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Meet Mike
by Jay Rose       Published July, 2000

Knowing what's inside a microphone can help you choose the right one for the job.

I used to be a shill for AKG. I'd say nice things about the mic and headset company, and in return they'd send me to trade shows in Geneva, Amsterdam, and other nifty cities. One time, I lost the ID badge that let me into the show area. The product manager told me not to worry; he always ordered a spare with a fictitious name. For the rest of that show I was Mike Rafone.

It was sort of an honor to wear that name. Few gadgets are as essential to production as the microphone. Mics are classified two ways, by their pickup pattern and how they create a signal. We covered patterns in "The Art of the Pickup" (March, 1999). Add this column's explanation of how they work inside, and you're on the way to being a mic expert.

DYNAMIC MICS

The simplest mics used regularly in production are really tiny electric generators. Just like a power plant's dynamo, dynamic mics use a coil of wire suspended in a magnetic field. The coil is attached to a plastic or foil diaphragm (figure 1a). Soundwaves vibrate the diaphragm, move the coil through the field, and -- as you learned in grade school science -- that generates electricity.

fig01a
fig01b
Figure 1. A dynamic mic (A) uses magnetism to generate electricity. A condenser one (B) relies on electric charges.

Since the electricity is converted directly from sound pressure, it takes a relatively large amount of the stuff to get a usable signal. Dynamic mics can't be as sensitive as condensers -- the other common kind used in production -- and don't create enough voltage to be plugged directly into a camcorder. On the other hand, their simple mechanical design and relatively large tolerances makes them more rugged and less sensitive to humidity or temperature changes. And since they convert sound directly into electricity, they can handle loud noises without distortion and don't contribute any electronic noise. They're usually also good at rejecting handling noise, which makes them a favorite for hand-held interview use.

Dynamic mics and ordinary headphones are very similar inside, just as electric generators and motors use basically the same construction. In fact, you can connect a headphone to a sensitive mixer input as an emergency slate mic. Back when I was a radio station engineer, I'd occasionally fuddle announcers by doing the reverse, connecting a studio mic to an intercom output. While they were just about to read the news, I'd whisper into the 'com. Their mic -- now working as a headphone -- would say "Eww. You had garlic for lunch!"

CONDENSER MICS

Condenser mics are preferred in film and video production. Since they don't have large magnets, they can be smaller and lighter -- an advantage for both booms and lavalieres. High-quality condensers can be more sensitive to subtle sounds than dynamics. And since it's cheaper to build an adequate-quality condenser than a similar dynamic, manufacturers prefer them for low-cost bundled systems.

A condenser mic doesn't turn sound into volts; instead, it modulates a voltage from some other source. A metalized plastic diaphragm, with an electric charge on it, is mounted close to a rigid plate (figure 1b). As sounds vibrate the diaphragm near the plate, a tiny but varying stream of electrons can jump across. This signal is too weak to send to a recorder -- even a few feet of extra cable would destroy it -- so a small preamp is mounted in the mic itself.

Traditional condenser mics charge the diaphragm with up to couple of hundred volts, stepped up from the preamp's power supply. This means they can be very sensitive (or create very little electrical noise of their own, which amounts to the same thing) so this design is preferred for high-quality studio and boom mics. But you can also charge small items chemically instead of electrically, using a process something like the way they make static-charged dust mops. The result is the ubiquitous electret condenser, found everywhere from cell phones to film sets. Tiny electret lavs can be so small they're virtually invisible (see last January's Audio Solutions). Full-size electrets, costing $75 - $500 depending on construction and pickup pattern, are fine as boom mics on video shoots.

All condenser mics require power for the preamp, but there are different and incompatible ways of getting it to them.

•       If a camera has a mini-jack mic input, it probably puts a low DC voltage on the audio conductor itself. This can cause trouble with standard mics, and distort a mic-level signal from a mixer or wireless receiver. You need an adapter with a blocking capacitor, or a transformer adapter (which also gives you the advantage of balanced wiring). By the way, the terms /ital/ condenser and /ital/ capacitor mean the same thing in electronics, but the former usually refers to microphones and the latter to circuit components.

•       Full-sized production mics are powered either by an internal battery or by phantom powering, a scheme that puts a low DC voltage on both wires of a balanced audio cable and returns it through the cable shield. Dynamic mics, which don't require power, won't see a phantom voltage at all if they're properly wired for balanced connection. (Improperly-wired mics or adapters can be damaged by the voltage, so it's a good idea to leave the phantom turned off if you're not sure.)

•       Tiny electret lavalieres have batteries in their plugs or use phantom power. If they're part of a wireless system, they get their power from the transmitter's battery.

When power is first applied to a condenser mic, it can produce a loud "thunk". As a general rule, leave the mic's volume turned down when you change the battery or plug it in to a phantom supply.

OTHER MIC ELEMENTS...

You may encounter a ribbon mic when recording voice-overs. Ribbons use the same basic principle as dynamics, but without a diaphragm. Instead, a delicate foil strip is suspended in a very strong field and is vibrated directly by the sound waves. Because of this, a ribbon can be more accurate -- particularly on relatively loud mid-rangy sounds like voices -- and is often preferred by announcers. But ribbons are almost never found on a shooting set: they're too heavy to use on a hand-held boom, and not very sensitive. They're also fragile: a strong wind can knock the ribbon out of position, requiring an expensive repair.

Piezo electric mics attach a crystal or ceramic element to the diaphragm, which generates electricity as it's flexed. Carbon mics rely on the principle that grains of carbon conduct electricity better when they're squeezed together. Both types are low fidelity, noisy, and have been largely replaced by cheap electrets.

...AND A NEW MIC DESIGN

The acoustic principles in this series of columns are based on rules of physics, which aren't likely to change until someone rebuilds the universe. But clever equipment designers find ways to get around the rules, playing one against the other to achieve new results.

Case in point: when I wrote about pickup patterns sixteen months ago, it was absolutely true that directional mics got less directional with lower frequencies and that their off-axis sound was uneven. This was due to the physics of acoustic cancellation, and even the best mics suffered from it.

Now the folks at Audio-Technica are using electronics to do what acoustics can't. They've put five standard mics in a single housing, and send the separate signals to a belt-pack processor. It compares relative strengths and phases to reduce unwanted signals cleanly and smoothly. The processor's output has far better off-axis rejection than any single mic's can ever be. When things are just right, it can virtually eliminate noises coming from behind.

fig02 Their AT895 isn't a cure-all, but it is amazing for outdoor handheld work, and for booming in noisy but non-reverberant environments (echoes confuse the processor). Figure 2 shows me holding one alongside a standard short shotgun. When I used the pair for an interview, the result blew me away: my off-axis but nearby questions were almost full-volume on the standard mic, but almost disappeared on the AT895. The system is expensive -- $2500 to $3000, depending on accessories -- and noisier than other directional mics. I'm sure Audio-Technica will work to refine both the technology and the price for videographers.

By the way, I don't shill for AKG any more but I'm still using their mics -- that's one of theirs next to the Audio-Technica in the photo. Don't think I'm shilling for that company either... though I sure think their new mic is cool.

***

Jay Rose (jay@JayRose.com) is a Boston-based sound designer whose recent work includes documentaries for A&E and Discovery networks, ID packages for PBS, and a book about sound for video producers. Details at www.JayRose.com.