The document specifies a reference threshold of hearing for the calibration of audiometric equipment used under the following conditions. a) The sound filed in the absence of the listener consists of either a free progressive plane wave (free filed) or a diffuse sound field. In the case of a free progressive plane wave, 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 band of (white or pink) noise in the case off 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. ect.