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Physics of moisture-induced mic noise (was MS-stereo and figure-8 mi

Subject: Physics of moisture-induced mic noise (was MS-stereo and figure-8 mi
From: "Rob Danielson" danielson_audio
Date: Sun Jan 3, 2010 11:00 am ((PST))
Hi umashankar and Scott--

Some questions sparked by your comments:

(1) Audio Technica describes the AT-3032's element as, "fixed-charge back p=
permanently polarized condenser." I asked this
before and I don't recall an answer, is this an
electret mic capsule, not a "true" condenser? I
think the Sennhesier ME series mics might fall
into this category too. If so, its
well-documented that recordists can get moisture
related issues with all of these mics--
especially in the first several hours of
operation under provoking humid conditions. (I
feel that extreme hot and humid as well as
high-humidity and temps just below freezing are
the two most vulnerable conditions. RF mics in
particular often act-up just under freezing). I
think we may be concentrating on capsule design
"explanations" too much.

(2) The worst kinds of moisture-related noise are
"fizz" and continuous "Sputtering." A much less
objectionable type is a "pop."  These can occur a
few times per second to one or two every few
hours or even weeks.  I've observed 6 or more
condenser mics make transitions from fizzing to
sputtering to regular popping to very rare
popping over the course of 2-3 months of
continuous, outdoor use. Could it be that small
dirt particles are collecting or some other
physical change is happening with these exposed
capsules that is affecting the nature of the gap,
the nature of the electric discharge and thus
audible qualities? The audio performance of the
mics does not seem to be compromised.=A0

(3) When I first reported getting extreme
moisture immunity with first one and then a pair
of Rode NT2000 mics in Summer/Fall 2008, Richard
Lee of the miclist, posed that the performance of
my particular mics was "exceptional" and that the
popping noise probably wasn't moisture-related.
I'm far from an electronics engineer, but I've
since observed the transition from  Fizzing to
irregular popping too many times to think there's
not some consistency. Its rare for recordists to
leave mics on and outdoors continuously, but it
occurs to me that one could actually study the
duration-induced phenomenon, maybe with a
microscope of some capsules, to find out what is
producing the physical change and build this
"change" into the mic at the outset. Maybe one
could "see" the discharges with the right tools?
I have no idea about the science that would be
involved but I thought it might be worth
mentioning again.

Its counter-intuitive, but it might make sense to
set one's new shiny, new mics outdoors to
condition them with the imperfections of air. :-D
Rob D.

  =3D =3D =3D =3D =3D =3D

At 4:04 PM -0800 1/2/10, umashankar wrote:
>...generally speaking, electrets recover more
>reliably than true condensors, partly because
>the charge=A0on th electret is evenly distributed.
>a lot of design=A0work goes into externally
>polarised=A0condensors to distribute the charge

Scott Fraser described.

>... while in moist air a sufficiently high voltage potential &
>a sufficiently short distance between elements can cause an arc across
>the gap. ...From what I can tell,  electret
>condenser mics are not susceptible to popping in
>humid air=A0since they don't rely upon an external
>polarization voltage to charge
>the capacitor.
>Scott Fraser


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