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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Hello everyone.
So I'm thinking about how it would be possible to make a capacitive microphone. As I understand it, a capacitive microphone is a mic that uses two thin plates to make a capacitor, and when pressure waves hit the plates it causes a capacitance change, thus creating a voltage difference.
Would it be possible to make a capacitive microphone yourself, using some plastic that is conductive? I know anti static bags are technically conductive, but I don't know if they are conductive enough for the process to work right. Alternatively I thought maybe aluminum foil would work but that's probably too thick.
Is there some thin film plastic with metal deposition on it that would work? I'd love to hear your ideas. Reasoning behind why I'd like to try this out, is because all of the microphones available cheaply are small capsule; if you could create your own you could potentially create a decent sized microphone that has good noise resistance and a low noise floor. Potentially, though. I don't know how hard this is going to be!
I'd think the difficulty would be in spacing, mounting, and securing the "plates" such that they can move from incident pressure waves at realistically usable amplitudes. I'm sure you could easily make one with household materials, but you might need a concussion grenade to get any signal out of it!
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
inducktion yes of course you can. this is the reverse of a electrostatic speaker. two perforated metal plates holding a thin membrane that is slightly conductive. I used 0.00025" mylar film that i hand rubbed with graphite powder until I got around 1meg per sq resistance. You only need to do one side but this side is the output so you need to attach a fine copper wire with silver glue. the outer metal plates are biased in my case @ 2kV +- so no movement of film is 0v any movement ether way causes a voltage to be induced very high impedance source of course
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
If it is just a low noise microphone that you require why not use a dynamic microphone with a low noise pre-amplifier ?
e.g. a not particularly sensitive Shure M58 microphone (or a Maplin DM-13 microphone insert cartridge, -76 dB usa rating), 1.85 mV @ 1 pa = 94 dB SPL, so 0 dB SPL = 37 nV rms
excellent audio op-amps (e.g. LT1115) have around 1 nV/SQRT(Hz), for a full 20 kHz bandwidth that's 141 nV rms
thermal noise voltage of 600 Ohms at 27C, 20 kHz bandwidth, is SQRT(4.k.T.B.R) = 446 nV
equivalent dB SPL of noise = 20.log(468/37) = 22 dB SPL equivalent
I doubt that you will find background noise environments lower than 22 dB SPL in nature
even an old TL071 op amp at 18 nV/SQRT(Hz) would be around 37 dB SPL (741 = c43 dB SPL ?)
(I calculated these numbers ... not verified)
After a little wandering around the 'net have a look at page 12
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Making a capacitive microphone
