Thanks very much, Klas. You've improved my understanding considerably!
Excuse me for raising my feeble question again:
Over the course of the last 2.5 years of making
recordings with several mic rigs that STAY
outside all the time, I have observed NT1-A's,
NT2000's, MKH30/40's, MKH80's and AT3203's that
have exhibited fizzing, sputtering and popping in
high humidity, initially, LOSE this behavior as I
leave them powered-up and outside for weeks,
months and years. There is usually some audible
improvement after 5-12 hours but there is a high
percentage of continued improvement until the
mics produce only very brief noise events which
can disappear again for weeks or even months.
This is clearly different mic-moisture behavior
from what recordists get when operating gear for
much shorter durations. (I'm running the mics on
long cables to pres that are inside so the mics
are always powered-up.)
My question is, what could be happening to the
mics? Isn't it likely that something consistent
and "physical" would have to happen to the mics
to make them so immune to ill-effects? The
temperature here has ranged from -34F to 104 F
and the humidity gets very high.
Folks keep pointing-out that dirt, corrosion and
oxidation are possible _causes_ of problems,..
but these mics that are exposed to the elements
24-7 and they no longer have problems! :-) :-)
It seems its worth looking into. I might try to
study 6 or more mics that exhibit problems and
track the changes they go through with outdoor
exposure process. Can you think of visible
details within the mic capsules themselves that
one could study as well?
Richard Lee wrote that the gap in true condenser
mics is sealed. Does this mean that no dirt or
moisture can get into the gap? Rob D.
=3D =3D =3D =3D
At 6:46 PM +0100 1/6/10, Klas Strandberg wrote:
>"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