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.
These could include the National Physical Laboratory in the UK, PTB in Germany, NIST in the USA and the National Measurement Institute, Australia, where the reciprocity calibration (see below) is the internationally recognised means of realising the primary standard.
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 sound levels beyond the device’s operational range.
The technique exploits the reciprocal nature of certain transduction mechanisms such as the electrostatic transducer principle used in condenser measurement microphones.
to act as the source of sound and the other responds to the pressure generated in the coupler, producing an output voltage
Having determined the product of the transmission factors for one pair of microphones, the process is repeated with the other two possible pair-wise combinations
The set of three measurements then allows the individual microphone transmission factor 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.
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.
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.
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 dependent 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, 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.
Sound calibrators are different from pistonphones in that they work electronically and use a low-impedance (electrodynamic) source to yield a high degree of volume independent operation.
Sound calibrators tend to be less precise than pistonphones, but are (nominally) independent of internal cavity volume and ambient pressure.