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Boundary microphone
A boundary microphone is a small omnidirectional condenser mic capsule positioned near or flush with a boundary (surface). The arrangement provides a directional half-space pickup pattern while delivering a relatively phase-coherent output signal.
The boundary microphone can be used as a piano mic or on hockey boards for body check sound effects. It is also commonly used to record full room sound by being mounted on a wall. When used to record a soloist or small musical ensemble along with the room acoustics, a boundary microphone prevents phase interference between direct and reflected sound, resulting in a natural sound with a flatter frequency response than can be obtained with a stand-mounted microphone at the same distance (explanation below).
==Development==
A conventional microphone placed on, or a few inches above, a hard boundary surface will pick up the desired direct sound as well as delayed sound reflecting off the boundary surface. The direct and delayed reflected sounds will combine at the microphone to create comb filtering, with constructive and destructive interference causing peaks and valleys in the frequency response.〔Bartlett, Bruce (2009). "(The Boundary Microphone )".〕 The delay time of the reflection for most microphones would be in the range of 0.1 to 1 milliseconds, corresponding to cancellation frequencies of a few kilohertz and octave multiples. Since these frequencies are audible, the cancellation effects are also audible and are said to "color" the resulting audio signals.
By placing the diaphragm of the microphone capsule parallel to and facing the plate boundary provided by the microphone package, the reflected sound delay is reduced, and the resulting comb filter interference frequencies are high enough that they are outside the audible range.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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