This part of IEC 61094 gives a polynomial function derived from a least square fit to data from several laboratories, for the differences between free field and pressure sensitivity levels of laboratory standard microphones as specified in IEC 61094 1; enables determination of the free-field sensitivity level of a laboratory standard microphone for zero-degrees incidence in air by adding values of these differences to the pressure sensitivity level; gives tabulated values for the polynomial function for a range of frequency and temperature; is applicable when a suitable free field calibration is not available.