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Boundary Mikes or PZMs (pressure zone microphones) look very different to a normal mic and work by exploiting a particular characteristic of sound, in that when sound bounces off a hard surface, in the small zone immediately next to the surface the sound pressure is doubled meaning that any mic in that zone will have a higher output. Sounds simple, and it is, but of course there are limitations. I'm not going to get into the physics here but as always there are some great sites on the net particularly The Crown Microphone website. but what this technology means for mic design is that a very small mic capsule is fixed in a low profile metal housing which you place directly onto a hard reflecting surface such as a floor or wall and it's this surface (boundary) that makes the mic work. Stick one on a stand or hold it in your hand and you don’t get the effect. To prevent phase cancellation the mic capsule has to be very small (less than ¼ inch) and traditionally boundary mics had an omni capsule though today there are a range of cardioid and hypercardioid versions.

The physics states that the doubling effect of the pressure zone will happen equally right across the frequency spectrum if the “Boundary” is infinitely long and infinitely wide but unfortunately, infinite surfaces are difficult come across even in the most expensive studios and microphones exhibit different frequency response dependant on the size of the surface they are placed. Most applications for boundary microphones are in the conference or audio visual field for sound re-enforcement where they make great lectern mics and table top mics for discussion groups but studio engineers being what they are have tried using them over the years on all kinds of instruments, sometimes to great effect. Boundary mics can be taped to the lid of a grand piano or placed on the floor infront of a drum kit and the only application where they can't realistically be tried is on vocals. One of the first studios I worked in had a pair of cheap Tandy PZM mics Gaffer taped to the walls of the drum room and with a bit of eq they made a pretty good job as drum overheads. Most companies make a boundary mic and prices range from a few pounds to a few hundred depending on application and quality. At the top end Crown produce a very high quality stereo boundary mic, the SASS system based on a form of dummy head priced around $1000.

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