Provides a reference threshold of hearing for the calibration of audiometric equipment used under the following conditions: - 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). - 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. - 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. - Listening is binaural.