- Measurement microphone calibration
In order to take a scientific measurement with a microphone, its precise sensitivity must be known (in volts per Pascal). Since this may change over the lifetime of the device, it is necessary to regularly calibrate measurement microphones. This service is offered by some microphone manufacturers and by independent certified testing labs. All microphone calibration is ultimately traceable to primary standards at a National Measurement Institute such as the National Physical Laboratory in the UK, PTB in Germany and NIST in the USA, where the reciprocity calibration (see below) is the internationally recognised means of realising the primary standard. Laboratory standard microphones calibrated using this method are used in-turn to calibrate other microphones using comparison calibration techniques (‘secondary calibration’), referencing the output of the ‘test’ microphone against that of the reference laboratory standard microphone.
A microphone’s sensitivity varies with frequency (as well as with other factors such as environmental conditions) and is therefore normally recorded as several sensitivity values, each for a specific frequency band (see frequency spectrum). A microphone’s sensitivity can also depend on the nature of the sound field it is exposed to. For this reason microphones are often calibrated in more than one sound field, for example a pressure field and a free field. Depending on their application, measurement microphones must be tested periodically (every year or several months, typically), and after any potentially damaging event, such as being dropped or exposed to sounds levels beyond the device’s operational range.
Reciprocity calibration is currently the favoured primary standard for calibration of measurement microphones. The technique exploits the reciprocal nature of certain transduction mechanisms; measurement microphones are usually capacitor microphones (also called condenser microphones) which exhibit this behaviour. Reciprocity calibration is carried out using an acoustical coupler (to give the microphone’s pressure response), but can also be implemented in a free field (to give the free-field response). In order to carry out ‘pressure field’ reciprocity calibration, three uncalibrated microphones A, B and C are used. Microphones A and B are placed facing each other in an acoustical coupler designed to create a cylindrical cavity between their diaphragms, allowing the space to be easily modelled. One of the microphones is then driven electrically to act as the source of sound, and the other responds to the pressure generated in the coupler, producing an output voltage. Provided that the microphones are reciprocal in behaviour (the open circuit sensitivity in V/Pa as a receiver is the same as the free sensitivity in m³/s/A as a transmitter), it can be shown that the combined sensitivity product of the coupled microphones is given by the ratio of electrical transfer impedance to acoustical transfer impedance. The first is measured in the calibration procedure, and the second deduced from transmission line analysis. Having determined the sensitivity product for one pair of microphones, the process is repeated with the other two possible pair-wise combinations (AC and BC). The set of three sensitivity product measurements then allows the individual microphone sensitivities to be deduced by solving three simultaneous equations. The technique provides a measurement of the sensitivity of a microphone without the need for comparison with another previously calibrated microphone, and is instead traceable to reference electrical quantities such as volts and ohms, as well as length, mass and time. Although a given calibrated microphone will often have been calibrated by other (secondary) methods, all can be traced (through a process of dissemination) back to a microphone calibrated using the reciprocity method at a National Measurement Institute. Reciprocity calibration is a specialist process, and because it forms the basis of the primary standard for sound pressure, many national measurement institutes have invested significant research efforts to refine the method and develop calibration facilities. A system is also commercially available from Brüel & Kjær.
For airborne acoustics, the reciprocity technique is currently the most precise method available for microphone calibration (i.e. has the smallest uncertainty of measurement). Free field reciprocity calibration (to give the free-field response, as opposed to the pressure response of the microphone) follows the same principles and roughly the same method as pressure reciprocity calibration, but in practice is much more difficult to implement. As such it is more usual to perform reciprocity calibration in an acoustical coupler, and then apply a correction if the microphone is to be used in free-field conditions; such corrections are standardised for laboratory standard microphones (IEC/TS 61094-7) and are generally available from the manufacturers of most of the common microphone types.
A comparison calibration determines a microphone’s sensitivity by comparing its electrical response to a sound field (ie. its sensitivity) against that of a previously calibrated microphone (a reference microphone). Since the sensitivity of the reference microphone is already known, if the 2 microphones are exposed to the same stimulus (sound-field), then the responses can be directly compared. Comparison calibration can be carried out against a microphone which was itself calibrated by comparison method (or another secondary method); however it would more normally be referenced against a reciprocity calibrated microphone, due to the added uncertainty of measurement introduced by each ‘step’ in the calibration chain.
A common method of comparison calibration requires the two microphones to be placed facing each other, closely spaced so that the pressure on both of the microphones’ diaphragms can be assumed to be equal at the frequencies of interest (the audible range). Both microphones are subjected to a broadband noise signal from a loudspeaker (typically white noise) so as to include every frequency of interest, and the electrical output signals from both microphones are subject to frequency analysis, resulting in the frequency response of the microphones to the specific sound they were exposed to. By this method the ratio of sensitivities of the two microphones can be determined. This can then be multiplied by the known sensitivity of the calibrated reference microphone in order to obtain the absolute sensitivity of the test microphone. Comparison calibration may be carried out in a free-field environment (typically in an anechoic chamber), a diffuse field (a reverb chamber) or a coupler, depending on the application the microphone it is intended for.
Calibration using Pistonphones and Sound Calibrators
A pistonphone is an acoustical calibrator (sound source) that uses a closed coupling volume to generate a precise sound pressure for the calibration of measurement microphones. The principle relies on a piston mechanically driven to move at a specified cyclic rate, pushing on a fixed volume of air to which the microphone under test is coupled. The air is assumed to be compressed adiabatically and the sound pressure level in the chamber can, potentially, be calculated from internal physical dimensions of the device and the adiabatic gas law, which requires that PVγ is a constant, where P is the pressure in the chamber, V is the volume of the chamber, and γ is the ratio of the specific heat of air at constant pressure to its specific heat at constant volume. Pistonphones are highly dependant on ambient pressure (always requiring a correction to ambient pressure conditions) and are generally only made to reproduce low frequencies (for practical reasons), typically 250 Hz. However, pistonphones can be very precise, with good stability over time.
However, commercially available pistonphones are not calculable devices and must themselves be calibrated using a calibrated microphone if the results are to be traceable; though generally very stable over time, there will be small differences in the sound pressure level generated between different pistonphones. Since their output is also dependent on the volume of the chamber (coupling volume), differences in shape and load volume between different models of microphone will have an influence on the resulting SPL, requiring the pistonphone to be calibrated accordingly.
Sound calibrators are used in an identical way to pistonphones, providing a known sound pressure field in a cavity to which a test microphone is coupled. Sound calibrators are different to pistonphones in that they work electronically and use a low-impedance (electrodynamic) source to yield a high degree of volume independent operation. Furthermore, modern devices often use a feedback mechanism to monitor and adjust the sound pressure level in the cavity so that it is constant regardless of the cavity / microphone size. Sound calibrators normally generate a 1 kHz sine tone; 1 kHz is chosen since the A-weighted SPL is equal to the linear level at 1 kHz. Sound calibrators should also be calibrated regularly at a nationally accredited calibration laboratory to ensure traceability. Sound calibrators tend to be less precise than pistonphones, but are (nominally) independent of internal cavity volume and ambient pressure.
- IEC 61094-2, edition 2. (February 20, 2009) "Measurement Microphones, part 2". IEC Standard for Pressure Reciprocity Calibration of Measurement Microphones
- IEC 61094-5, edition 1. (October 16, 2001) "Measurement Microphones, part 5". IEC Standard for Comparison Calibration of Measurement Microphones
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