On Tue, 5 Oct 1999, Vaughan Pratt wrote: > > >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. 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. The theory is that sounds coming from a long distance away (compared to the distance from mouth to mikes) will have roughly equal amplitudes, while the sound coming from the mouth will have a very slightly stronger amplitude at the close diaphram, and a slightly weaker amplitude at the far diaphram. This difference is measurable because the mike is so close to the mouth (very small r for the 1/r^2). Subtract the signal from one mike from the other mike, and what is left will be almost pure voice signal, since the roughly equal noise in both signals will cancel itself. However, since you're cancelling out *almost* all the voice signal by doing this, you wind up with a *very* low-gain mike. John Flanagan -- Subcription/unsubscription/info requests: send e-mail with subject of "subscribe", "unsubscribe", or "info" toWear-Hard Mailing List Archive (searchable): http://wearables.blu.org
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