What is "Boundary-Layer Microphone"?
A microphone placed on the surface of a large boundary. At the boundary, the direct sound is doubled, which yields +6 dB of the sound pressure. The diffuse sound is also doubled, but due to the random phase, the sum only yields +3 dB. In this way, the microphone emphasizes direct sound over the diffuse sound, providing higher directivity of the microphone. The principle is also known as PZMTM
(Pressure Zone Microphone), which is a trademark of Crown.