IEC 60655 Ed. 1.0 b:1979

Gives the values that are to be added to the pressure sensitivity level of a microphone in order to obtain the free-field sensitivity level when a very high accuracy is not called for or when conditions for carrying out an accurate free-field calibration are not available. The values given have been derived from measurements of the pressure and free-field sensitivity levels of a number of microphones carried out in accordance with IEC 60327 and IEC 60486 to which reference should be made concerning terminology, microphone preamplifier configurations, etc.