Boundary Microphone: PZM Microphone

Boundary microphones are used in some specialised applications. One version called the pressure zone microphone, or PZM microphone has been available at a very reasonable cost.

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The boundary microphone, also marketed as a pressure zone microphone or PZM microphone is able to provide very good performance in a number of applications.

Boundary microphones are often used with pianos that are normally difficult to record successfully. They are also often mounted on walls to record a full room sound. They may also be used to record music in a room along with the room acoustics. By its operation the boundary microphone is able to prevent the phase interference between the direct and reflected sound and this results in a more natural sound.

Difference between PZM and standard boundary microphone

Essentially both the boundary microphone, which is the generic name for this type of microphone and the PZM or pressure zone microphone are the same.

PZM was used by the Radio Shack / Tandy and other associated companies for a particular design of microphone. In the PZM, the microphone is mounted slightly above the surface plate ‘looking’ onto it. Traditional boundary microphones mount the microphone capsule flush with the boundary plate

There is no difference with the way the type types of boundary microphone work, although the PZM does provide more protection of the microphone capsule as it is protected y the housing of the overall assembly.

Boundary microphone basics

The boundary microphone uses the varying pressure at a boundary between the air and a hard surface caused by a sound wave as the basis of its operation.

It is found that when a sound wave arrives at a boundary like a wall or other obstacle, a pressure maximum occurs along with a velocity minimum.

The result of the pressure maximum is that placing a microphone in this region gives an acoustic gain or increase in sound level that can be captured by a microphone of +6dB.

In addition to this the boundary microphone is less prone to the colouration effects caused by reflections as the microphone is placed on one of the boundaries. As a result boundary microphones, including the pressure zone microphone variants are able to provide a high quality of sound.

Most pressure zone microphones are mounted on a small plate, typically around15 centimetres square. For it to perform well at all frequencies, the boundary surface should be similar in dimensions to the lowest frequency to be covered. As a result it is always best to mount boundary microphones or pressure zone microphones on a wall or a floor. Alternatively it may be possible to mount the microphone on a stiff wood or perspex panel – this should have the largest practical dimensions. The large surface enables the operation of the pressure zone microphone to extend down to the lowest frequencies.

Boundary pressure area

The distance from the boundary at which the in-phase reinforcement takes place is a small fraction of a wavelength. Thus for high frequencies t is particularly important that the microphone element is very close to the boundary.

It is found that for a 1dB reduction in acoustic loss, the distance between the boundary and the microphone diaphragm must be only 1/13 of a wavelength. In real terms this equates to a distance of only 0.13 centimetres for frequencies of 20kHz at the top of the normal accepted frequency range. Then for a 3dB drop the distance must be less than 1/8 wavelength which equates to 0.22 cemtimetres.

Off axis colouration

One of the effects that is experienced by traditional microphones is that of colouration by sounds that arrive at an axis to the diaphragm. It occurs because sounds arrive at one side of the diaphragm before the other. This results in a phase difference across the diaphragm and this in turn leads to a lower excursion and hence lower output.

The effect is more noticeable at high frequencies where wavelengths are shorter and phase differences greater. In fact low frequencies are not noticeably affected.

The overall result is that off-axis sounds tend to be less sharp on a traditional microphone.

This off-axis colouration does not occur when using a boundary or pressure zone microphone. The boundary plate blocks the effects of the phase differences across the diaphragm – sounds reach all areas of the diaphragm at the same time. As a result boundary microphones do not suffer from the same sound colouration for off-axis sounds.

Properties for microphone boundary

The key to the operation of the boundary microphone, or pressure zone microphone is the plate on which it is mounted.

The boundary plate must be sufficiently sized to enable rear rejection, good bass response and the pressure doubling effect.

For most music the boundary should be 11a minimum of about 1.5 x 1.5 metres, but for speech this figure can be reduced to about 0.5 x 0.5 metres.

To reduce the visual impact for video and other applications of the boundary clear acrylic plastic may be used. If the edges pick up the light then these can be painted black.

It is fund that diffraction around the edges can give rise to comb filter effects. These can be minimised by placing the microphone off-centre, and making the boundary rectangular or irregular in some way. The shape to be avoided at all costs is a circular one as this places the microphone the same distance in all directions from the edge.

The microphones with their plates may be supported in a number of ways – piano wire may be used if they are to be supported in mid air, or they can be placed on tables, walls or even on the floor.

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