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

How To Test A Capsule For Contamination Defects


Originally Posted: Tue, 07 February 2006

Andres Gonzalez wrote on Mon, 06 February 2006

...He said the intention was to actually get some breath moisture on the capsule and listen for any crackling sounds. If he heard crackling sounds, then he would figure the capsule has actually been damaged by moisture. But if there were no such crackling sounds, then the capsule was good.
Is there any merit to this kind of test?
The explanation is straightforward, but whether to follow his recommendation is a bit more controversial.
The purpose of the breath test (no spit involved, so only minimal amount of debris from the mouth would get to the capsule) is to trigger a capsule which is already contaminated by dirt, dust or injury to the diaphragms to collapse electrically:
The capsule's two sides (diaphragm and backplate) form the two plates of a capacitor which can only hold its charge (i.e. the capsule can only perform noise free and to spec) if there is enough isolation resistance between the plates, so they won't conduct electricity across and lose their function as capacitor.

As this resistance needs to be in the gig Ohms (several thousand milllion Ohms) it follows that any sizeable deposit of contamination (dirt) across the two plates will lower that resistance and short the capacitor.
Therefore, to test how close a capsule is to that point of electrical short (collapse) one can add a little humidity:

If the dirt build up is high enough, breath moisture will enable conductivity across the dirt particles. (It likewise may bridge between diaphragm and back plate if there is a hole in the diaphragm.) This will show up as severe and prolonged thundering or crackling discharge sounds, often accompanied by fading audio, either across the whole frequency band or severely reduced low end response.

If there is not enough dirt on the capsule, and if the capsule is otherwise healthy as well, there may be a bit of a crackle and pop, but within a second or two, the capsule will perform again as it did before the test.
Now is this test advisable?
I'd only recommend it if there would be serious consequences if you failed to do it:
Pre-purchase checkout of an expensive mic qualifies in my opinion, so does a last minute check before an important recording session.
However, if there is no compelling reason to pump a bunch of warm, moist air into the capsule, leave it alone.


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