Any microphone experts out there?
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From: GregS on 20 Jul 2010 09:37 In article <2pmdnYo3jbWvqdjRnZ2dnUVZ_rydnZ2d(a)posted.localnet>, Robert Baer <robertbaer(a)localnet.com> wrote: >Phil Hobbs wrote: >> Jim Thompson wrote: >>> On Sun, 18 Jul 2010 08:37:46 +0100, "Midge" <midge(a)somewhere.com> >>> wrote: >>> >>>> Have a small microphone mixer http://www.audiomate.co.uk/PDF/G105C.pdf >>>> >>>> I've been using a couple of dynamic microphones on it, but I wondered >>>> if it was possible to either get a tie-clip microphone (these seem to >>>> be electret condenser) which can be plugged straight in OR match the >>>> different technologies somehow. >>>> >>>> Thanks, Midge. >>> >>> If the electret has a built-in FET buffer, all you'll need is power >>> for the FET. >>> >>> Otherwise the Audiomate input impedance is too low. >>> >>> ...Jim Thompson >> >> As an aside, does the standard explanation of how electret mics work >> bother anyone but me? It's usually stated that the 'frozen in' >> electric polarization in the material works just the same as the applied >> voltage in a real capacitive mic. This is of course nonsense--charges >> such as air ions migrate around and eventually neutralize any E field >> external to the electret material, so that waving it around does nothing >> whatever. (Leakage--that's why you have to keep the capacitance mic's >> supply connected for it to work, duh.) >> >> The reality appears to be that they work by a combination of the >> piezoelectric effect (like a quartz crystal) and the variation of >> capacitance with stress (like coaxial cable with a DC voltage on it). >> >> Any comments from audio folks? >> >> Cheers >> >> Phil Hobbs >> > Regarding presumed "frozen in" charge.. > About 50 years ago, in high school, there was this "true" rumor of a >related experiment: > Take a sheet of glass and place a metal plate on each side to make a >capacitor, then charge it (2KV to 20KV if i remember correctly). > Disconnect completely and then remove the glass sheet. > Measure the voltage (virtually zero). > Remove volt meter, replace the glass sheet and measure again. > According to the story, one reads near the original charging voltage. > But it gets better.. > Short this cap and verify zero voltage. > Wait..(minutes, i think an hour max i think) then re-measure and find >a rather shocking (literally) voltage present. >** > Naturally, that second aspect is well known - most especially to TV >repairmen dealing with those CRTs. Mylar tape is made with a charge holding the sticky on. You can use this to make an electrostatic speaker. All you need is a step up transformer. Unfortunately the sticky weighs down the tape too much for efficiency. I tried taking the sticky off and the charge disapears. I wonder if that great roll of OD military mylar tape I have from the 60's still works. greg |