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Re: Physics of moisture-induced mic noise (was MS-stereo and figure-

Subject: Re: Physics of moisture-induced mic noise (was MS-stereo and figure-
From: "Eric Benjamin" ericbenjamin2
Date: Sun Jan 3, 2010 3:58 pm ((PST))
Rob Danielson wrote some really good questions:

(1) is this (AT3032) an electret mic capsule, not a "true" condenser?
It's an electret capsule.  I make no distinction between electret and non-e=
lectret microphones in terms of their being "true" condensers.  They're bot=
h condenser microphones, and they both have air gaps between the diaphragm =
and the back plate.  One type may have an advantage over the other, but as =
you write below:

> I think we may be concentrating on capsule design "explanations" too much=
I'm quite sure that you are right.  Microphone manufacturers try to make th=
eir designs resistant to humidity, but the results seem to be all to variab=
le in practice.  Sometimes mfgrs. use a conformal coating on the PCB in the=
 area right at the capsule input.  But I wouldn't be surprised if this does=
n't make problems worse under some circumstances.  The impedance at this po=
int is a gigOhm or higher in most designs (excluding RF microphones) and th=
e conformal coating may act as a sort of matrix to accumulate various sorts=
 of crud.

(2) Could it be that small dirt particles are collecting or some other phys=
ical change is happening with these exposed capsules that is affecting the =
nature of the gap?
Possibly, but it should be nearly impossible for dust to get in the gap (un=
less it was there when the mic was assembled) with an omni microphone.  A l=
ittle easier for dust to get in the rear of a cardioid microphone, but most=
 mfgrs. take good care to make that difficult.

(3) one could actually study the duration-induced phenomenon, maybe with a =
microscope of some capsules, to find out what is producing the physical cha=
nge and build this "change" into the mic at the outset.
An interesting experiment would be to substitute a capacitor with the same =
capacitance as the capsule and test to see if the discharge phenomena occur=
 under those circumstances.  Assuming that the problem occurs reliably (is =
that a contradiction in terms?), it could be determined whether the problem=
 is occuring within the capsule, or afterwards in the circuit.

Let me think out loud here for a moment.  These kinds of problems (popping,=
 fizzing, moisture induced failure) occur because moisture prompts a discha=
rge of current in the input circuit of the microphone.  This could occur in=
 the gap of the capsule, in the circuit from the capsule to the input to th=
e FET, across the bias resistors, across the input coupling capacitors (if =
any).  My gut feeling is that most of these problems occur in the path from=
 the capsule to the gate of the FET.  But I'm really talking from my nether=
 orifice.  I don't know without doing some experiments.

I recall Carl Countryman, manufacturer of Countryman lavalier microphones (= ), demonstrating his products at AES conventions=
 by immersing his microphones in a glass of Cola and then withdrawing them =
to show that they worked instantly after being withdrawn from the beverage.=
  This is one of the reasons that I've occasionally recommended using laval=
iers for nature rcording in the past, the others being durability and relat=
ively low cost.  But the quietest of the Countryman designs have a self noi=
se of about 24 dBA.  That's not really in the same category as a microphone=
 like the Rode NT1-A or Audio Technica AT-3032.  But it does lead one to th=
ink about a design that uses lavalier construction principles around a very=
 quiet capsule and circuit.  The basic idea here is to keep _all_ the moist=
ure out of the sensitive parts of the microphone.

Any one want to work on that?


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