Measurement microphones are most commonly condenser microphones in which sound pressure actuates a diaphragm that is one plate of a capacitor. The relative motion of the capacitor plates causes a change in capacitance and the resulting voltage is measured and converted into a reading such as sound pressure level (SPL).
Measurement microphones are described by type of sound field. Pressure field measurement microphones are used to measure sound pressure that has the same magnitude and phase throughout the field. Free field microphones are used to measure a plane wave that is propagating freely in one defined direction. Diffuse field measurement microphones, also known as random incidence microphones, measure sound waves from all directions.
The critical specifications for measurement microphones are frequency response, dynamic range, microphone diameter and sensitivity. Frequency response is the range of frequencies for which the microphone maintains a constant sensitivity within defined boundaries. Dynamic Range is the range of sound pressure levels (dB) for which the microphone will meet its performance specifications. It is limited on the low end by the inherent noise of the acoustic system and on the high end by the maximum sound pressure level. The standard unit for measuring sound pressure level is the deciBel (dB) with a reference pressure of 20 micro Pascals, which is the human hearing threshold. Microphone diameter has an effect on the dynamic characteristics. In general, larger diaphragms are more sensitive and have narrower frequency response. Sensitivity is typically given in millivolt output per Pascal of sound pressure. A correctly specified signal should exceed the inherent noise of the system yet not overload the preamplifier.
Many features are available for measurement microphones. An integral preamplifier boosts the microphone output from a small to intermediate level. Probe style measurement microphones are designed for measuring near-field levels or for use in hard-to-reach areas. Array style devices can be used to measure a 3-D sound field around a test object. Other features include ratings for outdoor use, a wind or turbulence screen to minimize noise and an accessory sound intensity probe with closely matched microphone pairs.
Measurement condenser microphones may have a prepolarized electret backplate or utilize external capacitor polarization. External polarization requires a separate power supply.
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