Preventative and restorative maintenance of a condenser mic capsule is such an important topic and often so easy to do, that I ask you to read the following all the way through. It may save you the money of a capsule replacement!
Wouldn't a wind screen do the job?
It would do an excellent job keeping microphone specialists employed cleaning capsules!
Please use wind socks and screens only when nothing else will do to prevent popping or wind noise from over-exciting the capsule’s diaphragm. Aside of their propensity to kill high frequencies, the open-cell foam of which most wind socks are made, start to deteriorate soon after manufacture; once disintegration accelerates, fine foam particles will shred off and penetrate the mic’s head basket, and deposit all over the electrostatically charged, and therefore attractive, capsule area. In then end, the foam turns to a semi-liquefied goo, which is very hard to remove from the capsule’s fragile diaphragms.
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 wind screen away. It will be harmful to the capsule, and more costly to use than to replace.
The Best Capsule Protection: A Plastic Bag
The best, cheapest (free) and all around easiest way to preventing a condenser capsule from deteriorating is getting a plastic produce bag from your supermarket. You may have to look a little harder to find a bag that is made of thicker, longer-lasting plastic material. A thousand 2-3 micron-thick, clear 3” x 12” plastic bags from a local vendor cost $20, and will cover capsule protection of all of your mics for the rest of your life.
How to use the bag:
Just put the bag over the mic on the stand, if you cannot disturb the mic’s recording set-up. After the session is finished, permanently store the bagged mic in its proper case. No need to seal the bag tight. You can let the mic breathe.
If humidity in the recording 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 present on the capsule toward the (drier) amp region around the tube, so that the bag will act as a capsule dehumidifier.
The main purpose and usefulness of the bag over the capsule is that of a physical barrier, preventing the electrostatically attractive capsule from sucking in airborne particles. These contaminants act similarly to salt crystals which aid fog formation: dirt particles accelerate formation of water droplets. The moisture added to the contaminants in turn will form a conductive path between diaphragm and backplate, thus shorten the highly insulated plates of the capsule, and cause capacitive discharge noise: rumbling, whistling and hissing, and ultimately no sounds at all.
The proper way to dehumidify and restore a condenser capsule
Silica-crystal bags are an excellent way of drying a humidified capsule, but only if you severely restrict the volume of humid air getting to the moisture-hungry silica.
Placing a silica bag next to the mic grill without otherwise restricting its access to air may be well intended, but futile: silica is hygroscopic (moisture absorbent), therefore, exposure to am unlimited amount of air will neutralize the silica within minutes, rendering it ineffective.
First, pre-dry the silica as directed, which is usually printed on the bag. I place it in a conventional or toaster oven, set to ca. 350-400° Fahrenheit (175-200° Celsius) for 3-6 hours. Silica crystals are usually dyed and turn dark blue when the material is completely dried out.
As soon as the silica is fully dried, place two bags (buy several, they are cheap!) against the grill of the mic, and immediately enclose the mic in a plastic bag which you tightly seal. As the sealed plastic bag is limiting the amount of air from which the silica can draw moisture, it forces the crystals to suck any excess moist air from the mic’s capsule. Save the unused silica bags in a sealed ziplock bag, and place it in a canning jar until needed.
I want to confirm that there is no danger in using plastic bags over my condenser mics...
Wouldn't plastic trap moisture or allow for condensation to occur? ...Is there any advantage to using plastic versus a cloth bag with a tight weave?
Several issues need to be considered when choosing cloth or plastic, sealed or open, to protect your mic.
Cloth needs to be absolutely lint-free to avoid capsule contamination. Cloth will not prevent excess ambient moisture to enter the capsule area.
A tightly closed plastic bag will trap moisture present inside the capsule head at the time of wrapping, and may even cause condensation as temperature changes. So choose your level of protection according to the environment present:
If the recording room is low or average in ambient humidity (up to 60%-70%) a lint-free cloth bag is fine. When using plastic, tightly wrap the mic before storage or shipping.
If the mic has been exposed to excess moisture while in use, and if dehumidifying is not possible, I would temporarily use a lint-free cloth bag for storing, or a plastic bag that is open enough to let excess moisture inside the mic head equalize with the lower ambient humidity in the storage area.
A Stern Word About Capsule Cleaning
All capsules exposed to dust and other airborne contaminants for an extended period will eventually need to be cleaned. However, the impact of contamination on capsule performance will vary with type and degree of contamination.
In professional hands, in most cases a contaminated condenser capsule can be returned to factory specs, and often with a substantial savings over purchasing a replacement capsule.
To restore the capsule to factory performance, regain its high impedance, and lighten the diaphragm of grime and other weighty foreign materials, all dirt and moisture must be removed.
Capsule cleaning is an art, based on science, and I continue to caution mic owners not to try to use mechanical means or water in attempting to clean the diaphragm, backplate, or other high-impedance capsule components.
Using a brush, regardless how fine or soft to try to remove contaminants from the diaphragm is a bad idea. Any mechanical contact with the membrane’s gold sputtering will press and scrape dirt particles right into the few angstroms thin conductive layer. The resulting partial loss of gold will result in increased capsule noise.
Using distilled water as cleaning agent is also fraught with peril: contaminants dislodged and floated by the water can penetrate into the minute gaps between diaphragm and backplate, where they permanently settle.
There are methods to restore a capsule to factory specifications that minimize harm to its fragile components. I have developed such method, and others may have as well. If you suspect capsule contamination, it is vitally important that you interview service providers about the method they use to remove the contaminants. In case of doubt and lacking access to a qualified capsule restorer, I recommend to buy a new or clean used capsule, rather than let the contaminated capsule be ruined by an amateurish cleaning attempt.
Finally, a word about capsule replacement and its sonic consequences. A replacement capsule, even if identical in design to the original it replaces, usually will sound different, so it is always a good idea to try capsule restoration first and replacement last, especially when the sound of the original capsule was particularly pleasing.
My aversion to "reskinning" (i.e. re-diaphragming) as solution for a contaminated capsule will be subject of another column.
© Klaus Heyne 2018