Recording - June 2010 - 20

Which leads us to two more myths (which contradict one another):

Myth #3: The only way to get good sound is to use a smaller-diaphragm mic with nice flat response. Myth #4: Mics with flat response are no good for recording music. Status of both myths: Emphatically not true.

Dynamic magic Surprisingly, E. C. Wente seems to have invented the dynamic mic as well; at least, his is the first patent I could find. It’s basically a loudspeaker run in reverse—in fact you can use a speaker as a microphone. Remember back in science class, when the teacher connected a loop of wire to a meter, and moved the loop through the field of a magnet? That made the meter’s needle jump; moving a conductor through a magnetic field makes current flow. That’s what happens in a dynamic mic. The magnetic field is focused on a coil with many turns (a lot of wire). The coil is attached to a circular or dome-shaped diaphragm, with a flexible edge connected to the capsule’s body. When sound waves hit the diaphragm, it moves back and forth, moving the coil through the magnetic field, generating a signal which follows the coil’s velocity. As you can see from the picture, the mechanism is a lot more complex than a condenser mic’s capsule, with more moving parts—diaphragm, coil, the diaphragm’s surround, and of course the coil’s connecting wires. You might guess that the more complex mechanism might have more chances for resonances to mess up the mic’s response, and you’d be right. Even more than with a condenser mic, the designer’s art consists of juggling one resonance against another until the result sounds

We’re in the business of smoke and mirrors—creating illusions to bring music to the listeners. No microphone hears the way ears do. But we can sometimes use miking technique to fool our ears into thinking they’re hearing something like a real instrument or voice. Or, if not, to give the ears something they like hearing, whether or not it’s “accurate”. Sometimes the way to do that is with a super-flat smalldiaphragm microphone; good ones have been available since the 1960s, when the signal-tonoise problem was solved with quieter tubes and FETs. But other days, that “ideal” micropone won’t give you the story at all. That’s partly cultural conditioning; we’ve all listened to recordings made using large-diaphragm condenser mics for most of our lives, and we’ve learned that “that’s what a recording is supposed to sound like”. But that’s not the whole story. I can go listen to somebody singing in the studio, mike them with an “accurate” smallFigure 2: The mechanism of a dynamic mic, from the original 1930 patent papers. diaphragm condenser, go back into the control room, and it’ll sound nothing like they sounded good. Because the moving mechanism is filled with elastic in the studio. The same singer, miked with a large-diaphragm devices that don’t always move in a simple fashion, dynamic condenser mic with a bit of lift at 12 kHz (that drumhead reso- mics are also more prone to nonlinearities, a fancy word for nance, not completely damped), off-axis response like a roller- things that produce distortion. coaster, and bass lift from proximity effect—back in the control So with all these drawbacks, why mess with dynamic mics? room, it sounds a lot more like what my ears heard in the stu- Because back in the 1930s, condenser mics were big, bulky, and dio. Smoke and mirrors! fragile, and needed big, clunky power supplies. Dynamic mics In other words, there’s a place for all sizes of mics: big, small, were smaller, and a lot tougher, and didn’t need a power supply in-between, flat, bumpy, whatever. There are magical micro- at all. That’s still true (except for “smaller”—since then, condenser phones with big diaphragms and small. (Of course, there’s junk mics have shrunk a lot). But there are other attractions to dynamic in all sizes too.) mics... namely, some of their sonic characteristics.
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RECORDING JUNE 2010

Recording - June 2010

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