Ron Wickersham and Ed Long invented the Pressure Recording Process PRP), which uses a pressure-response omnidirectional microphone facing down toward, and very close to, a sound-reflecting surface. The goal was to:
1. Prevent phase interference between direct and reflected sounds, resulting in a flatter frequency response.
2. Provide identical frequency response for direct sound and random-incidence sound.
3. Cancel sounds above the audio range by phase interference, reducing the need for steep anti-aliasing filters in A/D converters used to record the mic output.
4. Protect the mic capsule (compared to a flush-mounted mic capsule aiming up).
Ken Wahrenbrock built the first commercially available PZM prototype using a hearing-aid type electret condenser microphone and a support structure.
At Syn Aud Con classes, Don Davis showed PZM prototypes built by Ken Wahrenbrock and took orders for Ken. You could buy just a pair of capsules, or a kit to make a pair of mics, or actual finished mics. Don came up with the PZM name.
In a Syn Aud Con mail list, Crown alumnus Don Eger had this to say about the history of PZM microphones:
"I went to Ken Wahrenbrock at an AES show and ask if Crown could manufacture the mics for him to sell. Ken said he would get back to me. By the end of the show Ken ask if Crown would take over the development, manufacturing, and sales of the PZM mics. This was a surprise to me but Ken wanted more time to do experiments with different designs.
"Crown agreed to take on the challenge. My boss asked who was going to take on this project and then looked at me and said, 'You have $X to engineer and put the mics into production.' So I designed the first four models and put them into production. We used a TDS system made of an oscillator and HP analyzer to measure the microphones. The cantilever was make of 30% carbon fiber filled nylon, very new at the time.
"Thanks to Don Davis for his work in helping us get the contracts all worked out for the PZM.
"One other quick note, PRP is not just the microphone but is the entire system from the microphone to the speakers and room."