The SM91 Microphone is designed specifically for surface-mounted applications. It is a professionalquality permanently-biased condenser microphone with a half-cardioid directional pattern (cardioid in the hemisphere above the mounting surface.) The SM91 takes advantage of the well-known principle 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 SM91 surface-mounted microphone discriminates against sounds originating from the rear, suiting the SM91 for conditions where an omnidirectional pattern makes other surface-mounted microphones impractical. The intrinsic unidirectionality of the SM91 can be a great benefit when it is desirable to isolate a particular vocalist, instrument, or group from the rest of an ensemble being recorded. Because of a cardioid pickup pattern, no physically isolating barriers are required, and directionality is maintained to low frequencies. The SM91 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 placement and critical listening will demonstrate the best location for any particular purpose or effect desired. The meticulously optimized design of the SM91 includes a totally new cartridge, developed at Shure. The result is high output, notably accurate sound reproduction over the entire audio frequency range, and off-axis performance comparable to the finest unidirectional microphones. The supplied low-distortion, highclipping-level preamplifier provides switch-selectable Flat or Low-Cut response for further assistance in obtaining the best possible microphone signal even under difficult conditions.