Free-field, Pressure or Random incidence
There are three types of measurement microphones: Free-field, Pressure, and Random incidence.
You can see the differences between these three types of measurement microphones at the higher frequencies, where the size of a microphone becomes comparable with the wavelengths of the sound being measured.
What is a free-field microphone?
A free-field microphone is designed essentially to measure the sound pressure as it was 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 free field microphon has been carefully adjusted to compensate for the disturbances to the local sound field.
We recommend that you use free-field microphones for most sound pressure level measurements for example with sound level meters, sound power measurements and sound radiation studies.
What is a pressure microphone?
A pressure microphone is for measuring the actual sound pressure on the surface of the microphone’s diaphragm.
A typical application is in the measurement of sound pressure in a closed coupler or 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.
We recommend that you use pressure microphones with couplers like GRAS RA0045 IEC 60318-4 and RA0038 IEC 60318-5, 2cc coupler and for studies of sound pressures inside closed cavities.
Random Incidence microphones
What is a random incidence microphone?
A random incidence microphone is for measuring in sound fields, where the sound comes from many 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 standardized random incidence microphone.
We recommend that you use random incidence microphones for sound pressure level measurements according to ANSI standards.