Recording - June 2010 - 19

bump and hollow in the capsule housing affects the sound—and the art of condenser mic design becomes a process of juggling resonances until you get something that sounds good. It isn’t easy—and doing it repeatably, so that every mic coming off the assembly line sounds the same, really isn’t easy. Even with computer-controlled milling machines, building a capsule remains hard; it’s rumored that even Neumann, which continues to build capsules by hand because it’s more accurate, throws away fully half the capsules they make. But if you’re building a mic to sell for $100, you can’t afford to reject too many capsules. So the cheaper mics vary, and getting a good one becomes something of a crapshoot. But it can be done! Some companies, including KEL Audio and Groove Tubes, have managed to produce good mics at astonishingly low prices. Even the sometimes-maligned little Oktavas, if you buy them from www.oktava.com, can be finesounding mics in the right applications. (Their dubious reputation came from importers who sold stores mics with no quality control, or occasional Chinese knock-offs rather than real Oktavas.) So what’s a user to do? The answer, as is often the case in audio, is to use your ears. You can’t really judge a mic at the store; you need to take the mic home, listen to it in your room with your gear, and bring it back if it doesn’t sound right. This means buying from a store with a robust return policy; some places, especially musical instrument stores, refuse to accept returns on mics, sometimes claiming that Health Department restrictions prohibit the practice. This is horsefeathers; a store that won’t take a mic back if you don’t like it doesn’t deserve your business. Diaphragms large and small The first condenser mics all had large capsules (up to 3"). Smaller capsules produce less output (less charge flowing in and out), and with the relatively noisy tubes of the day, lower-output capsules had unacceptable signal-to-noise ratios. Small-diaphragm mics were only used for measurements, where a little noise didn’t matter so much. Why were they used for measurements? Well, a good measurement mic needs a very even frequency response, and the response needs to be the same from the sides and back. Large-diaphragm microphones get squirrelly off-axis. If a mic with a 1.5" capsule has a 1.1" diaphragm (the rest is the rim), that distance is roughly the wavelength of a 12 kHz sound wave. When such a wave passes the diaphragm sideways, one part of the diaphragm is under the compression part
RECORDING JUNE 2010

of the wave, while the other part is under the rarefaction. The diaphragm goes in on one half, out on the other; the net effect is very little, and the mic produces little output. At other frequencies the compressions or rarefactions might reinforce one another, producing peaks. The net result is a frequency response that’s very uneven. In general, the smaller the microphone, the less weird it gets off-axis; if it’s 1/2" or less, it’s smaller than the wavelengths of the highest frequencies we can hear. That’s why measurement mics are small. Sound from the back gets even messier; at higher frequencies, in an end-address microphone, the mic’s body actually casts an acoustical shadow on the capsule at

some frequencies, again making the frequency response squirrelly. Side-address mics get around this—the capsule’s main axis is at right angles to the body, and behind the capsule, instead of a solid body, there’s nothing but a (hopefully acoustically innocuous) screen, the same as in front. But why should off-axis response matter? Don’t we usually put our singers and instruments in front of the microphone? Sure, but even if there’s only one instrument or singer playing at a given time, there’s still the room. Even a close-miked recording picks up a surprising amount of room sound, which gives the voice/instrument a lot of its characteristic tone. If the mic’s off-axis response is weird, the room sound will be weird.

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Recording - June 2010

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