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Natural Science Forum / Physics / Acoustics / December 2003Anechoic Tube
In lieu of an anechoic chamber for testing microphones over their full frequency range I've been wondering if it would be possible to make a variant of the impedence tube with absorbant walls (to catch reflections off the microphone under test) and a sufficiently absorbant dead end to stop reflections from coming back. Can anybody address the feasability and/or construction of such a thing? Also, does anyone know if there is an anechoic chamber at U.C. Santa Cruz and who might manage it if so? Thanks, Bob  Signature"Things should be described as simply as possible, but no simpler." A. Einstein > In lieu of an anechoic chamber for testing microphones over > their full frequency range I've been wondering if it would [quoted text clipped - 3 lines] > reflections from coming back. Can anybody address the > feasability and/or construction of such a thing? Fine, as long as you can prove that the absorption is good at low frequencies. If you can't, then it is good only down to the frequency wher TOTAL absorption is assured, perhaps 1kHz or higher. Angelo Campanella. > > In lieu of an anechoic chamber for testing microphones over > > their full frequency range I've been wondering if it would [quoted text clipped - 7 lines] > frequencies. If you can't, then it is good only down to the frequency > wher TOTAL absorption is assured, perhaps 1kHz or higher. Right. The question is how best to get very high absorption across the band 20 Hz to 20 kHz on a much smaller area than a typical anechoic room, just the end of the tube. Maybe it's a nutty idea, I dunno, but if it could be done I'd sure like to give it a try. It would probably involve some kind of compound structure at the end but I'm no absorption guru (by a very long ways) and the details are well beyond me. Bob Signature"Things should be described as simply as possible, but no simpler." A. Einstein > > > In lieu of an anechoic chamber for testing microphones over > > > their full frequency range I've been wondering if it would [quoted text clipped - 17 lines] > > Bob My students do a laboration on this, measuring nicrophones. We have a tube, 2m long and 30 cm diameter, and a loudspeaker IN EACH END. The signal to the second speaker is a copy of that to the first speaker, but delayed. The second speaker cancels the reflections that would otherwise occur. If you are able to read swedish, the lab instructions are available at: http://www.speech.kth.se/~svante/elak/miclab.pdf ...or you can just look at the pictures. :-) The results are not terribly good, it is difficult to cancel standing waves completely. However, I think there is room for improvement, in particular regarding which signal is sent to the second speaker. We will see if I ever fix that. Also, our speakers are pretty old, I would assume that there are better ones around now. > The results are not terribly good, it is difficult to cancel standing > waves completely. However, I think there is room for improvement, in > particular regarding which signal is sent to the second speaker. We > will see if I ever fix that. Also, our speakers are pretty old, I > would assume that there are better ones around now. What about a hybrid approach where the end of the tube prior to the terminating speaker would contain some kind of fill which would reflect little incident sound and attenuate all but the lower frequencies as it passed through and then using a small omni mic in front of the terminating speaker to generate a cancelling signal to that speaker for the remaining LF? Feasable? Bob Signature"Things should be described as simply as possible, but no simpler." A. Einstein > > The results are not terribly good, it is difficult to cancel standing > > waves completely. However, I think there is room for improvement, in [quoted text clipped - 9 lines] > to generate a cancelling signal to that speaker for the > remaining LF? Feasable? Nope. The sound pressure is not supposed to be zero, it is supposed to be that of the propagating, but not the reflected wave. I have a hard time imagining hoe a microphone could differentiate between the two. Possibly a cardioid mic directed towards the reflected wave. Hmm, that's an idea... But for the moment my greatest hope is a delayed copy of the signal to the first speaker (as we have it) plus some filter to mimic the filtering effect of the tube. Actually we DO absorb higher frequencies with a small amount of mineral wool near the second speaker, and turn the speaker off above 1kHz (we use sine sweeps). > Bob |
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