R/E/P > Klaus Heyne's Mic Lab Stickies

How To Keep A Condenser Capsule Clean and Dry

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Klaus Heyne:
How to keep a condenser capsule clean and dry

Preventative maintenance of condenser mics is such an important topic and often so easy to do, that I ask you to read the following, and add your own tips to it. It may save you money over capsule restoration or replacement!

 Wouldn't a wind-screen do the job?

It would do an excellent job to keep microphone specialists employed with capsule cleaning jobs!

Please use wind socks/screens only when nothing else will do to prevent popping or wind noise from exciting the capsule. Wind socks are made of open-cell plastic foam which starts deteriorating within 1-2 years after manufacture; thereafter its disintegration accelerates.  Fine foam dust and flakes will quickly penetrate through the screen and be deposited all over the (electrostatically charged and therefore attractive) capsule area. The final stage of this material is that of a semi-liquefied goo, and then the fun begins...

Try this test with your windscreen: rub your fingers against it over a white piece of paper. If foam dust is deposited on the paper, throw the screen away. It will be harmful to the capsule, and be more costly to use than to replace.

The best, cheapest (free) and all around easiest way to preventing a condenser capsule from deteriorating is the good old plastic bag from your supermarket's produce or cereal/candy department (you may have to look a little harder for a bag that is made of a bit thicker plastic material, but it will last longer.)

Just put the bag over the mic on the stand, if you cannot tear down the recording set up for positioning reasons. Then, after the session is finished, permanently store the mic in this bag inside its proper case. No need to seal the bag tight.  You can let the mic breathe.

If humidity in the room was excessive, you would already have had moisture back- down on the capsule, so no need to worry if the bag fits too tightly. If anything, a bag over a warm tube mic will force redistribution of the higher humidity sitting on the capsule towards the drier amp region of the mic, so the bag will somewhat act as a capsule dehumidifier.

The main idea of the bag over the capsule is that of a physical barrier- to prevent the electrostatically attractive capsule from sucking in particles from as far as six feet away, and letting these contaminants act like salt crystals which catalyze fog formation: If there is any amount of a dirt layer, it becomes that much easier for moisture to form droplets and start a conductive path to short out the two plates of the condenser (diaphragm and backplate.)

Silica-Gel bags are an excellent way of drying an overly humidified capsule, but only if you tightly restrict the volume of humid air getting to the moisture-hungry silica crystals.

Placing a silica bag next to the mic grill without otherwise restricting its access to air may be well intended, but is useless: Silica is so hygroscopic that any access to unlimited amounts of air will neutralize the silica within minutes.  

So here, then, is the proper way to dehumidify a condenser capsule:

First, dry the Silica bag as directed, usually printed on its side. I place it in a conventional oven or toaster oven, set to ca. 350-400

John Etnier:
Until reading this sticky on capsule cleanliness I had regularly used the original-issue foam windscreens on a pair of Neumann KM-84's.

In some circumstances the guitarist will inadvertently breathe into the mics (rear ports?), causing a big LF noise. The foam filters eliminate this problem, whereas a hoop-style pop-screen doesn't really work because it gets in the way of an already crowded situation, particularly when the guitarist is singing and playing.

Any problems with that?

Klaus Heyne:
I understand that some circumstances require the dreaded windscreen/sock.

Use your foam screens until they don't pass the rub test anymore, then buy new ones, and test regularly.

bgavin (Bruce Gavin):
Klaus Heyne wrote on Fri, 07 May 2004 15:01
...(electrostatically charged and therefore attractive) capsule area.

Does this imply electret capsules attract dust at all times, compared to externally polarized types only when they are active?

Considering your comments on the deterioration of foam material in windscreens, do you recommend against using an original wood box for storage if the interior is made of "hard molded foam"?

Stephen Goebel


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