Original Source:http://www.uneeda-audio.com/pzm/

In 1982, Ron Wickersham and Ed Long were granted US Patent 4,361,736, "Pressure recording process and device."  

A process for transducing acoustical signals, including music, speech, noise, etc. which embodies a method which eliminates the usual discrimination between the frequency spectra of the direct and random incidence acoustical components of the sound being transduced and allows the frequency range of interest to be controlled by adjusting the spacing between the diaphragm or major entry port of an acoustical to electrical transducer and a boundary. The invention requires that the diaphragm or major entry port of the transducer be oriented substantially parallel with and proximate to a boundary. A formula is disclosed by which the proximate relationships between the diaphragm and a boundary can be determined for any desired frequency range, which eliminates the discrimination between the frequency spectra of the direct and random incidence acoustical signals on the transducer while allowing the rejection, at the transducer, of unwanted higher frequency acoustical signals, by means of acoustical cancellation, which causes a filtering action without the undesirable effects of non-uniform phase or group delay caused by high order, sharp cut-off filters normally employed to reject these higher frequency signals. In the device, an acoustical-to-electrical transducer, having a uniform pressure versus frequency characteristic in the desired frequency range, is mounted with its diaphragm substantially parallel with and proximate to a surface forming part of a boundary and supported by a structure which is adjustable, or fixed during manufacture, to control the proximate distance between the diaphragm and its substantially parallel boundary. A surface connected to the adjustable or fixed structure, which forms part of a boundary, can also be used when it is only slightly raised from the boundary by shock mountings to mechanically isolate it from the boundary under which circumstances it functions as part of the larger substantially parallel boundary.

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