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ISO 389-1:2017 specifies a standard reference zero for the scale of hearing threshold level applicable to pure-tone air conduction audiometers, to promote agreement and uniformity in the expression of hearing threshold level measurements throughout the world. ISO 389-1:2017 states the information in a form suitable for direct application to the calibration of audiometers, that is, in terms of the reference equivalent threshold sound pressure levels of generic supra-aural earphones specified in 4.2, measured on an ear simulator complying with IEC 60318-1 and in terms of model-specific data given in two additional tables for the IEC 60318-3 acoustic coupler and the IEC 60318-1 ear simulator, respectively. The data are based on an assessment of the information available from the various standardizing laboratories responsible for audiometric standards and from scientific publications. Some notes on the application and derivation of the reference levels are given in Annexes A and B.