The PZM Microphone: One Of Recording’s Strangest And Most Useful Tools

It turned out to be an amazing little microphone, and it sounded far better than it had any right to. One of my favourite tricks was to tape it to a wall and have the vocalist sing directly into the wall. They were usually a bit taken aback at first, but once they heard the result, they were just as surprised as I was.

Of course, the PZM microphone is far more versatile than that. In fact, if you could only own one microphone, there is a strong argument that it should be a PZM.

The initials PZM stand for Pressure Zone Microphone, which explains the basic idea behind the design. A PZM is a microphone intended to be used on a surface such as a table, floor, wall, hard baffle, stage, piano lid, or panel. Instead of placing a conventional microphone in front of a source, the PZM uses a miniature condenser microphone cartridge mounted face-down very close to a sound-reflecting plate or boundary.

That arrangement is what makes the microphone work. The diaphragm is positioned parallel to the reflecting surface and placed extremely close to it. This puts the diaphragm inside the pressure zone, where the direct sound from the source and the sound reflected from the surface arrive at almost the same time. Because the two sounds arrive together, the microphone avoids the phase cancellations that can occur when a normal microphone is placed close to a hard reflective surface.

This is the clever part of the PZM design. In many recording and sound reinforcement situations, microphones have to be placed near reflective surfaces. This might happen when recording an instrument surrounded by hard baffles, placing microphones near a stage floor, or working close to the open lid of a piano. With a conventional microphone, sound reaches the diaphragm in two ways. Some sound travels directly from the source to the microphone, while some reflects off the nearby surface first.

The reflected sound travels a slightly longer path, so it arrives a little later than the direct sound. When those two versions of the same sound combine at the microphone diaphragm, some frequencies add together while others cancel. The result is a comb-filter effect, a series of peaks and dips in the frequency response that can colour the sound and make the recording less natural.

The PZM was designed to solve that problem. By placing the diaphragm very close to the surface, the reflected path is made effectively equal to the direct path. The direct and reflected waves reach the microphone at the same time, which removes the delay that causes the comb-filter effect. The result is a smoother frequency response and a clearer sound.

This is also why simply putting a normal microphone on the floor is not the same thing. A conventional microphone still has its diaphragm slightly above the surface, and that small distance is enough to create a short delay in the reflected sound. The result can be cancelled high frequencies and a duller sound. The PZM is specifically built so the diaphragm can sit close enough to the boundary for the pressure zone effect to work properly.

The plate on a PZM is not a pickup. It does not convert sound into an electrical signal. The plate is simply a hard surface that reflects sound into the microphone diaphragm. The actual microphone element is the small condenser cartridge mounted next to the boundary.

A PZM has a hemispherical pickup pattern. It is equally sensitive to sound arriving from any direction above the surface plane. This is one of the reasons it can be such a useful microphone. It does not need to be aimed in the same way as many conventional microphones, and it does not suffer from the same kind of tonal change when the sound source moves around it.

With many conventional microphones, the sound changes depending on where the source is in relation to the microphone. The on-axis response can be different from the response to sounds arriving from the side or rear. The PZM avoids much of this because the cartridge is very small and sound enters through a tiny, radially symmetrical opening. This means the effective frequency response remains more consistent, regardless of the angle from which sound arrives.

That lack of off-axis colouration is especially useful when recording ambience. Room reflections and reverberation usually arrive from many directions. With some conventional microphones, reverberant sound can be reproduced with less high frequency detail than the direct sound. This can make distant microphone placements sound duller as more room sound is picked up. A PZM captures direct and ambient sound with a more consistent tonal balance, which helps the overall recording remain clearer.

This also explains the sense of reach that PZMs can have. Reach means the ability to pick up quieter or more distant sounds clearly. A PZM has several qualities that help here. Its on-surface mounting increases the direct sound level by 6dB, while reverberant sound is increased by only 3dB because it is incoherent. This improves the direct-to-reverberant sound ratio and can make distant sources sound closer and more defined than they might through a conventional omnidirectional microphone.

The size of the surface matters. The larger the boundary, the more extended the low frequency response. If a PZM is used only with its own small plate on a stand, the low frequency response shelves down by 6dB at 377Hz. Mounted on a 610mm x 610mm panel, the response shelves down by 6dB somewhere between 83Hz and 124Hz, depending on the panel angle. For the best bass and flattest response, the PZM should be mounted on a large hard boundary such as a floor, wall, table, or baffle at least 1220mm x 1220mm.

That boundary can be made from around 6mm plywood, masonite, or clear plastic. If less bass is desired, the microphone can be used on its own plate and kept away from other reflective surfaces. If a suspended PZM and plate are placed very close to the sound source, the full bass response returns.

This surface-dependent behaviour is one reason the PZM is such a creative studio tool. It can be used on the floor, on a wall, inside or near a piano, against a hard baffle, on a table, or mounted to a panel. Each placement changes the practical use of the microphone while still retaining the basic pressure zone principle.

