For a white paper discussion of acoustic noise canceling try this:
http://www.computeraudio.telex.com/in03a.htm
It is biased in favor of Telex's acoustic approach but offers some valuable
info about how things work theoretically.
thanks,
-chris thompson
gtri eoeml 404-894-6143
Work like you don't need the money.
Love like you've never been hurt.
Dance like nobody's watching.
Pray like there's no tomorrow.
----- Original Message -----
From: Vaughan Pratt <
>
To: <
>
Sent: October 06, 1999 2:04 PM
Subject: Re: Microphones
>
> >> >A non-electronic solution would be to use a microphone which has its
> >> >diaphram open to both sides. In theory ambient noise cancles out
(equal
> >> ^^^^^^^^^
> >> >pressure on the front and back of the diapram), but directional sound
> >> >(e.g. voice) does not.
> >>
> >> In whose theory? There is no such thing as nondirectional sound.
> >
> From: john flanagan <
>
> >Actually, Pratt is correct on this one, but perhaps isn't conveying it
> >clearly. Noise-cancelling microphones operate on the principle that the
> >farther you get from a sound source, the quieter the sound (remember,
> >1/r^2). NC mikes have two diaphrams, a small distance apart (less than
> >1cm) from one another.
>
> My point was that the proposed *single*-diaphragm solution couldn't work
> as envisaged. My comments weren't relevant to the two-diaphragm setup,
> if that wasn't clear then what I said would certainly seem very unclear.
>
> The only noise-relevant property of a mike with both sides of the
> diaphragm open is its polar (gain or response) pattern, which (in plan
> view) is a figure-eight. Thus it hears louder fore-and-aft than to the
> sides. It will therefore "cancel" all sounds, near or far, coming from
> the side but not from front or back. It makes no distinction between
> near and remote sounds other than the obvious one that remoter sounds
> are quieter, which is true for all microphones.
>
> Vaughan
>
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