Externally-polarized microphones

These microphones are used with standard preamplifiers such as the Type 26AK, which has a 7-pin LEMO connector. The preamplifier should be connected to a power module (for example Type 12AK) or an analyzer input which can supply the preamplifier with power as well as 200 V polarization.

Externally-polarized microphones are the most accurate and stable and are to be preferred for very critical measurements.

Prepolarized microphones

These microphones are used typically with CCP (Constant Current Power*) preamplifiers such as Type 26CA. Prepolarized microphones must be connected to an input stage for CCP transducers or be powered by a CCP supply, for example the Type 12AL.

CCP preamplifiers use standard coaxial cables, thus reducing costs. On the other hand the long term stability and high temperature stability of prepolarized microphones are not as good as for externally polarized microphones.

Free-field microphones

A free-field microphone is designed essentially to measure the sound pressure as it existed before the microphone was introduced into
the sound field. At higher frequencies the presence of the microphone itself in the sound field will disturb the sound pressure locally. The frequency response of a freefield microphone has been carefully adjusted to compensate for the disturbances to the local sound field. (See also random-incidence microphones)

Free-field microphones are recommended for most sound pressure level measurements for example with sound level meters, sound power measurements and sound radiation studies.

Pressure microphones

A pressure microphone is for measuring the actual sound pressure, as it exists on the surface of the microphone’s diaphragm. A typical application is in the measurement of sound pressure in a closed coupler or, as shown below, the measurement of sound pressure at a boundary or wall; in which case the microphone forms part of the wall and measures the sound pressure on the wall itself.

Pressure microphones are recommended for use with couplers like RA0045 IEC 711 coupler, 2cc coupler and for studies of sound pressures inside closed cavities.

Random-incidence microphones

A random-incidence microphone is for measuring in sound fields, where the sound comes from many different directions e.g. when measuring in a reverberation chamber or in other highly reflecting surroundings

The combined influence of sound waves coming from all directions depends on how these sound waves are distributed over the various directions. For measurement microphones, a standard distribution has been defined based on statistical considerations; resulting in a standardised random-incidence microphone.

Random-incidence is used typically for sound pressure level measurements according to ANSI standards.

Upper limit of dynamic range

The highest levels that can be measured are limited by the amount of movement allowed for the diaphragm before it comes into contact with the microphone’s back plate.

As the level of the sound pressure on a microphone increases, the deflection of the diaphragm will accordingly be greater and greater until, at some point, the diaphragm strikes the back plate inside the body of the microphone. This is ultimately at the highest level the microphone can measure.

Lower limit of dynamic range

The thermal agitation of air molecules is sufficient for a microphone to generate a very small output signal, even in absolutely quiet conditions. This “thermal noise” lies normally at around 5 μV and will be superimposed on any acoustically-excited signal detected by the microphone. Because of this, no acousticallyexcited signal below the level of the thermal noise can be measured.

The dynamic ranges of various G.R.A.S. microphones are shown in the chart below. Different colours are used to distinguish between pressure (dark grey), free-field (orange) and random-incidence (light grey) microphones.

Upper-limiting frequency

The upper-limiting frequency is linked to the size of the microphone, or more precisely, the size of the microphone compared with the wavelength of sound. Since wavelength is inversely proportional to frequency, it gets progressively shorter at higher frequencies. Hence, the smaller the diameter of the microphone, the higher are the frequencies it can measure. On the other hand, the sensitivity of a microphone is also related to its size which also affects its dynamic range. 

The frequency ranges of various G.R.A.S. microphones are shown in the chart below. Different colours are used to distinguish between pressure (dark grey), free-field (orange) and random-incidence (light grey) microphones.

Lower-limiting frequency

The lower-limiting frequency of a microphone is determined by its static pressure equalisation system. Basically, a microphone measures the difference between its internal pressure and the ambient pressure.

If the microphone was completely airtight, changes in barometric pressure and altitude would result in a static deflection of its diaphragm and, consequently, in a change of frequency response and sensitivity. 

To avoid this, the microphone is manufactured with a static-pressure equalisation channel for equalising the internal pressure with ambient pressure. On the other hand, equalisation must be slow enough to avoid affecting the measurement of dynamic signals.