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).
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. 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.
Directional boundary microphone
A directional boundary microphone is a boundary microphone with a directional polar pattern such as cardioid or supercardioid. One method to create a directional boundary mic is to mount panels (hard surfaces) next to an omnidirectional mic capsule.
Another method to create a directional boundary mic is to use a small-diameter (1 cm or less) directional mic capsule mounted on a boundary surface, with the axis of the microphone parallel with the surface. As shown on manufacturers' datasheets, the mic capsule retains its directionality (cardioid or supercardioid polar pattern) and prevents comb filtering by keeping phase interference above the audible range. Examples include the Crown PCC-160 introduced in January 1985, and the Shure SM-91, which is also commonly used inside a kick drum for sound reinforcement.
The predecessor of the directional boundary microphone was a directional microphone placed in an Electrovoice Mic Mouse, a foam block that suspended a conventional microphone horizontally just above a surface. Because conventional microphone diaphragms are relatively large (> 1 cm diameter), phase interference from surface sound reflections caused a rolloff in the high frequencies of a microphone in a Mic Mouse.
Some applications for directional boundary mics are picking up actors' voices onstage in drama or musicals, picking up the footwork of dance troupes, picking up speech at conference tables and boardrooms, and recording small musical ensembles or soloists.