From: dpierce@cartchunk.org   
      
   Audio Empire wrote:   
   > On Sat, 17 Apr 2010 14:58:30 -0700, Norbert Hahn wrote   
   > (in article ):   
   >>There is two more type of condensor mic which don't use DC at all:   
   >>Both types use a low voltage RF generated by a low-noise RF oscillator.   
   >>   
   >>The mic capsule of a RF condensor mic is part of a resonant circuit   
   >>that modulates the frequency of the oscillator signal. Demodulation   
   >>yields a low-noise audio frequency signal with a very low source   
   >>impedance.   
   >>In the other type of RF condensor mic the capacitance of the mic   
   >>capsule is used to modulate the amplitude of the RF oscillator.   
   >>   
   > These were somewhat common back in the 1940's and 1950's but are   
    > rare (if not outright non-existent) today.   
      
   Rare is a relative term: potable water is rare in the middle   
   of an ocean. But non-existant? Hardly.   
      
   In addition to the other uses cited elsewhere, so-called RF   
   microphone carrier systems are used for instrumentation purposes   
   notably where extremely low frequency reponse is required.   
   Referring again to Bruel & Kjaer, their microphone carrier   
   system with an appropriate capsule is capable of measuring   
   down to frrequencies approaching DC (i.e., it can act like an   
   electronic baramoeter), while still maintaining response up to   
   150 kHz. Used with something like a B&K 4147 capsule, you have   
   an acoustica measuring system with response from under 0.01 Hz   
   to over 20 kHz within a 2 dB envelope.   
      
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