The free-field correction of laboratory standard microphones

The free-field sensitivity of a microphone is related to the product of its pressure sensitivity, the scattering factor, and the load of the radiation impedance on the diaphragm. The product of the last two factors is called the free-field correction and describes how much the sensitivity of the microphone differs from its pressure sensitivity. By definition the free-field sensitivity implies a plane wave impinging on the microphone. However, in practice the free-field reciprocity technique is based on the substitution of the microphones by point sources with a given diffraction factor. At sufficient distances spherical waves approximate plane waves, but this may not be the case when the microphones are calibrated using the reciprocity technique where the microphones are usually quite close to each other because of the very poor signal-to-noise ratio. An analysis of the resulting deviations is required for determining an accurate estimate of the microphone sensitivity. The problem has been analysed numerically. The numerical results are complemented with experimental data.