Re: electret microphone low frequency response
- From: John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 21 Jan 2009 21:02:53 -0800
On Thu, 22 Jan 2009 15:34:17 +1100, "Phil Allison"
<philallison@xxxxxxxxxx> wrote:
"John Larkin"
I did a system for monitoring the noise generated by the static tests
of the Saturn S1B rockets, in the vicinity of the Mississippi test
site. FM on leased phone lines. We used closed-loop-heated GR electret
mikes
** GR = Gen Rad - right ??
Yup. Late 60's, when I was still in school, and there was still a
General Radio.
We heated the mikes for amplitude stability and to keep moisture out.
The things were put on telephone poles in grey steel Hoffmann boxes.
The locals liked to shoot at them. They shoot at most anything in
Mississippi.
dumping directly into a jfet follower, no gate resistor at all.
They were flat down to at least 0.1 Hz.
** But the OP hs no such exotic electret capsule with external FET.
In the cartoon schematics I've seen, there's nothing in the capsule
but the electret and a jfet. Makes sense... you don't need a gate
resistor, as it will self-bias close to zero volts.
Instead - he is trying to use capsules that not only use inbuilt FETS
with deliberate gate leakage PLUS have air bleed holes to equalise
atmospheric pressure changes.
As Arie noted, you've may have to do something to make sure the
pressure doesn't sneak around to the back of the diaphragm. Are any
electret mikes inherently back-sealed? If not, you could glue one into
a hole in a plastic pill bottle or something.
Anyhow, actual measurements would be interesting.
John
.
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