Measurement Microphone Guide
Your guide to selecting the right measurement microphone
Measurement microphones are available in many types covering various frequency ranges, dynamic ranges and application situations. GRAS measurement microphones are amongst the world's best.
With this guide, you can find help to select the right measurement microphone for your applications. Get introduced to selecting externally polarized or prepolarized microphones, free-field, pressure or random incidence microphones as well as learning about the dynamic range of a microphone and the frequency range of a microphone
Externally polarized microphone or prepolarized microphone
Can you tell the difference?
All GRAS measurement microphones are of the condenser type.
This requires a polarization voltage which can either be supplied from an external power supply or the microphone itself can be polarized by injecting a permanent electrical charge into a thin PTFE layer on the microphone backplate.
Externally Polarized Microphones
These microphones are used with standard preamplifiers such as the GRAS 26AK, which has a 7-pin LEMO connector. The preamplifier must be connected to a power module (for example GRAS 12AK) or an analyzer input which can supply the preamplifier with power as well as 200 V polarization for the microphone capsule.
Externally polarized microphones are the most accurate and stable and are to be preferred for very critical measurements.
These microphones are used typically with CCP (Constant Current Power) preamplifiers such as GRAS 26CA. Prepolarized microphones must be connected to
an input stage for CCP transducers or be powered by a CCP supply, for example the GRAS 12AL. CCP preamplifiers use standard coaxial cables, thus educing costs. On the other hand the long term stability and high temperature stability of prepolarized microphones are not as good as for externally polarized microphones.
Free-field microphone or Pressure microphone or Random incidence microphone
Which one should you choose?
There are three types of measurement microphones: Free-field, Pressure, and Random incidence.
You can see the differences between these three types of measurement microphones at the higher frequencies, where the size of a microphone becomes comparable with the wavelengths of the sound being measured.
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 freefield microphone has been carefully adjusted to compensate for the disturbances to the local sound field. (See also random incidence microphones). Free-field microphones are recommended 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, as shown to the left, 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. Pressure microphones are recommended for use with couplers like GRAS RA0045 IEC 60318-4 and RA0038 IEC 60318-5, 2cc coupler and for studies of sound pressures inside closed cavities.
Random Incidence Microphones
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. Random incidence is used typically for sound pressure level measurements according to ANSI standards.
Dynamic range of a microphone
Which levels can the microphone handle?
The dynamic range of a microphone is defined as the range between the lowest level and the highest level which the microphone can handle.
This is not only a function of the microphone alone, but also of the preamplifier used with the microphone. The dynamic range of a microphone is, to a large extent, directly linked to its sensitivity.
Frequency range of a microphone
Which one should you choose?
The frequency range of a microphone is defined as the interval between its upper limiting frequency and its lower limiting frequency.
With today’s microphones you can cover a frequency range starting from around 1Hz and reaching up to 140 kHz.