The following text is from Lou Burrough's excellent book,
'Microphones: Design and Application'. Burroughs was the founder of
ElectroVoice (EV) and certainly knew a thing or two about microphone
design, care and maintenance. He was also the founder of the 3:1 rule
for when multiple microphones are picking up the same sound source.
Although the book is a firm favourite with many pro engineers, it has
been out of print for many years. So, I am pasting this here because
the information is otherwise unavailable. From a copyright point of
view, it would fall under the category of 'fair use', considering the
small amount of text (relative to the size of the book) and its
educational value.
Here it is, I hope you find it helpful:
"Condenser microphones employ two types of diaphragms. The
omnidirectional unit is usually equipped with a stretched metal
diaphragm, and the cardioid with a stretched metallic-coated plastic
diaphragm. Both are vulnerable to degraded response, due to a
collection of dust on the diaphragm. A great deal of this accumulation
is due to electro-static attraction, accumulating non-ferric dust in
much the same way as the magnet in the dynamic microphone attracts the
ferric type. Many quality condenser diaphragms are protected by a very
fine mesh screen, but some are not. However, regardless of how small
the mesh, full range sound must be allowed to enter; therefore, the
openings are sufficiently large to allow fine dust to pass through and
sooner or later frequency response will be altered. Few units are
designed to allow for cleaning. Should you be able to get to the
diaphragm, it will probably be found that the dust is pretty well
cemented to the diaphragm due to the oily film usually found on
surfaces everywhere, or from the cementing effect of breath moisture
and saliva particles.
High temperature and humidity may have a detrimental effect on
response, but this is usually temporary. When condensers are allowed
to reach below-freezing temperatures and are then brought into a warm,
normally humid, room they will sweat internally as well as externally,
resulting in a noisy or dead unit. Both the head and electronics are
usually affected and often require dehydrating time. Placing the unit
in a desiccator will hasten its return to normal.
Store condensers in a dry place and you will be rewarded with quieter
operating units. Those employing batteries should have their batteries
removed and reinserted several times before each use to make certain
noise will not occur from corroded contacts.
The lives [of your microphones] may be prolonged for years if you will
simply keep them covered when not in actual use. Each microphone
should have a container that can be tightly closed for storage. When a
microphone is mounted but temporarily not in use, it should be covered
with a cloth bag, such as a canvas money bag, with a drawstring or
zipper.
To emphasize the above, here is a case history that I have found to be
typical. Some time ago, the chief engineer of a recording studio and I
had a discussion about microphone maintenance. The subject was
triggered by our walking into a studio where several microphones were
lying on a dusty table in preparation for being mounted on stands.
Investigating, I found that all of the microphones on the dusty table
were about two years old. They were in daily use and considered to be
in normal condition. After examining each microphone, I offered to run
curves on all of them and service those that needed it in return for
the opportunity to find out what had happened to their frequency
responses.
The following week, the studio was closed for vacation and
twenty-eight microphones were sent to me. First, curves were run; then
all diaphragms and grilles cleaned.
Eight condenser microphones were received and the response of each was
degraded. After cleaning, the two containing metal diaphragms returned
to normal response. The six metallized plastic diaphragm units
improved considerably after cleaning, but the responses of no two were
alike and none were equal to a new microphone of the same make and
model.
Eleven dynamic microphones were examined and all were degraded. After
the dust filters protecting the diaphragm were cleaned, eight of them
returned to normal response. Three of another make had permanently
warped diaphragms due to ferric dust accumulation on the diaphragm
above the voice coil gap. Of the nine ribbon microphones, all were
found loaded with ferric dust and the ribbons stretched beyond repair.
Here was a professional recording studio depending on degraded
microphones to reproduce quality sound! If equalization had not been
available, several of the microphones could not have been used. In
comparing curves of the new units of each model with those run of the
ones sent to us, only a few were recognizable as the same microphones
they were before cleaning."
That's the end of the quoted excerpt from Burrough's book, but I'll
add the following. It's mostly related to studio work (it's something
I teach my audio students), but is equally applicable to nature
recording:
Never swing a microphone through the air when you're carrying it (i.e.
holding it in your hand and swinging your arm as you walk). Always
think about the force of air that the diaphragm is being exposed to,
and the dust and other particles it may pick up as it moves through
the air. Carry your microphones to the microphone stand in their boxes
or bags, and don't open them until you need them.
Don't leave your microphones on stands overnight, or if you have to,
be sure to put covers over them with either drawstrings or zippers to
close them. The soft fabric covers that are supplied with Oakley
sunglasses work well with most microphones. Don't keep them in plastic
bags if there is high humidity or temperature changes, in case
moisture condensates inside the bag.
I hopet that is helpful...
- Greg Simmons
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