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RIBBON MICS: Blast from the Past
Ribbon microphones are a welcome holdover from the early days of broadcasting. Ignored during the '70s and '80 for being old-fashioned and excessively delicate, ribbon mics have since been rediscovered, resulting in the reissues of several old favorites and a high price tag on vintage pieces. They employ a fine, corrugated metal strip suspended in a strong magnetic field. With air movement, the strip wiggles, and electrical current is generated. You'll find these mics to be very heavy, in part because of the large magnets they require and also because they need special signal transformers to communicate properly with the mixing desk. They usually have a figure-eight pickup pattern, which means that sound coming from both the front and the rear of the mic is picked up, while most of the sound from the side of the mic is rejected. This pattern is also sometimes called "bipolar" or "bidirectional." You will occasionally find cardioid ribbons, but they are rare. Figure 4 shows a typical ribbon mic with its associated pickup pattern.
Condensers: Cadillacs of the Studio
The most versatile and often most expensive mics in the arsenal are the condensers. These use the electronic principle of capacitance (condenser is the old way of saying capacitor) to generate signal. A very fine membrane of electrically charged, metallic coated mylar is stretched in front of a metal plate, and as air movement pushes the mylar back and forth, the capacitance across the the two elements changes. A special amplifier turns these changes into electrical current. Because the membrane is extremely light, it is more sensitive to microscopic air pressure changes; thus, condenser mics tend to have greater detailing of sound. With their in-mic amplifiers they can develop tremendous gain with minimal noise, the only drawback being that they require some sort of power source, such as phantom power from the mixing desk. The pricier models are usually "multi-pattern" - with the on-board electronics and a pair of diaphragms, all of the possible mic pickup patterns can be realized on a condenser, so you get a lot for your money even if the price runs high. Figure 5 shows a rough schematic of a condenser mike.
Last, and probably least, are crystal microphones, which use a piezo-electric crystal element to do their work. This technology is pretty amazing: a crystal generates electric current when vibrated. Unfortunately, though, the frequency response characteristics are dismal, with nothing picked up below 400 Hz and a major spike around 2-3 kHz (painful!). This construction is the basis of most contact mics (such as acoustic instrument transducers) and there are ways to temper it with plastic backings and such, but when you need a mic that sounds cheap in that special way - tinny, small and telephonic - reach for a crystal mic. They're usually unidirectional, though Midland made some dual element designs in the '60s (they look something like the RCA DX-77 pill-mics). They use two crystal elements facing each other to get a sort of donut-shaped omni pattern with horrific phase cancellation and chainsaw-like response. Heaven!! You can find them on eBay for a song.
That pretty much covers the basics of mic construction and mic jargon. In future articles we'll talk about the why's and how's of microphone selection, and where to best put these different types to use.
Copyright 2000 Douglas B. Henderson
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