Importance of the free-field calibration of microphones

Abstract

Numerous regulatory requirements, standards, and product characterization and quality control procedures important for industry, commerce, health, and safety rely on practical measuring instruments including sound level meters, personal sound exposure meters (noise dosimeters), and standardized measuring microphones. Such instruments are usually large enough that the effects of diffraction must be considered throughout a significant portion of the operating frequency range. These effects cause differences between the free-field, the pressure, and the diffuse-field sensitivities of instruments, particularly at high frequencies. Differences can also result from other phenomena, such as the effects of a microphone’s static (barometric) pressure equalization vent, especially at low frequencies. Different calibration procedures have evolved, so that an appropriate calibration is available for each of the field types: free, pressure, and diffuse. Critically important free-field procedures and some of the most accurate and widely used diffuse-field procedures should be, but are not always, traceable to primary free-field calibrations of microphones by the reciprocity method. Future international key comparisons at the highest level of accuracy should include determinations of the free-field sensitivities of laboratory standard microphones such as IEC type LS2 by the reciprocity method.