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Physics of moisture-induced mic noise

Subject: Physics of moisture-induced mic noise
From: "Klas Strandberg" klasstrandberg
Date: Wed Jan 6, 2010 9:46 am ((PST))

"True" condensor microphones:

A dc voltage, often 30 - 60 v , is applaid between a metalized
membrane and a plate, electrode, often out of pure brass.
(Corrosion??) The distance between the membrane plate and the plate
is in the 2-10 thousands of a millimeter region only!

Consider the distance between electrode and membrane, 2 - 10 mil? How
big is a pile of dust / bacterial / fungus and what will such "piles"
do to the noise performance of a mic when leak currents go over them?

A lot, I=B4m afraid. Even very small leak currents will be heard, due
to the very high input impedance and the high gain. Such leak
currents can also go at the outside surface of the capsule and
humidity and dirt makes everything worse, of course.

Sometimes one can find design errors. Both my NT1A had poor input
capacitors soldered at the PCB. This could easily be detected by
spraying cooling spray. After replacement, the mic=B4s got as low noise as =

I am not criticizing R=F6de. To record Soundscapes at minus 20C is not
one of their test situations, neither is it to spray cooling spray at
the PCB. They have no reason to detect such an error.  Besides, both
my mic=B4s were made during the transition period between soldering
with a lead/tin alloy and pure tin. It may very well be that the
capacitors couldn=B4t take the higher temperature needed.


A electret membrane is a metalized plastic. Then you "spark" a
certain voltage peak through it, with a special curve over a special
time. Then ions will then separate. After heating and cooling the
material in a special way you can make this ion separation permanent.
So instead of a voltage gap between the membrane and electrode, which
easily cause leak currents, you have a "charge" in the membrane,
which does not.

That is the basic explanation why electrets are less sensitive to
humidity and dirt than "true" condensor mic's: Leak currents.

Electret omnis are safer that cardioids, as they are closed. Dirt
cannot so easily come inside.
It is better if the capsule is followed by a FET-IC with a internal
bias resistor. When the bias resistor is on a PCB, followed by a FET,
the FET may get out of rail under humid conditions.

Noise caused by corroded and dirty plugs and jacks is often much
"scratchier" than leak currents.

So the "physics of moisture-induced noise" is roughly and basically a
matter of leak currents.
However, why the leak currents appear, or do not appear, is a more or
less random procedure, due to all kinds of pollution at different
places of the capsule, as of the age and use of the capsule in question.


Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01

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