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RE: Aye, cynical I am

Posted by Jon Risch (B) on October 22, 2009 at 18:41:46

In Reply to: RE: Aye, cynical I am posted by Ethan Winer on October 22, 2009 at 13:15:59:

Ethan,

Thank you for a reasoned reply.

I just have one comment, and one correction.

First, regarding:
"When mounted straddling a corner, my company's MiniTraps measured just under 3-1/2 sabins at 63 Hz. This is for 2 by 4 foot panels placed in the best locations possible. Looking at the tube trap sabins graph in Everest's book (page 487 for the popular 4th edition), it is claimed that a 16-inch diameter tube trap has about 15 sabins of absorption at 63 Hz. The definition of one sabin is a one square foot opening to the outdoors. But a 16-inch diameter tube has only 12.56 square feet! So even if 1-inch thick 703 rigid fiberglass could absorb 100 percent at 63 Hz, which we all know it cannot, the highest value theoretically possible is 12.56 sabins...."

A: We both know that there ARE absorption coefficients that go above 1, this happens all the time, and it can either be due to measurement 'artifacts', or other acoustic absorbing actions, such as diaphragmatic sheets or barriers included in the absorbing device, or a combination of the two. The tube trap does indeed include such a diaphragm, in the form of a perforated metal barrier in the earlier retail traps, and later on, in the form of a limp plastic sheet which seems to have minute perforations in it. I know this because of being present when some traps were dissected by an interested party (who sought my advice on how to effectively put them back together without having "broken them"). Thus a tube trap can achieve an absorption coefficient greater than 1, and indeed, the data at their web site shows this, with the absorption coefficient reaching greater than 1.4 at middle bass frequencies.

Data at ASC's site shows a 20" trap reaching an absorption coefficient of 0.94 at 63 Hz. Using the surface area I calculate for a 4 foot tall 20" diameter trap of 20.94 sq. ft., this would equate to 19.68 sabins, for a single trap.

At your retail website, the data you present shows the mini-traps hung on a wall reach an absorption coefficient of 1.86 at 400 Hz, way more than 1. Is this then bogus or false too? You apparently use that same greater than 1 absorption coefficient data to calculate the Sabins to reach 14.88 at 400 Hz.
There doesn't seem to be any disclaimer, or "correction" to an absorption coefficient of just 1.

You cite an absorption of 3.5 sabins at 63 Hz when your mini-trap in in the corner, but we don't know that the tube trap was measured this way, in fact, if the data in Mr. Everest's book were presented in an industry standard manner, it should have been with the tube trap laying down on the floor, rather than in a corner.
Nowhere in my edition of "The Master Handbook of Acoustics" does it say that the measurement represents more than one tube trap, or a corner measurement.

My own experiences with actual ASC tube traps, with DIY versions, and with simple 2X4 foot fiberglass panels tends to back-up the numerical data I am familiar with, and my perception that a simple fiberglass panel is just not a bass trap, even if you place it in a corner. Yes, it does SOMETHING, especially when you compare it to nothing, but it is not doing the same thing as what I consider a bass trap to be doing for the control of the lower bass room modes, and the smoothing and defining of the bass musical notes.

So I believe that I will try to contact Mr. Noxon to further clarify the numerical performance of the larger ASC bass traps. While I am satisfied with the numbers I have seen, I wouldn't mind getting further reinforcement of that data.

Finally, regarding my website, Geocities is closing ALL of their free web site hosting as of Oct. 26th. For some reason, I was remembering it as this Friday, which is only the 23rd. I want to re-locate my site to another (free) host, but just haven't had the time yet. Maybe this week end, if I get the time.


Jon Risch

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