This part of ISO 389 specifies a standard reference zero for the scale of hearing threshold level applicable to pure-tone air conduction audiometers, in order to promote agreement and uniformity in the expression of hearing threshold level measurements throughout the world.It states the information in a form suitable for direct application to the calibration of audiometers, that is, in terms of the response of two different standard models of earphone measured on a coupler complying with IEC 60303 and in terms of other supra-aural earphones of models specified in 4.3 measured on an artificial ear complying with IEC 60318.It is 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 derivation and application of the recommended reference levels are given in annex A.