In a boundary mics, also know as a Pressure Zone Microphone or PZM, an acronym trademarked by Crown International, a tiny opening allows sound waves to reach a small mic diaphragm immediately adjacent to a hard surface. Sound waves a few millimeters away from a hard surface form a pressure zone, or boundary layer, in which interaction between the sound wave molecules and the surface bring the waves into phase.
This pressure zone also results in sound pickup that changes little in relation to its distance from the sound source. It also results in an increased sensitivity of the mic and an increases in high frequency response. So by placing a necessarily small mic diaphragm against a hard surface you get a inconspicuous one that has increased sensitivity, virtually no phase coloration and little change in sound quality due to direction it in relation to the sound source you want to pick up.
In comparison, a conventional vocal mic has a very directional pickup pattern and is relatively insensitive so that a vocalist has to sing directly into the microphone’s diaphragm to avoid the pickup of sound reflections and off axis coloration.
The Boundary Microphone is usually a very small condenser microphone mounted to a flat surface to serve as the boundary layer. In use the microphone will often be mounted against a flat surface, like the floor of a stage or a conference table, so that the whole surface area will act as a large boundary layer.
The ability of these microphones to pick up sound with little coloration due to direction of the sound source and there excellent sensitivity make them perfect for several disparate applications. They are often used at the front of a theater stage to pickup all of the actors voices or on a conference table so that individual microphones are not needed for the participants in a conference call.
Another common application is on percussion instruments. The microphone’s ability to pick up sound from different directions makes it very good for miking a drum kit. It’s excellent frequency response will pick up the wide variation in sound levels produced by percussive instruments like a piano. Due in large part to its patent expiring Boundary Microphones have become much more common and diversified in recent years. Many manufacturers now produce them in a variety of configurations making them more common and less expensive.