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Re: Microphones

From: "Chris Thompson" <>
>For a white paper discussion of acoustic noise canceling try this:
>http://www.computeraudio.telex.com/in03a.htm

Looking at this (thanks, Chris!) I see that John Flanagan was too
kind when he said I was correct but unclear.  Figure 5 of Lake's very
informative article clearly shows that I was incorrect in deducing
from the figure-eight polar pattern of a mike with both sides of the
diaphragm open that it could not separate near sounds from far.  Sigh,
well at least I got the figure eight part right.

What I didn't reckon on was the propagation path from front to back (for
a sound coming from the front), which does a nifty juggling act of being
short enough to keep the phase shift low (so that the noise-cancelling
effect will work up to the important frequencies in speech) while being
long enough for the inverse-square law to kick in.  The article didn't
make clear what the acoustic mechanism for this path was but presumably
it's two diffractions, one at each turn of the path as it goes around to
the back.  Clearly it works (taking Figure 5 at face value), but surely
a twice-diffracted wave would be so attenuated as to make it thoroughly
unclear from the theory how you'd obtain Figure 5 other than empirically.
Lake's treatment while very illuminating raises as many questions as
it answers.

Vaughan

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