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Re: (RIVES or Knowledgeable Inmates) How Can I Measure the Effectiveness of Bass Traps?

Posted by Jon Risch (B) on March 14, 2004 at 21:00:51

In Reply to: (RIVES or Knowledgeable Inmates) How Can I Measure the Effectiveness of Bass Traps? posted by gmo on March 10, 2004 at 19:19:23:

Effective action of bass traps seldom show up in the usual sorts of frequency response curves available to the non-professional. It is not necessary to significantly flatten the bass FR via the traps in order for them to do some good acoustically. Most people get quite confused about this subject, and seem fixated on some sort of holy grail of flat FR in the bass. This is one of those nearly impossible quests, and flat bass FR can not truly be achieved in most domestic listening room situations. EQ is a false fix, see:
http://www.AudioAsylum.com/audio/tweaks/messages/17541.html
All you can do is get it close, and damp the room modes enough to make it listenable.

However, there ARE ways to measure bass performance in a room that is not tied strictly to the FR.

ASC has a very helpful test, called the MATT test, see:
http://www.asc-hifi.com/matt_test.htm

They even have a test tone to download, see:
http://www.asc-hifi.com/matt_test.zip
The test tone is also avaialable on the Stereophile Test disk #2.

The test tone is a series of tone bursts, that are spaced closely together. The test frequency rises with time, allowing one to pinpoint which frequency is garbled.

If one wished to use this as a metric for bass trap performance, then I would use a full duplex sound card to record the output via a good quality microphone (electret condensor type), and then examine it in a wav file editor to see how long it takes the tone bursts to decay down to some arbitrary level, etc.

This will provide some indication of bass trap effectiveness, that is not mired in the FR domain.

You could make your own home-made MATT type test using a wave tone generator by generating some tonebursts at 1/3 octave frequency intervals, spaced apart by approx. 0.0625 seconds silence. At the lower frequencies (below 80Hz), use only 4 complete sinewaves per toneburst. Above 80 Hz, use an integer number of complete sine waves closest to the spacing interval of 0.0625 seconds.

I would not go above 200 Hz, thus, you would have around 10-11 test frequencies. Most systems and rooms will not have significant output or issues below approx. 33-40 Hz, so you cut down the number of tones even further.

One could take this even further, and create a sequence of the appropriate tone bursts, with varying silent intervals, starting out at say, 0.040 seconds of silence and incrementing up through 0.10 seconds, to see where the tones became distinct individual bursts, and thus, provide a track by track reference to how much silent interval was needed for this.

Seven different silent intervals, by approx. 8 frequencies, is 56 tracks, no too bad to generate and burn.

Jon Risch

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