On grand piano, a PZM can be taped to the underside of the lid in the middle. With the lid on the long stick, this can produce a clear piano sound. For greater control, reduced leakage, or less feedback, the lid can be placed on the short stick or closed, with the piano covered by a heavy blanket. For stereo piano recording, two PZMs can be taped under the lid, with one over the treble strings near the hammers and the other over the bass strings away from the hammers. Placement near the hammers emphasises attack, while placement farther away gives more tone.

A PZM can also be used at some distance from the piano to include more room ambience. One method is to place a PZM on a panel about 1830mm to 2440mm from the piano and about 1220mm high, with the panel facing the piano and parallel to the open lid. Another method is to record the piano close and then mix in separate ambience microphones placed on walls farther away.

On drums, a PZM can be placed on a panel or hard gobo around 300mm to 610mm in front of the kit, just above tom height. Two microphones placed around 915mm apart can be used for stereo. A PZM can also be placed inside the kick drum against the shell, with a pillow or blanket pressing against the beater head.

For horns, a PZM can be mounted on a wall, control-room window, or hard-surfaced gobo. The players perform towards the wall or gobo from a short distance away. Because their sound reflects back from the surface, the players can hear each other and create a natural acoustic balance. A clip-on PZM can also be used on a music stand between every two players.

In theatre, drama, and opera, PZMs can be placed across the front edge of the stage, about 300mm from the edge. One or two microphones can be enough for smaller stages, while several can be used across a wider stage. A directional PZM design can help reject sound from the audience or orchestra pit.

For orchestras, bands, and choirs, PZMs can be used on panels, on the floor, or as suspended microphones depending on the setup. A pair of panel-mounted PZMs can be spaced apart and placed in front of the ensemble, while floor-mounted PZMs can be positioned several metres from the first row of players. If the stereo spread becomes too wide, a centre-fill microphone can be added.

PZMs are also useful for ambience. One or two PZMs mounted on a wall can capture room sound in a clear and relatively uncoloured way. They can also be used for audience reaction, which makes them useful in live recording, broadcast, and performance spaces.

For conferences, group discussions, and interviews, a PZM can be placed on the centre of a table. For more control and less room reverberation, one PZM can be used for every four to six people, with no person more than about 915mm from the microphone. Because the microphone has low vibration sensitivity, it can be used on conference tables without the same need for shock mounting, while still reducing table thumps.

The low-profile design is another important part of the PZM’s appeal. It is visually unobtrusive and does not point directly at performers. This can reduce microphone anxiety and make people feel less like they are performing into a microphone. PZMs can also be hidden in theatre sets, used on camera without drawing much attention, or placed on lecterns and tables with minimal visual clutter.

Of course, the PZM is not the perfect microphone for every possible recording job. No microphone is. It is a particular tool with a particular way of working. But it is a very useful tool, especially when microphones need to be placed close to reflective surfaces, when a broad pickup area is needed, or when a less intrusive microphone setup is desirable.

A PZM can also be made more directional in several ways. A piece of thick carpet folded fuzzy side out can be placed behind the capsule to increase rear rejection, particularly at higher frequencies. A sheet of acoustic foam behind the microphone can also help reduce sound from the rear. A PZM can be taped to the centre of a panel, with sounds from the front side picked up and sounds from the rear side rejected. Placing the microphone off-centre on the panel can help produce a smoother response.

Because the PZM is a condenser microphone, it requires power. Power supplies can use built-in batteries or phantom power adapters, depending on the model. Some models have attached cables, others have detachable cables, and some are designed for permanent flush mounting. There are also lavalier versions and models designed for directional pickup.

What made that cheap Realistic PZM so memorable was not just that it worked, but that it encouraged a different way of thinking about recording. Instead of asking where to put the microphone stand, you started asking what surface could become part of the microphone. A wall, a floor, a piano lid, a table, a panel, or a baffle could all become part of the recording setup.

That is why the PZM remains such an interesting microphone. It is simple, strange, and surprisingly effective. It can capture vocals, piano, drums, horns, ensembles, ambience, speech, and stage sound in ways that conventional microphones often cannot. It is not just a budget microphone trick, although it can certainly be that. It is a genuinely clever design that solves a real acoustic problem.

And for anyone starting out with a limited microphone collection, that makes the PZM a powerful thing to have. It may look odd, and it may ask performers to do things they are not used to, such as singing into a wall, but the results can be far better than expected. For a microphone that once looked like a cheap curiosity in a Tandy store, the PZM turned out to be one of the most useful recording tools I ever stumbled across.

PZM-style boundary microphones are still available today from manufacturers such as AKG, Shure, Sennheiser and Beyerdynamic, among others. While they are no longer as cheap as the old Tandy Realistic-branded PZM, which was made by Crown, they remain relatively affordable compared with many studio microphones and are still just as effective when used in the right situation. For anyone looking for a microphone that can be placed on a wall, floor, table, piano lid or hard panel, the basic idea remains every bit as clever and useful as it ever was.