Development
A conventional microphone placed on, or a few inches above, a hard boundary surface will pick up the desired direct sound as well as delayed sound reflecting off the boundary surface. The direct and delayed reflected sounds will combine at the microphone to create comb filtering, with constructive and destructive interference causing peaks and valleys in the frequency response. The delay time of the reflection for most microphones would be in the range of 0.1 to 1 milliseconds, corresponding to cancellation frequencies of a few kilohertz and octave multiples. Since these frequencies are audible, the cancellation effects are also audible and are said to "color" the resulting audio signals. The goal of the PZM design was to avoid this coloration.
By placing the diaphragm of the microphone capsule parallel to and facing the plate boundary provided by the microphone package, the reflected sound delay is reduced, and the resulting comb filter interference frequencies are high enough that they are outside the audible range.
Crown International owns the trademark on the term "Pressure Zone Microphone" and PZM. The patent (U.S. Patent 4,361,736) for the microphone invention expired in 1999.
PZM is the trademark for Crown International's line of omnidirectional boundary microphones. Most other microphone manufacturers offer that generic type of microphone (as shown in their product catalogs).
Read more about this topic: PZM (microphone)
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