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Re: diaphragm output & phase: HC selectivity ... and the lateral organ of the fish

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 (http://www.bksv.com/3095.asp) 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!