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. In all cases, the microphones discussed in the following are condenser microphones.
Pressure microphones
A pressure microphone is for measuring the actual sound pressure as it exists 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.
Free-field microphones
A free-field microphone is designed essentially to measure the sound pressure as it existed 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. In general, the sound pressure around a microphone cartridge will increase because of reflections and diffraction.
The frequency characteristics of a free-field microphone are designed to compensate for this increase in pressure; hence the output of a free-field microphone is a signal proportional to the sound pressure as it existed before the microphone was introduced into the sound field. A free-field microphone should always be pointed towards the sound source (0° incidence) as shown above. In this situation the presence of the microphone's diaphragm in the sound field will result in a pressure increase in front of the diaphragm depending on the wavelength of the sound and the diameter of the microphone, see curve (a) below.
For a typical ½" microphone, the maximum pressure increase will occur at 26.9kHz, where the wavelength of the sound (λ) coincides with the diameter of the microphone, i.e.:
| λ = | 342 m/s26.9 kHz | ≈ 12.7mm = ½” |
By design, the sensitivity of the microphone must decreases accordingly by an amount which compensates for this increase in acoustical pressure in front of the diaphragm. This is done by increasing the acoustical damping within the cartridge of the microphone so as to obtain the pressure response shown below in curve (b).
The result is an output from the microphone cartridge which is proportional to the sound pressure as it existed before the microphone was introduced into the sound field, see curve (c) below. Curve (a) above is also called the free-field correction curve for the microphone and must be added to the pressure response of the microphone shown in curve (b), to obtain the characteristics of a free-field microphone shown in curve (c).
Random-incidence microphones
In principle, a free-field microphone requires to be pointed towards the sound source and that the sound waves travel, essentially, in one direction. In some cases, e.g. when measuring in a reverberation chamber or in other highly reflecting surroundings, sound waves will not have a well defined direction of propagation, but will arrive simultaneously at the microphone from various directions.
Sound waves arriving at the microphone from the front will cause a pressure increase as described above for a free-field microphone, whereas sound waves arriving from behind the microphone will, to a certain extent, cause a pressure decrease because of the shadowing effects of the microphone. The combined influence of sound waves coming from all directions depends, therefore, on how these sound waves are distributed over these various directions. For measurement microphones, a standard distribution has been defined based on statistical considerations; resulting in a standardized random-incidence microphone.
Pre-polarized or externally-polarized microphones
Externally-polarized microphones are used with standard preamplifiers, which have a 7-pin LEMO connector. The preamplifier should be connected to a power module or an analyzer input which can supply the preamplifier with power as well as 200 V polarization.
Pre-polarized microphones are used typically with CCP (Constant Current Power) preamplifiers. These microphones must be connected to an input stage for CCP transducers or be powered by a CCP supply.
National Instruments recommends pre-polarized microphones and constant current powered (CCP) preamplifiers for use with NI devices that provide IEPE (Integrated Electronic Piezo-Electric) signal conditioning.
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.
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 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.
The frequency ranges of various G.R.A.S. microphones are shown in the chart below. The Type or Model number of each microphone is shown. The microphones are grouped according to size of external diameter, i.e. 1”, 1/2”, 1/4” and 1/8”.
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.
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 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.
The sensitivity of a microphone is determined chiefly by the size of the microphone and the tension of its diaphragm. Generally speaking, 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.
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.
In fact, as the deflection of the diaphragm becomes large, the relationship between diaphragm deflection and the consequent change in microphone capacity becomes non-linear and results in distorting the output signal of the microphone. Because of this, the upper limit of the dynamic range is described as that level where distortion reaches 3%. A distortion limit of 10% usually occurs at about 6 dB higher.
The dynamic ranges of various G.R.A.S. microphones are shown in the chart below.
The Type or Model number of each microphone is shown. The microphones are
grouped according to size of external diameter, i.e. 1”, 1/2”, 1/4” and 1/8”.
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 5µV 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.
This 5µV output signal is equivalent to a certain "apparent" sound pressure level which can be calculated from the sensitivity of the microphone. For a microphone with a sensitivity of 50mV/Pa, this would correspond to an apparent sound pressure of:
5µV 50mV/Pa | = 0.0001 Pa; in other words around 14 dB re. 20 µPa |
Similarly, for a microphone with a sensitivity of 4mV/Pa, this would correspond to an apparent sound pressure of:
5µV 4mV/Pa | = 0.00125 Pa; in other words around 36 dB re. 20 µPa |
Hence, a microphone with a sensitivity of 50mV/Pa can measure down to about 14 dB whereas a microphone with a sensitivity of 4mV/Pa can only measure down to about 36 dB.
In practice, a microphone needs to be connected to a high-impedance preamplifier in order to handle the very weak output signal from the microphone. A preamplifier also has a certain amount of noise which will be added to the thermal noise generated by the microphone. See Preamplifiers.
