PZM is Crown's trademark...

The cavity effect on a microphone decides whether it is omni, cardioid or hypercardioid. It has nothing to do with the boundary effect.

The noise cancelling on a boundary mic is determined by the difference in signal between direct and reflected sound, and the flat surface upon which it is mounted is the 'boundary'.

The way it works is to shorten the delay between direct and reflected sound.

In 1978 sound engineers Ed Long and Ron Wickersham recognised the effects of a boundary layer in sound recording. In studying the behaviour of flushmounted microphones, they discovered that within a few millimeters of a large surface, sound levels from a pair of equal level signals add coherently because, in close proximity to the surface, the particles are still in phase as they accelerate after being brought to a stop by the boundary, thus creating what is called a pressure field, or pressure zone, in the boundary layer.

The result is a 6 dB increase in acoustic pressure.

In this pressure field, the instantaneous pressure is uniform in all directions, with no direction of propagation. A microphone diaphragm will therefore have 6 dB higher sensitivity when placed in the pressure zone than an equal microphone placed in the free field. Diffuse sound will not be reflected at a plane boundary because diffuse sound has no direction (per definition). This microphone will therefore give direct sound 3 dB higher level than diffuse sound. If used outdoors, these microphones will perform better than conventional microphones with regard to wind noise, as the wind velocity in principle is zero on the surface of the ground.

The pressure zone can be defined another way: The pressure zone is the distance from the boundary that the microphone diaphragm must be placed to achieve the desired high-frequency response. The closer the diaphragm is placed to the boundary (up to a point), the more extended is the high-frequency response.