a basstrap device!) A thicker, high density fibretube would increase bass absorption even more? Is there an optimum amount of air (volume) that should be enclosed in the trap? Also, for first reflection dampeners, how large (area) should these be to be effective? Is there any rule of thumb?
Thanks in advance!
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
Thanks for your help! So you don't rec'd any of these "pre-made" mineralfibre tubes? That's sad since I found one that I thought should work: 2.4pcf, inner dia. 219-813 mm, isolation 40-160 mm.
No, the "pre-made" pipe insulation may have the density (often way too much), but it does not seem to work as well as the compressed fiberglass.They would probably work almost as well as the Super Quick & Dirty versions, but why pay more for less performance?
Jon Risch
Thank you Jon! Last Q: is there an easy way to measure the performance of a DIY basstrap or absorbers in general? Tia.
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