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).