When introducing an acoustic microphone into a sound field, it is important to note that any microphone will disturb the sound field you are trying to measure. The reflections and diffraction due to the physical presence of the microphone can affect the measured pressure level which result in an inaccurate measurement reading. Therefore knowing which field you will be operating in will help you choose the right type of measurement microphone for your application.
Measurement microphones are divided into three different categories according to their response in the sound field: Free-Field, Pressure, and Random-Incidence. The main differences occur at the higher frequencies where the physical size of a microphone becomes comparable with respect to the wavelengths of the sound being measured. With this each their own capabilities and applications.
The different types of measurement microphone are as follows:
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 microphone has been carefully adjusted to compensate for the disturbances to the local sound field.
TME Systems recommends 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.
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.
TME systems recommends that you use pressure microphones with couplers such as the G.R.A.S. RA0045 IEC 60318-4 and RA0038 IEC 60318-5, 2cc coupler and for studies of sound pressures inside closed cavities.
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.
TME Systems recommend that you use random incidence microphones for sound pressure level measurements according to ANSI standards.
For a free consultation to your sound measurement needs, contact TME Systems today!