bbystrek wrote:
>
> Hello Everyone,
>
> What are the physical principles behind why are condenser microphones
> are adversely affected by high moisture? Is it a matter of humidity,
> condensation, moisture absorption, or some other factor? Why does it
> create crackling noises? Are there any design questions or specified
> ratings that can be asked of a manufacturer to characterize this?
> Perhaps in terms of maximum guaranteed operating (not storage)
> humidity at a particular operating temperature, or temperature
> range. Is moisture the basis of issues pertaining to operating at
> cold temperatures?
Several questions.
A condenser microphone works by forming a electrical condenser from the
diaphragm and some other conductive surface. A condenser consists of two
conductive plates with a insulator between. In this case the insulator
is nearly always air. It picks up sound because the spec of a condenser
changes with distance between the two surfaces and the diaphragm is thin
enough that sound moves it. In order to work there has to be a
electrical charge difference between the two. The charge can be a
relatively high voltage. Moisture increases the conductivity of the air
and along surfaces and can even result in minute arcs. These are one of
the sources of crackling. In some cases the extra conductivity may
simply keep the mic from maintaining a charge.
In addition the material of the diaphragm may change character as it
picks up moisture. This may cause it to sag slightly toward the other
plate or other effects. Of course if it touches no charge can be
maintained and the mic will go dead. Absorbing moisture is generally
reversible. Note that I'm talking humidity here, another issue would be
actual droplets of moisture on the diaphragm, which as well as other
things change the weight and thus the sensitivity of the diaphragm.
The sensitivity of a diaphragm to moisture depends on all kinds of
things, like the material it's made of, thickness, how tightly it's
stretched.... Within a given manufacturer's series of mics the material
specs are usually very similar if not the same.
If they give a operating humidity, it can be a guide to this sort of
thing, or a partial guide. But it's really something that's only found
by experience. There's no specific spec other than that.
Moisture may be part of cold temperature problems, but there are
separate issues there. Materials change character at different
temperatures, for instance a diaphragm may become stiffer. They also
change actual size.
> Apparently the susceptibility to moisture varies between mic
> manufacturers and the group's experience vouches that the Sennheiser
> K6 and MKH are field proven to perform better than most. Better than
> what, I'm not exactly certain. Telinga products appear from the
> posts to be slightly more sensitive to moisture in contrast to
> Sennheisers.
The K6 is a power supply, it's the ME series it powers. The MKH series
adds in radio frequency at the diaphragm and reads the modulation of
that frequency. That's a quieter method, and part of the reason for
their good self noise specs.
Yes, Sennheiser's are favored because actual field experience by many
recordists is that they stand up to the conditions encountered better
than most other mics. And they are darned good mics on top of that. They
may or may not be the only mics this good at standing the environment,
but carting a unknown mic out there is a risk and good mics are
expensive, so most just stick with what everybody else is using. We
definitely need to organize a database of all these special things about
mics for nature recording. Ordinary mic specs and reviews fall well
short of our needs.
I'm not sure Telinga is any more sensitive to moisture. I believe Klas
reported testing their mics by leaving them out for months/years at a
time. I avoid carting mine out into the rain, but it's seen a lot of
moisture without any problems.
> Did I miss anything or are any of my thoughts wrong?
Mics do have temperature effects as well as the moisture ones. Ideally
you want your mics at environmental temperature before starting recording.
The type of noise you get from moisture can vary for different mics, not
always crackling. Some just abruptly quit with little warning.
In fact the type of self noise a mic makes varies a lot from one
manufacturer to another. We often talk about hiss, but some of them
sputter and crackle instead, or other sounds.
Walt
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>From Tue Mar 8 18:22:40 2005
Message: 9
Date: Sun, 25 Aug 2002 18:22:06 -0000
From: "bbystrek" <>
Subject: Noise Sample of Noise Reduction Algorithms
Hello,
Reading some of the back posts regarding using an omnidirectional
microphone with a parabolic setup in an attempt to cancel ambient
noise got me wondering...
The omni was either hard wired in an antiphase fashion or recorded as
a second channel and later applied during post recording editing.
>From what I gather, everyone seems to have abandoned the experiments
due to an increased perceived noise floor speculated to be due to
spatial "time of arrival" (phase) issues.
Could anything be gained from collecting a noise sample to later be
used to feed the noise reduction algorithms incorporated into many of
our digital audio workstations? Perhaps either recorded using an
omni, or simply pointing the dish at say the barking dog in the
distance for a few seconds. If nothing else, I bet it would help
guide any digital manipulations later; or at least provide critical
information as to what spectral components are present in each noise
source. Though I suspect that a lot of the unwanted noises we record
are reverberant in nature, modified by both the environment and the
parabola characteristics. Directly pointing a parabola at the noise
source would not necessarily represent the spectral content of the
reverberations as they appear in the recording of our subjects.
Anyway, has anyone tried anything along these lines? Perhaps some of
the experts amongst the software manufacturers would be willing to
discuss how to optimize their algorithms.
Brian Bystrek
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