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.