How to Select 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. If you conduct very critical measurements we recommend an externally polarized microphone which is the most accurate and stable.
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, which will reduce your costs. The long term 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.
What is a free-field microphone?
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 microphon has been carefully adjusted to compensate for the disturbances to the local sound field.
We recommend 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.
What is a pressure microphone?
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.
We recommend that you use pressure microphones 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
What is a 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.
We recommend that you use random incidence microphones 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.
The dynamic ranges of various GRAS microphones are shown in the chart below:
Sensivity of a microphone
The sensitivity of a microphone is determined by the size of the microphone and the tension of its diaphragm. A large microphone, with a loose diaphragm, will have a high sensitivity and a small microphone, with a stiff diaphragm, will have a low sensitivity. A microphone with a high sensitivity will be able to measure very low levels, but not very high levels, and a microphone with low sensitivity will be able to measure very high levels, but not very low levels.
Upper limit of dynamic range
The highest levels that can be measured are limited by the amount of movement allowed for the diaphragm before it comes into contact with the microphone’s back plate. As the level of the sound pressure on a microphone increases, the deflection of the diaphragm will accordingly be greater and greater until, at some point, the diaphragm strikes the back plate inside the body of the microphone. This is ultimately at the highest level the microphone can measure.
Lower limit of dynamic range
The thermal agitation of air molecules is sufficient for a microphone to generate a very small output signal, even in absolutely quiet conditions. This “thermal noise” lies normally at around 5 μV and will be superimposed on any acoustically excited signal detected by the microphone. Because of this, no acoustically excited signal below the level of the thermal noise can be measured.
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.
Low frequency measurements require a microphone with a well controlled static pressure equalization with a very slow venting. High frequency measurements are very sensitive to diaphragm stiffness, damping and mass as well as diffraction.
The frequency ranges of various G.R.A.S. microphones are shown in the chart below. Different colours are used to distinguish between pressure (dark grey), free-field (orange) and random incidence (light grey) microphones.
Upper limiting frequency
The upper limiting frequency is linked to the size of the microphone compared with the wavelength of sound. Since wavelength is inversely proportional to frequency, it gets progressively shorter at higher frequencies. The smaller the diameter of the microphone, the higher are the frequencies it can measure. The sensitivity of a microphone is also related to its size which also affects its dynamic range.
Lower limiting frequency
The lower limiting frequency of a microphone is determined by its static pressure equalization system. Basically, a microphone measures the difference between its internal pressure and the ambient pressure. If the microphone was completely airtight, changes in barometric pressure and altitude would result in a static deflection of its diaphragm and, consequently, in a change of frequency response and sensitivity. To avoid this, the microphone is manufactured with a static pressure equalization channel for equalising the internal pressure with ambient pressure. Equalization must be slow enough to avoid affecting the measurement of dynamic signals.
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