Re: diaphragm output & phase: HC selectivity ... and the lateral organ of the fish (Bob Masta )

Subject: Re: diaphragm output & phase: HC selectivity ... and the lateral organ of the fish
From:    Bob Masta  <audio@xxxxxxxx>
Date:    Sun, 7 Oct 2007 09:03:51 -0400

On 6 Oct 2007 at 20:13, Steve Beet wrote: > Hi Kevin, > > I'll keep this short because it's getting a bit off-topic, but if you look > at B&K's web site ( and search for very low > frequency microphones, you'll find the only ones which go down to DC are > "piezoresistive" designs. The lowest frequency they quote for a condenser is > 0.07 Hz. Pretty low, but not DC. > > I believe RF condenser microphones can extend down to DC too, but the most > common condenser mikes rely on a high impedance DC bias voltage, with that > (resistive) impedance combining with the capacitance of the microphone > itself to make a low-frequency time constant which prevents a true DC > response. > > Steve > > Actually, the basic condenser technology does go down to DC. B&K mics have a special vent hole specifically to prevent DC response. In general, DC response would be a Bad Thing for most users, since changing room pressure, etc, could bias the diaphragm position and increase distortion, or limit maximum SPL. (Overdriving a condenser mic can cause the diaphragm to short to the backing plate, sometimes spot-welding together.) However, somewhere I have seen in B&K literature instructions for sealing the vent for special purposes. Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

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