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 typically used to measure loudspeakers or outdoor sound.
In order to make measurements in a free-field, totally without reflecting objects, the measurements must be made outdoors at the top of a flagpole (or equivalent) or in an anechoic chamber.
In an anechoic chamber the ceiling, floor and all the walls are covered by a highly absorptive material which eliminates reflections. Thus, the sound pressure level in any given direction from the noise source may be measured without the presence of interfering reflections.
