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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: "Rob Danielson" danielson_audio
Date: Sun Jan 3, 2010 7:23 pm ((PST))
At 11:06 AM +0000 1/4/10, Richard Lee wrote the below to me (Rob D.)
He attempted to post it to the natrec list.

Rob D. asked initially:

>>AT-3203.."fixed-charge back plate permanently polarized condenser."
>>is this an
>>electret mic capsule, not a "true" condenser?

Richard replied:

>This is a back electret. Some of the best mikes in the world including the
>Shure SM81 & DPAs are back electrets.

Richard wrote (trimmed):

>...The most stringent condition most studio mikes face is being taken from=
>boot of a cold car into a warm & humid room. Moisture condensing on the me=
>backplate & other bits forms a leakage path.

Yes, moving from cold to hot and vice versa greatly increases the
chances but the examples of several types of mics with almost perfect
immunity to all types of moisture induced noise have stayed outdoors
2-3 months. A few have taken a couple of months longer to completely

Richard wrote (trimmed):

>...Cleanliness is a major factor. If you bring a scrupulously clean but co=
>backplate into a warm humid environment, a very fine mist of condensation =
>form & go away quickly as the capsule warms.  If there is the slightest
>contamination, the condensation will bead & form much bigger drops. These =
>carry contaminants into the gap & other places they shouldn't go

I and the other recordists used no special covering/protection from
airborne materials other than typical wind screens.  It has been dry
and dusty at times too. After being out for months, the capsules
appear quite a bit dirtier. I don't know what to say except that more
dirt doesn't seem to be the cause of more noise, because the noise
lessens as the dirt accumulates.

Rob wrote initially:
>>...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.

>I don't know what this is. Never (have) come across it.

We discussed it on the miclist  a little over a year ago.

>What mikes have done this for you?  Electrets?  RF mikes?  I still
>think its an electrical phenomena.

Rob :
NT2000's, NT1-A's AT3203's and MKH's.

Rob wrote initially:
>>Maybe one could "see" the discharges with the right tools?

>At the voltages involved, there really isn't a "discharge".

Rob wrote initially:
>>.. outdoors to condition them with the imperfections of air ..

>It may be the mike has simply achieved thermal equilibrium with its
>surroundings. It's sudden changes in temperature/humidity which leads to

Thanks for the input. Maybe there's a form of "equilibrium" that
takes months to fully achieve?" Rob D.


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