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-- by John Woram
If you look at a microphone's spec sheet, you'll notice that it probably shows the mike's polar response. This isn't an indication of how well it performs up North-it's a graph of sensitivity to sounds originating in the 360-degree space surrounding the microphone diaphragm.
The graph is produced by pointing the microphone at a fixed sound source and then rotating it through 360 degrees while plotting its output level. At zero degrees, the output signal plots a point at some convenient distance above the intersection of the graph's origin-in effect, the beginning of a circle whose radius represents output signal level. As the microphone rotates, so does the plotter pen, which traces a "circle" whose radius varies according to the microphone output. I'll describe three such patterns.
Omnidirectional. The simplest to design, this microphone consists of a diaphragm stretched across an otherwise-sealed enclosure. If a sound occurs anywhere in the surrounding area, pressure fluctuations strike the diaphragm, and it vibrates accordingly. The microphone can't tell where the sound originates, so it responds without regard to its physical location with respect to the diaphragm-hence the omnidirectional designation.
Bidirectional. The bidirectional microphone's diaphragm is exposed to the air at its front and rear. Thus, a sound originating directly in front, or directly behind, can strike the diaphragm. However, a sound that arrives at a right angle to the diaphragm strikes its front and rear simultaneously. With no pressure difference between front and rear, the diaphragm doesn't budge, and the sound is therefore ignored.
Unidirectional. The earliest unidirectional microphone contained two diaphragms-one omnidirectional, the other bidirectional. With the outputs combined within the microphone housing, both diaphragms moved in the same direction in response to a sound arriving from the front, and the microphone signal was the sum of the two outputs. If a sound arrived from the side, the bidirectional component didn't respond (as above), so the output was now lower in level, because it consisted of the omnidirectional signal only. If a sound arrived from the rear, the omnidirectional diaphragm continued to respond as before, while the bidirectional diaphragm moved in an opposing direction. With the two outputs now equal (in amplitude) but opposite (in polarity), the combination produced zero output.
The modern unidirectional microphone is no longer a dual-diaphragm system. Design engineers have developed sophisticated means to combine the effects of both in a single-diaphragm design.
The polar trade-off. The cardioid microphone is often first choice. Its ability to favor sounds originating in front of it is a big advantage in a noisy environment, even if that noise is nothing more than your computer's fan. However, an inexpensive cardioid microphone may have undesirable side effects, such as a rising low-frequency response as a sound moves closer to the diaphragm, sensitivity to handling and vibration, and so on. By contrast, the omnidirectional microphone's simple design gives you a better-quality microphone for the same price. If you do a lot of face-to-face interviews, you may prefer a bidirectional microphone.
If you're considering an upgrade from the inexpensive microphone that came with your sound card, think about an omnidirectional polar pattern. In a reasonably quiet environment, you may not need built-in directional sensitivity, and you certainly don't need the built-in side effects of the unidirectional model.
Copyright (c) 1997 CMP Media Inc.