ISO/TC 43/WG 1 - Threshold of hearing

This document 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 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. The threshold data differ from the audiometric zero specified in ISO 389‑1, ISO 389‑2, ISO 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 document is therefore not appropriate.

ISO 389-1:2017 specifies a standard reference zero for the scale of hearing threshold level applicable to pure-tone air conduction audiometers, to promote agreement and uniformity in the expression of hearing threshold level measurements throughout the world. ISO 389-1:2017 states the information in a form suitable for direct application to the calibration of audiometers, that is, in terms of the reference equivalent threshold sound pressure levels of generic supra-aural earphones specified in 4.2, measured on an ear simulator complying with IEC 60318‑1 and in terms of model-specific data given in two additional tables for the IEC 60318‑3 acoustic coupler and the IEC 60318‑1 ear simulator, respectively. The data are based on an assessment of the information available from the various standardizing laboratories responsible for audiometric standards and from scientific publications. Some notes on the application and derivation of the reference levels are given in Annexes A and B.

ISO 7029:2017 provides descriptive statistics of the hearing threshold deviation for populations of otologically normal persons of various ages under monaural earphone listening conditions. It specifies the following, for populations within the age limits from 18 years to 80 years for the range of audiometric frequencies from 125 Hz to 8 000 Hz: a) the expected median value of hearing thresholds given relative to the median hearing threshold at the age of 18 years; b) the expected statistical distribution above and below the median value. For the frequencies from 3 000 Hz to 8 000 Hz, the median and statistical distribution for populations above 70 years are presented for information only. ISO 7029:2017 also provides for information the expected median values at audiometric frequencies from 9 000 Hz to 12 500 Hz within the age limits from 22 years to 80 years.

ISO 389-1:2016 specifies the following data applicable to the calibration of bone vibrators for pure-tone bone-conduction audiometry: a) reference equivalent threshold vibratory force levels (RETVFL), corresponding to the threshold of hearing of young otologically normal persons by bone-conduction audiometry; b) essential characteristics of the bone vibrator and the method of coupling to the test subject, and to the mechanical coupler; c) essential characteristics of the masking noise and the baseline masking noise level applied to the ear not under test. Guidance on the practical application of this part of ISO 389 in the calibration of audiometers is given in Annex B. RETVFL is the vibratory force level transmitted to a mechanical coupler of specified characteristics by a vibrator when applied to the mechanical coupler under stated conditions of test and when energized at the voltage level corresponding to the normal threshold of hearing for location on the mastoid prominence. NOTE 1 Values for the differences in reference equivalent threshold vibratory force levels between location on the forehead and mastoid are included for information in Annex C. NOTE 2 Recommended procedures for carrying out bone-conduction audiometry are specified in ISO 8253‑1.

ISO 1999:2013 specifies a method for calculating the expected noise-induced permanent threshold shift in the hearing threshold levels of adult populations due to various levels and durations of noise exposure; it provides the basis for calculating hearing disability according to various formulae when the hearing threshold levels at commonly measured audiometric frequencies, or combinations of such frequencies, exceed a certain value. The measure of exposure to noise for a population at risk is the noise exposure level normalized to a nominal 8 h working day, LEX,8h, for a given number of years of exposure. ISO 1999:2013 applies to noise at frequencies less than approximately 10 kHz which is steady, intermittent, fluctuating, irregular. Use of ISO 1999:2013 for sound pressures exceeding 200 Pa (140 dB relative to 20 µPa) is recognized as extrapolation.

ISO 8253-3:2012 specifies basic methods for speech recognition tests for audiological applications. In order to ensure minimum requirements of precision and comparability between different test procedures including speech recognition tests in different languages, ISO 8253-3:2012 specifies requirements for the composition, validation and evaluation of speech test materials, and the realization of speech recognition tests. ISO 8253-3:2012 does not specify the contents of the speech material because of the variety of languages. Furthermore, ISO 8253-3:2012 also specifies the determination of reference values and fulfilment requirements for the realization and manner of presentation. ISO 8253-3:2012 specifies procedures and requirements for speech audiometry with the recorded test material being presented by air conduction through an earphone, or from a loudspeaker for sound field audiometry. Methods for using noise either for masking the non-test ear or as a competing sound are described. Some test subjects, for example children, can require amended test procedures not specified in ISO 8253-3:2012. Specialized tests such as those used for evaluating directional hearing and dichotic hearing are outside the scope of ISO 8253-3:2012.

