Recording - May 2012 - 47

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large ensembles such as orchestras, omnis capture the sound more as listeners hear it: musically balanced and bathed in the natural ambience of a performance space. The frequency response of omnis also makes them popular when recording classical and other acoustic musics. It is simply impossible to make a directional mic with a frequency response as flat as a quality omni. For audiophile purists seeking the minimum of coloration, omnidirectional microphones are the choice. Bidirectional (Figure-8) Fig. 4 shows a popular bidirectional mic, the Royer R-121. Fig. 5 is its polar chart. “Bi” mics are also commonly referred to as figure-8, due to the shape of their polar pattern. Unlike an omnidirectional mic, in which the capsule is sealed so sound can enter from the front only, bidirectional mics are sensitive to sound reaching both sides of the diaphragm. Figure-eight mics are also known as pressure gradient microphones, as they respond to the difference (or gradient) in pressure at both sides of the diaphragm. What exactly does this mean? Sound sources directly in front of or behind the mic are captured with maximum sensitivity, as both sides of the diaphragm respond to different pressures. However, sound sources at the sides of the mic (90˚ to the diaphragm), are almost entirely cancelled, since they produce the same pressures (equal magnitude and direction) against both sides of the diaphragm. Put as simply as possible, offaxis sounds push equally on both sides of the capsule, canceling each other out. The two-sided nature of bidirectional mics makes them useful in some of recording’s more creative applications. Employing the Beatle trick and placing a figure-8 between two singers can sometimes lend just the camaraderie required for a duet or background vocals. Positioning one between two rack toms not only eliminates a mic on a crowded drum set, but also offers needed off-axis cancellation of the snare drum. For a left-of-normal approach, mixing a bidirectional with a unidirectional or omnidirectional microphone makes the variable stereo effect known as Mid-Side. Visit www.recordingmag.com/videos/videoDet ail/18.html for a great video on this topic. Also, two coincident (in the same place) bidirectional mics angled 90º with respect to each other make up a Blumlein Pair, another interesting stereo miking technique. Both of these procedures embrace the fact that sounds entering opposite sides of a bidirectional mic generate electrical signals of reversed polarity. I’ll return to this concept in a moment when discussing unidirectional microphones. Oddly, it isn’t the polar pattern of bidirectional mics that ensures their regular usage in the recording studio. Most ribbon microphones are figure-8 by default, and
RECORDING MAY 2012

Fig. 3: Omni mics become increasingly directional at higher frequencies. The Neumann TLM50’s polar pattern exemplifies this, shown here at 125 Hz, 8 kHz, and 16 kHz. (Polar patterns courtesy Neumann)
Similar to the ear, an omni mic captures sound from all directions. The term pressure microphone is also used to describe this type of mic. This nomenclature stems from the fact that omnis are sensitive to any sound pressure change, regardless of location of origin. Building a mic with an omnidirectional pattern is relatively easy and thus the earliest microphones were all omni. To construct the pattern, designers seal the back of the diaphragm so sound can only enter from the front. Since sound waves bend (diffract) around small objects, sound from the mic’s rear twists around the microphone body and enters through the forward-facing diaphragm. The reliance on diffraction has one important caveat. At higher frequencies sound waves are shorter and diffraction is minimized as wavelengths approach mic capsule size. This makes omnidirectional mics, and all other patterns as well, increasingly directional at higher frequencies, where sound waves are less likely to bend around a microphone. Because of this, most polar plots chart sensitivity at multiple frequencies. Be sure to find and investigate the plots for any microphone you own or use, and don’t be surprised if you discover different responses at different frequencies. Fun with omnis Omnidirectional mics have a variety of uses in day-to-day life, such as speakerphones, video cameras, and dictation recorders. The pattern’s popularity is due to its many positive attributes. Omnidirectional mics are especially even in frequency response, and are less sensitive to wind noise, handling (vibrational) noise, and proximity effect than their directional cousins. Also important, omnis allow a sound source to move around with less level and tonal shift than that experienced with directional mics. Although omnidirectional microphones offer stellar performance, they see limited use in modern music production. For the most part, today’s recording techniques rely on directional mics, as it is usually desirable to isolate sound sources, record them to separate tracks, and manipulate them independently during mixdown. This said, omnidirectionals still have a place in music recording. When tracking

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Recording - May 2012

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