object. The place right next to the object was called the "boundary layer", because it was between the object (no movement of air) and the free-flowing stream of air beyond it. In 1978, audio engineers Ed Long and Ron Wickersham studied the effects of the boundary layer in sound recording. In a study of flush-mounted microphones, they realized if the mic is only a few millimeters away from a large surface, there is a boost in coherence of sound signals, as they are still in phase after being stopped at the boundary. The point where the waves were stopped at the boundary was called a "pressure field" or "pressure zone". They also noted that using a mic in this set-up leads to a six dB boost in sound pressure. Only two years after Long and Wickersham's research, in 1980 Crown Audio developed and marketed the first boundary mic. 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. 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. The terms "boundary microphone" and "pressure zone microphone" (or PZM) refer to the same general design. PZM is a trademarked by Crown. Crown's trademark on this approach is "Phase Coherent Cardioid" or "PCC," but there are other makers who employ this technique as well. Radio Shack and Tandy also use the PZM term for their particular design of a boundary mic, in which the condenser capsule was positioned slightly higher than the base plate. One 1986 source uses the alternate term "barrier microphone" to refer to the boundary microphone approach. Variant features Since one of the benefits of boundary microphones