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This part of ISO 389 specifies a reference threshold of hearing for the calibration of audiometric equipment used under the following conditions. a) The sound field in the absence of the listener consists of either a free progressive plane wave (free field) or a diffuse sound field, as specified in ISO 8253-2. In the case of a free field, the source of sound is directly in front of the listener (frontal incidence). b) The sound signals are pure (sinusoidal) tones in the case of free-field conditions and one-third octave bands of (white or pink) noise in the case of diffuse-field conditions. c) The sound pressure level is measured in the absence of the listener at the position where the centre of the listener's head would be. d) Listening is binaural. NOTE 1 Correction values for the threshold of hearing under free-field listening conditions and selected angles of sound incidence (45° and 90°) deviating from frontal incidence are given in ISO 8253-2 for information. NOTE 2 Other conditions are given in Bibliographic Reference [1]. The data are given in numerical form for the preferred frequencies in the one-third-octave series from 20 Hz to 16 000 Hz inclusive in accordance with ISO 266 and, in addition, for some intermediate audiometric frequencies up to 18 000 Hz. It should be emphasized that the threshold data differ from the audiometric zero specified in ISO 389-1, ISO 389-2, ISO/TR 389-5 and ISO 389-8, since the latter refer to monaural listening through earphones with sound pressure levels referred to specified couplers and ear simulators. Direct comparison between the data in the parts of ISO 389 mentioned above and in this part of ISO 389 is therefore not appropriate.