Calibration of a microphone in which the open-circuit voltage produced by a certain value of sound pressure variation is determined. The presence of the microphone upsets the free field that existed before the introduction of the microphone, causing difficulties in determining the pressure value. At higher frequencies, when the dimensions of the microphone are comparable with the wavelength, the actual pressure on the pressure-sensitive portion of the microphone can approach twice that of the actual pressure in the free wave. At lower frequencies, where the dimensions are small compared to wavelength, the pressures are substantially equal.