A pressure-field is sound pressure that has the same magnitude and phase at any position within the field. Sound fields with a close resemblance to the pressure field can be achieved in small cavities (small compared to the wavelength) such as artificial ears.
Pressure-field microphones are typically used in couplers, wind tunnels or in flush-mounted measurements.
The opposite of an anechoic chamber is the reverberation chamber where all surfaces are made as hard and reflective as possible and where no parallel surfaces exist. This causes a diffuse-field because all the sound waves are arriving simultaneously from all directions with equal probability and level.
Sound fields with a close resemblance to diffuse-field can be achieved in environments such as buildings with hard walls, where many simultaneous sound or noise sources exist, such as with in-cabin measurements and in churches.
If you are not sure about the sound field you are in, it is best to assume that it is diffuse to minimize measurement error. Diffuse-field microphones are typically used to measure vehicle interior noise or building acoustics.
Free‐field microphones are designed to have a flat frequency response in a free field. Free‐field microphones are commonly used for sound measurement in an anechoic chamber or far away from reflecting buildings, etc. Another application area for free‐field microphones is general electroacoustic purposes, like loudspeaker and microphone measurements.
