Boundary microphone

 A boundary microphone uses a small condenser microphone capsule mounted very near a sound reflecting plate, or boundary, so there is no delay in the reflected sound. Direct and reflected sounds add in-phase over the audible range of frequencies , resulting in a flat response, free of phase cancellations, excellent clarity and reach, and the same tone quality anywhere around the microphone. Boundary microphones have a directional response that is either half- omni, half- cardioid or half- supercardioid. An example of a boundary microphone is a PZM (pressure zone microphone).