The paper gives a brief description of the principles and the uncertainty of the acoustic
calibration methods that today are applied by National Metrology Institutes and
calibration service centers. Even if some of the calibration principles have been applied
over more than half a century, the methods and the instrumentation are still being refined
in order to minimize their uncertainty, to extend their frequency ranges, to include extra
parameters and to speed up slow processes. In addition to the traditional methods for
microphone sensitivity and frequency response calibration, new development areas, like for
example wind power, has created needs for low-frequency and infra-sound calibration, down
to 0.1 Hz. Other high-tech areas have lead to the development of methods for phase
response comparison calibration of microphones for large arrays, for sound intensity
measurement and for verification of dynamic linearity of microphones at very high sound
pressure levels, up to about 174 dB that corresponds to 10 kPa.