When enough dust, dried spit and other airborne contaminants have settled permanently on a 6µ or thinner diaphragm, there comes a point when the effects of that debris will affect the transient response, high frequencies, and ultimately the capsule's functionality.
The biggest problem caused by capsule contamination is break-down of capacitance.
A condenser microphone capsule consists of a diaphragm (or two, in case of multi-pattern mics) and a backplate (or two), against which the diaphragm is mounted at very close proximity (also measured in microns of millimeters), but with an incredibly high isolation resistance between the two plates: ten-thousand million (10gig) ohms or more. Close distance and high isolation between the two capacitor plates makes it imperative to keep both plates meticulously clean.
If the isolation resistance is reduced through dirt accumulation aided by the electrostatic attraction and settlement of airborne particles on the charged capsule, the capacitance formed by the two capacitor plates (condenser is another word for capacitance) is gradually reduced, and eventually shorted completely.
Capacitance collapse will show up as audio fading out, accompanied by lots of discharge sounds, from thunder to whistling noises. The process of deterioration is accelerated when moisture or humidity is present (for example through breath): moisture forms an electrically conductive path. And as soon as conductivity between diaphragm and backplate is created, the capacitance between the plates discharges, and renders the capsule inoperative.
Some capsule designs are more resistant than others to that phenomenon, but ultimately, all conventional condenser mic capsules are prone to be affected by dirt accumulation.
To restore the capsule to its original uncontaminated state, all dirt and moisture must be removed, to regain the super-high impedance between the relevant capsule parts.
I have always been reluctant to self-promote on my forum, the Mic Lab, and continue to resist it. But I am passionate about correct capsule cleaning, and its many amateurish, destructive attempts continue to irritate me.
Two examples: the frequent mentioning of distilled water as cleaning agent is fraught with peril. Contaminants dislodged and floated around through water can easily penetrate the minute gap between the plates and permanently settle there. Another peril: mechanical contact with the angstrom-thin layer of gold, by brushing a liquid over the diaphragm, often scrapes dirt particles into the gold which removes some of its conductivity and capacitance.
Capsule cleaning is an art, based on science, and I continue to warn mic owners to not try to use primitive means in attempting to clean a diaphragm, backplate, and its associated high-impedance components. There are methods to restore most capsules back to factory specifications that involve little or no mechanical contact with the diaphragm surface. I have developed such methods, and others may have as well.
It is vitally important that any microphone owner who suspects capsule contamination interviews service providers about the methods used to remove the contaminants. In case of doubt, I recommend to buy a new or used (but clean) capsule, rather than expose and lose the defective capsule to an amateurish cleaning attempt.
I have not found many capsules that could not be restored to factory specification through proper cleaning. My aversion to the alternative of "reskinning" (re-diaphragming) capsules, even those that are heavily contaminated and seemingly beyond hope, will be subject of another sticky.
© Klaus Heyne 2018