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Q. PZM and Boundary mics — what's the difference?

The capsule of Crown's PZM 30D points downwards towards the mic's metal plate.The capsule of Crown's PZM 30D points downwards towards the mic's metal plate.

I'm wondering what the difference is between PZM and boundary microphones. I've heard a lot of people say you should place boundary mics on a hard surface, so do you need to do this with a PZM as well, or is the plate normally sufficient? I often whack a Crown PZM in my kick drum to get the 'click' sound, but will I get an even more defined sound if I mount the Crown on a slab of, say, perspex inside the drum?

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Technical Editor Hugh Robjohns replies: The term PZM is a contraction of Pressure Zone Microphone, and was used by Crown (and Radio Shack/Tandy) to describe its particular incarnation of a 'boundary layer' microphone design, in which the mic capsule is mounted slightly above the surface plate, looking back down onto it. Other boundary layer mic designs simply mount the mic capsule flush with the boundary surface. Both designs work in the same basic way, and there is no significant difference between them, although the PZM design affords a higher degree of physical protection to the capsule.

When a sound wave reaches a fixed surface boundary — such as a wall — there is a pressure maximum (and velocity minimum) at the surface. Placing the microphone in this zone of pressure maxima gives a 'free' boost in sensitivity of 6dB, which is handy. Perhaps of more significance is the reduction in comb-filtering effects. If you consider a mic placed above a reflective surface like a wooden floor, the mic will 'hear' both the direct sound from the source and a reflected version of it, as it bounces off the floor and back into the mic. The extra distance travelled to the floor and back will delay that reflected sound relative to the direct sound and hence cause some coloration because of the comb-filtering effects. Placing the microphone flush with the reflective boundary ensures that those reflections can no longer interfere with the direct sound, and the sound quality improves as a result.

Most boundary layer (or PZM) mics have a relatively small housing or base plate, purely for practical reasons. However, to ensure that the boost in sensitivity is maintained uniformly across the entire frequency spectrum, the pressure zone must be maintained for all frequencies too, and that means that the boundary surface requires dimensions similar to the wavelength of the lowest frequency of interest, hence the advice to place boundary layer mics on the floor or on walls. The problem with that, of course, is that the wall might be a long way away from the sound source. So an alternative approach often used is to mount the boundary layer mic on a stiff wooden or perspex panel with the largest practical dimensions, in order to produce an area of maximum pressure near the mic.

Without this additional panel or boundary wall, the pressure maxima won't build up for lower frequencies, so the mic's sensitivity at low frequencies won't increase in the intended way. The result is effectively the acoustical equivalent of a 6dB high shelf boost, with the turnover frequency being determined by the dimensions of the mic's housing or base plate. So the mic will typically tend to sound a little bass light if used without a large boundary surface.

For the kick-drum application, the easiest and most practical solution is to place the PZM or boundary layer mic on the floor just in front of the kick drum, to maximise the mic's bass extension.