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Thursday, July 21, 2011

This Article Features Photo Zoom

Ultradirectional shotgun microphones have been an essential part of capturing audio for film and video for more than a half century, and they have come a long way, baby. Early models were based on dynamic moving-coil transducers and were heavy, awkward and had low output levels that required a substantial gain boost (which also increased the level of background noise) in order to provide a useable signal.

Anatomy of a Shotgun Mic

Modern shotgun mics are more compact, lighter and offer significantly improved performance over their earlier counterparts. Today, there are dozens of shotgun mics on the market, with models ranging from simple units that easily attach to the accessory shoe on your DSLR/camcorder for grab-and-go work to elaborate units intended for the most discriminating pro user.

MYTH VS. REALITY
Shotgun mics can provide a significant sonic improvement over a built-in camera microphone, although the laws of physics and acoustics dictate what they can and cannot do.

For an understanding of how these mics work, let's examine some basics. Shotgun mics operate on the principle of acoustical cancellation, a phenomenon where the action of one sound wave can alter or reduce the movement of another wave. In fact, the motion of sound waves is similar to ocean waves, with peak crests (high points) and troughs (low points). If two sound waves combine in the air and their peaks meet at the exact same time, the result is summation—an increase in the overall wave. If one wave is slightly delayed so that the peak high point of one meets the low trough of the other, the result is cancellation. One practical example that many people are familiar with involves hooking up a set of stereo speakers. When both speakers are properly connected, the result is full bass (the lows of both speakers are summed in the room); if the wires on one speaker are reversed, that speaker will emit a reversed waveform—cancellation occurs, and the bass sound is weak and thin.

Figure 1

Shotgun microphones use this principle to advantage. The microphone capsule is located at the end of an interference tube—a long cylinder with slots cut along its sides. Audio signals that reach the mic from the front are unchanged, while sounds arriving at the mic from the sides enter through the slots and are delayed just long enough to create some cancellation of those sounds. This increases the mic's directionality, with the net effect based on the length of the tube and the exact geometry of the slots. Generally, the longer the tube, the greater the effect.

One caveat about shotgun mics is that this interference effect varies with frequency. The polar response diagram (Figure 1) shows the directionality of a Sanken CS-2, a pro shotgun often used by film crews. To interpret the diagram, imagine the mic capsule is at the center, pointing toward the sound source (0 degrees). The colored lines indicate how the mic's directionality changes at different frequencies. Lower tones (250 Hz and below) are barely affected, while the rejection of sounds from the side of the mic is far more effective in the upper 4 kHz to 16 kHz ranges shown at the right of the diagram.

Shotgun mics also exhibit a lobing effect (as seen in the lower half of the diagram), where sounds—at a reduced level—are still picked up at the rear of the mic. This is another factor to keep in mind when using shotgun microphones, whether camera-mounted, handheld or on a boom.

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