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.
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.
Types 4160 and 4180 are both high stability condenser microphones for laboratory standard applications and in coupler measurements.
Type 4160 has been used for some time in such applications and conforms to IEC 327 and 402, ANSI S1.12-1967 and ANSI Sl.lO-1966, JIS C 5515, and the current IEC draft standard for 1” laboratory standard microphones. It has become established as a high precision reference standard and is used in the medium frequency ranges and for measurements at low sound pressure levels.
Type 4180 has been developed to meet a growing need for a laboratory standard ½" microphone which enables measurements at higher frequencies and at higher sound pressure levels. It has been designed to comply with the most recent drafts of laboratory standard microphone standards. Types 4160 and 4180 are delivered with individual calibration charts.
