At 3:58 PM -0800 1/3/10, Eric Benjamin wrote:
>
>>Rob Danielson wrote some really good questions:
>>
>>...I think we may be concentrating on capsule-design "explanations" too m=
uch.
>
>I'm quite sure that you are right. Microphone manufacturers try to
>make their designs resistant to humidity, but the results seem to be
>all to variable 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 doesn't make problems worse under some
>circumstances. The impedance at this point is a gigOhm or higher in
>most designs (excluding RF microphones) and the conformal coating
>may act as a sort of matrix to accumulate various sorts of crud.
Seems like the strong trend for all types of noise to greatly
diminish or disappear with continuous exposure over time would
suggest that the "change" is happening on a small but significant
scale. Seems like the ill-effects of conformal coating would be hard
to reverse so stably, right? We also begin to see positive change
after 12 hours or more pretty consistently.
>
>... it should be nearly impossible for dust to get in the gap
>(unless it was there when the mic was assembled) with an omni
>microphone. A little easier for dust to get in the rear of a
>cardioid microphone, but most mfgrs. take good care to make that
>difficult.
That's good to know the gap is sealed. Is it sealed air tight? What
are some changes the components might go through with temperature
swings, relative humidity swings and exposure to air borne pollutants?
>
>(3) 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.
>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.
It would be much easier if we could isolate the failing component(s).
To start testing, one would need 6 or more mics that have proven to
fail almost every time. Replacing the mic capsules with capacitors
in a couple of them might be a good idea.
>
>Let me think out loud here for a moment. These kinds of problems
>(popping, fizzing, moisture induced failure) occur because moisture
>prompts a discharge 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 the 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.
What kind of problems do you imagine happening, "in the path from the
capsule to the gate of the FET" and how could they change over time
with exposure outside? Note that the mics I'm referring to are not
being handled, carried around or taken inside, etc. The pres and
recorder are not outside and the cables aren't being changed so
they're not too many variables aside from the mics proper.
>I recall Carl Countryman, manufacturer of Countryman lavalier
>microphones (<http://www.countryman.com/>http://www.countryman.com/
>), 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
>lavaliers for nature rcording in the past, the others being
>durability and relatively low cost. But the quietest of the
>Countryman designs have a self noise 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 think about a design
>that uses lavalier construction principles around a very quiet
>capsule and circuit. The basic idea here is to keep _all_ the
>moisture out of the sensitive parts of the microphone.
Wouldn't studying the acclimation of the larger diaphragm mics be
sounder "method" than trying to apply what works with a small
electret capsule to much larger, non-electret capsules? Rob D.
>Any one want to work on that?
>
>Eric
>
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