Recording - June 2010 - 16

By Paul J. Stamler

So many microphones, so much to understand! Ribbons. Dynamics. Condensers. End address/side address. Tubes. Solid-state. Transformers/no Transformers. I can’t cover everything you need to know; instead, I’m going to spend most of this caffeinated ramble under the hood, talking about what actually happens inside mics, how they work (and don’t)—my colleague Joe Albano will handle the usage side of things in his “Studio Miking Primer” on page 26. Along the way, I’ll visit and bust a few audio myths, and point out a couple that survive because they’re, well, true—or at least partly so. Warning: If you’re a real engineer, with a B.S.E.E., you’d better quit reading now. I’m going to oversimplify a whole lot, and you’ll just get mad reading this.

where the pressure goes up, and rarefactions, where it goes down. When the air pressure changes, the diaphragm moves in and out in response. When the diaphragm moves closer to the backplate, the capacitance increases, and the capacitor will hold more charge. So the electrons rush in, which is another way of saying a current flows. Since they do it through a (big) resistor, that means a voltage appears across that resistor. (Ohm’s Law—a discussion for another day.) Now the diaphragm moves farther from the backplate; since that decreases the capacitance, charge flows back out, creating a voltage with the opposite polarity. Keep the diaphragm wiggling back and forth in response to sound waves, and you’ll generate a corresponding electrical signal at the output of the capsule.

Condenser—old and simple Mics are transducers—they convert changes in air pressure (sound) into changes in electrical voltage (signal), and they do this via different mechanisms. I’ll start with the oldest type of microphone in common studio use, the condenser microphone, and use it as an example to describe generalities that apply to all mics. The first practical condenser mic was invented in 1916 by Edward C. Figure 1: The mechanism of a condenser mic, from the original 1916 patent papers. Wente of Western Electric. The condenser has only one moving part, the diaphragm. This is a piece of thin plastic film coated on one side Now if you just hooked the capsule to a cable, and plugged with metal (sometimes it’s just a very thin sheet of metal), stretched that into your preamp, you’d get almost no signal—the cable tightly across a ring and clamped with another ring, with a metal also has capacitance (between its wires), which will bleed the plate (the backplate) a few millimeters behind the diaphragm. signal away. This happens because the capsule has a very high When you have two pieces of metal close to each other but impedance—again, a discussion for another time. But you can not touching, they create a device called a capacitor. (In fact, put a head amplifier into the body of the microphone, close to “condenser” is an older term for capacitor; in Europe these mics the capsule so there’s little chance for the signal to leak away. If are often called capacitor mics.) A capacitor can be charged up that amplifier has a nice low output impedance, it can send the by pumping electrons into it, from a battery or other source of signal through long cables without any significant losses. DC—like a phantom power supply (see below). How much The first condenser mics’ head amplifiers used vacuum tubes. charge a capacitor will hold is described by its capacitance, Tubes, unfortunately, are still pretty high-impedance (not as high and that changes depending on (among other things) how close as a condenser capsule, but high), so they’re not that happy drithe two metal pieces are to each other. ving long cables either. The solution was to use a stepdown transSo let’s say you have a condenser mic capsule all charged up former, with a high-impedance primary winding (keeping the tube and ready for action, when along comes a sound wave. Sound happy) and a low-impedance secondary winding (keeping the waves are really rapid changes in air pressure: compressions, cable happy). This had the added benefit of providing a balanced
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RECORDING JUNE 2010

Recording - June 2010

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