Used Shure SM91A Microphones

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Condenser cardioid boundary microphone

The SM91A Microphone is designed specifically for surface-mounted applications. It is a professional-qual- ity permanently-biased condenser microphone with a half-cardioid pickup pattern (cardioid in the hemisphere above the mounting surface.)
The SM91A takes advantage of the well-known princi- ple that, at a barrier or boundary, sound pressure doubles compared to its value if the boundary is removed. When placed sufficiently near the boundary surface, a microphone has effectively 6 dB higher sensitivity and approximately 3 dB greater rejection of random background noise.
Because of its half-cardioid polar pattern, the SM91A surface-mounted microphone discriminates against sounds originating from the rear, suiting the SM91A for conditions where an omnidirectional pattern makes other surface-mounted microphones impracti- cal. The intrinsic unidirectionality of the SM91A can be a great benefit when it is desirable to isolate a particular vocalist, instrument, or group from the rest of an en- semble being recorded. Because of a cardioid pickup pattern, no physically isolating barriers are required, and directionality is maintained to low frequencies.
The SM91A can be used for individual instrument pickup, e.g., mounted inside the lid of a grand piano or on the floor next to a bass drum. Experimental place- ment and critical listening will help to determine the best location for any particular purpose or effect desired.