VIBRATION
ACOUSTICS
Dynamic Range of a Microphone
The dynamic range of a microphone is defined as the range between the lowest level and the highest level that the microphone can handle. It not only depends on the microphone, but also on the preamp that is used with it. The dynamic range of a microphone is, to a large extent, directly related to its sensitivity.
Sensitivity of a microphone
The sensitivity of a microphone is determined by its size and the tension of its diaphragm. A large microphone, with a loose diaphragm, will have a high sensitivity and a small microphone, with a rigid 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 a low sensitivity will be able to measure very high levels, but not very low levels.
Lower limit of dynamic range
The thermal agitation of the air molecules is sufficient for a microphone to generate a very small output signal, even under conditions of absolute silence. This “thermal noise” is typically around 5 µV and will overlap any acoustically excited signal detected by the microphone. Therefore, no acoustically excited signal can be measured below the thermal noise level.
Upper limit of dynamic range
The highest levels that can be measured are limited by the amount of movement the diaphragm allows before contacting the microphone backplate. As the sound pressure level in a microphone increases, the deflection of the diaphragm will consequently increase until, at some point, the diaphragm hits the back plate inside the microphone body. Ultimately, this is the highest level the microphone can measure.
Frequency Range of a Microphone
The frequency range of a microphone is defined as the interval between its upper limit frequency and its lower limit frequency. Today’s microphones can cover a frequency range that starts at around 1Hz and goes up to 140kHz.
Low-frequency measurements require a microphone with well-controlled static pressure equalization with very slow ventilation. High-frequency measurements are highly sensitive to diaphragm stiffness, damping, and mass, as well as diffraction.
Upper limit frequency
The upper limit frequency is related to the size of the microphone compared to the wavelength of the sound. Since wavelength is inversely proportional to frequency, it progressively shortens at higher frequencies. The smaller the diameter of the microphone, the higher the frequencies it can measure. The sensitivity of a microphone is also related to its size, which also affects its dynamic range.
Lower limit frequency
The lower limit 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 were completely hermetic, changes in barometric pressure and altitude would cause a static deflection of its diaphragm and consequently a change in frequency response and sensitivity. To avoid this, the microphone is manufactured with a static pressure equalization channel to equalize internal pressure with ambient pressure. The equalization must be slow enough not to affect the measurement of dynamic signals.
Free Field, Pressure or Random Incidence Microphones
There are three types of measurement microphones: free-field, pressure, and random incidence.
The differences between these three types of measurement microphones are seen at higher frequencies, where the size of a microphone becomes comparable to the wavelengths of the sound being measured.
Free field microphones
A free field microphone is essentially designed to measure 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 local sound field disturbances. Free-field microphones are recommended for most sound pressure level measurements, for example with sound level meters, sound power measurements, and sound radiation studies.
Pressure Microphones
A pressure microphone is used to measure the actual sound pressure at the surface of the microphone diaphragm. A typical application is the measurement of the sound pressure in a closed coupler or the measurement of the sound pressure in a boundary or wall; in this case the microphone is part of the wall and measures the sound pressure on the wall itself. It is recommended to use pressure microphones with couplers such as GRAS RA0045 IEC 60318-4 and RA0038 IEC 60318-5, 2cc coupler and for sound pressure studies inside closed cavities.
Random Incident Microphones
A random incidence microphone is used to measure in sound fields where sound is coming from many directions, for example when measuring in a reverb chamber or other highly reflective environments. The combined influence of sound waves from all directions depends on how these sound waves are distributed in different directions. For measurement microphones, a standard distribution has been defined based on statistical considerations; the result is a standardized random incidence microphone. Random incidence microphones are recommended for sound pressure level measurements in accordance with ANSI standards.
