This International Standard specifies combinations of sound pressure levels and frequencies of pure continuous tones which are perceived as equally loud by human listeners. The specifications are based on the following conditions: a) the sound field in the absence of the listener consists of a free progressive plane wave; b) the source of sound is directly in front of the listener; c) the sound signals are pure tones; d) the sound pressure level is measured at the position where the centre of the listener's head would be, but in the absence of the listener; e) listening is binaural; f) the listeners are otologically normal persons in the age range from 18 years to 25 years inclusive. The data are given in graphical form in Annex A and in numerical form in Annex B for the preferred frequencies in the one-third-octave series from 20 Hz to 12 500 Hz, inclusive, in accordance with ISO 266.