Sound and Vibration Basics
Measurement Microphones
When you introduce a microphone into a sound field, the microphone disturbs the sound field you are trying to measure. The reflections and diffraction due to the physical presence of the microphone cause an increase in the measured pressure level. **
The maximum sound pressure 'build-up' occurs when the diameter of the microphone coincides with the wavelength of sound, for 1/2" microphones this occurs at 27 kHz. If you are measuring with the microphone as part of the structure i.e. flush mounted in a wall or inside an ear-muff and you need to know the sound pressure inside the coupler then this is the microphone you need and they are called pressure microphones.
In most measurement applications, however, you want to measure the noise levels without any interference from the microphone i.e. what the level was before you introduced the microphone. In this case the microphone designers introduce acoustic damping equal and opposite to the pressure increase and to the result is known as a free-field microphone.
However the build-up of sound pressure across the microphone diaphragm also varies with the angle of incidence of the sound wave so the compensation introduced by the designers is only 'true' at one angle of incidence. Therefore when using free-field, omnidirectional microphones the meter should be pointing towards the noise source - 0 degree angle of incidence.
If you happen to be inside a highly reflective room like a reverberation chamber the sound waves will bounce off multiple surfaces and arrive at the microphone simultaneously. For these measurements you need the third type of measurement microphone, called a random incidence microphone.
Most sound level meters are delivered with a 1/2-inch free-field microphone, suitable for most applications.
Measurement microphones are condenser or capacitive devices and therefore require a polarisation voltage. In the early days the polarisation voltage, usually 200 VDC, was supplied separately by the meter. Pre-polarised, also known as electret microphones, which do not require an external supply, were introduced later. Both versions are in current use although some traditionalists claim the externally polarised or air-condenser microphones are more stable and more accurate.
The upper frequency range of the microphone depends upon the diameter of the microphone and the wavelength of the sound. For 1/2-inch microphones the maximum pressure build-up occurs at 27 kHz giving an 'agreed' practical upper limit of 20 kHz
If you halve the size of the microphone you double the frequency range, but the area of the diaphragm has gone down by a factor of 4 so the sensitivity - signal out reduces by about 14 dB. Therefore 1/4-inch microphones are not suitable for low level measurements. However for measurements above 146 dB the practical limit for most 1/2-inch microphones, the smaller diameter, less sensitive microphones come into their own.
Measurement microphones may be referred to by their IEC 61094 Classifications, for example
WS1F = Working Standard, 1.0", free-field response.
WS2P = Working Standard, 1/2", pressure response.
WS3D = Working Standard, 1/4", diffuse field response.
LS instead of WS indicates Laboratory Standard
See also : Free-field Microphones, Pressure Microphones, Diffuse-field or Random-incidence Microphones
** The above notes assume the Acoustic Impedance of the microphone, meter and operator, if present, are similar to the air they displace. This is obviously not true but the effects are usually small compared to the sound pressure levels of interest. however it should be borne in mind.
Certified Measurement Microphones
Standards : BS EN 61094 Measurement Microphones
