Selecting a microphone involves a number of choices:
- externally polarized or pre-polarized,
- free-field, pressure or random incidence,
- dynamic range,
- frequency range.
Externally polarized or pre-polarized
Condenser type microphones require a polarization voltage that 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. The preamplifier must be connected to a power module or an analyzer input which can supply the preamplifier with power as well as 200 V for polarization. Externally polarized microphones are the most accurate and stable and are preferred for very critical measurements.
These microphones are used typically with constant current power preamplifiers. Pre-polarized microphones must be connected to an input stage for constant current power transducers or be powered by a constant current power supply.
Constant current power preamplifiers use standard coaxial cables. The long-term stability and high-temperature stability of pre-polarized microphones are not as good as for externally polarized microphones.
Free-field, pressure or random incidence
Measurement microphones can be divided into three groups: free-field, pressure, and random incidence. The differences between microphones from group to group are 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 free field microphone has been carefully adjusted to compensate for the disturbances to the local sound field.
Image 18: Free-field microphone
Free-field microphones are recommended for most sound pressure level measurements for example with sound level meters and sound power measurements.
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 below, 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 studies of sound pressures inside closed cavities.
Image 19: Pressure microphone
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.
Image 20: Random incidence microphone
The dynamic range of a microphone
The dynamic range of a microphone can be defined as the range between the lowest level and the highest level that 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.
In general, a microphone with 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. 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 low sensitivity.
The upper limit of the 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.
The lower limit of the dynamic range
The thermal agitation of air molecules is sufficient for a microphone to generate a tiny output signal, even in absolutely quiet conditions. This thermal noise normally lies at around 5 V 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.
The frequency range of a microphone
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, it is possible to 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. Special versions are available for infrasound measurements.
High-frequency measurements are very sensitive to diaphragm stiffness, damping, and mass, as well as diffraction.
Upper limiting frequency
The upper limiting frequency is linked to the size of the microphone, or more precisely, 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. Hence, the smaller the diameter of the microphone, the higher are the frequencies it can measure. On the other hand, 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 equalizing the internal pressure with ambient pressure. On the other hand, equalization must be slow enough to avoid affecting the measurement of dynamic signals.