- a) the sound field in the absence of the listener consists of a free progressive plane wave (free field);
- b) the sound source is directly in front of the listener (frontal incidence);
- c) the sound signals are pure (sinusoidal) tones;
- d) 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;
- e) listening is binaural;
- f) the listeners are otologically normal persons within the age range 18 years to 25 years inclusive.
NOTE 1 The xth percentile threshold is the value of threshold below which x % of the individual thresholds of population fall. The threshold distributions in this International Standard have been derived from the results of comprehensive statistical analyses. See Annex D.
NOTE 2 The applicability of the percentiles and the values of parameters given in this International Standard to diffuse-field listening conditions has not been examined. They are expected to be applicable to the conditions for the frequencies from 20 Hz to 250 Hz where the threshold difference between free-field and diffuse-field listening conditions is zero as specified in ISO 389-7:2005, Table 1.
The percentiles 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 for some intermediate audiometric frequencies. The percentiles are applicable to the assessment of an individual's hearing in relation to the distribution of hearing thresholds under the above conditions. The percentiles can also be used to evaluate the audibility of low-level noise around hearing threshold.NOTE 3 An application example of hearing threshold distribution to noise evaluation can be found in ISO 7779:2010, Annex D.