ISO 28961:2012 provides descriptive statistics, percentiles, of the hearing threshold distribution whose mean is the reference threshold value specified in ISO 226 and ISO 389-7. The mean and percentile thresholds are specified under the following conditions: 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. 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.

ISO 8253-1:2010 specifies procedures and requirements for pure-tone air conduction and bone conduction threshold audiometry. For screening purposes, only pure-tone air conduction audiometric test methods are specified. It is possible that the procedures are not appropriate for special populations, e.g. very young children. ISO 8253-1:2010 does not cover audiometric procedures to be carried out at levels above the hearing threshold levels of the subjects. Procedures and requirements for speech audiometry, electrophysiological audiometry, and where loudspeakers are used as a sound source are not specified.

ISO 8253-2:2009 specifies relevant test signal characteristics, requirements for free, diffuse, and quasi‑free sound fields, and procedures for sound field audiometry using pure tones, frequency-modulated tones or other narrow-band test signals presented by means of one or more loudspeakers. The primary purpose is the determination of hearing threshold levels in the frequency range 125 Hz to 8 000 Hz, but this range can be extended to 20 Hz to 16 000 Hz. ISO 8253-2:2009 does not include specifications for the use of hand-held loudspeakers. Speech as a test signal is not covered. The purpose of ISO 8253-2:2009 is to ensure that tests of hearing, using sound field audiometry, give as high a degree of accuracy and reproducibility as possible. Examples of graphical representations of the results are given.

ISO 389-9:2009 specifies test conditions for determining the hearing thresholds of subjects for the purpose of establishing standardized values for reference hearing threshold levels.

ISO 389-6:2007 specifies reference hearing threshold levels for tests signals of short duration applicable to the calibration of audiometric equipment where such signals are used.

ISO 389-5:2006 specifies reference equivalent threshold sound pressure levels (RETSPLs) of pure tones in the frequency range from 8 kHz to 16 kHz applicable to the calibration of air conduction audiometers for specific earphones.

ISO 389-8:2004 specifies reference equivalent threshold sound pressure levels (RETSPLs) for pure tones in the frequency range from 125 Hz to 8 kHz, applicable to the calibration of air conduction audiometers equipped with a particular model of circumaural earphones (SENNHEISER HDA 200). Some notes and references on the derivation and the test conditions used to determine the recommended reference levels are given in Annex A and the Bibliography. The sound attenuation of the earphone is given in Annex B. For speech audiometers of types A-E and B-E, the correction figures of the earphone for a free-field equivalent output are given in Annex C.

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: the sound field in the absence of the listener consists of a free progressive plane wave; the source of sound is directly in front of the listener; the sound signals are pure tones; 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; listening is binaural; the listeners are otologically normal persons in the age range from 18 years to 25 years inclusive.

Specifies reference levels for narrow-band masking noise presented by air conduction from an earphone in pure-tone audiometry. The data are given in terms of levels to be added to the reference equivalent threshold sound pressure levels for the corresponding pure-tone frequencies. Data are given for noise bandwidths of one-third and one-half octaves.

Specifies levels supplementary to those specified in ISO 389:1991 (to be re-issued as ISO 389-1). Values given are applicable to insert earphones of type Etymotic Research ER-3A, coupled to the human ear by ear inserts of type ER-3-14.

ISO 389-7:2005 specifies 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. The data in ISO 389-7:2005 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. 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 that in ISO 389-7:2005 is therefore not appropriate.

Specifies a reference threshold of hearing for the calibration of audiometric equipment used under definite conditions. The listeners are otologically normal persons in the age from 18 to 25 years inclusive. The data are given in numerical form for the preferred frequencies in the one-third-octave series from 20 Hz to 16 000 Hz in accordance with ISO 266.

Specifies the following data applicable to the calibration of bone vibrators for pure-tone bone-conduction audiometry: reference equivalent threshold force levels (RETFL), corresponding to the threshold of hearing of young otologically normal persons by bone-conduction audiometry; essential characteristics of the bone vibrator and of its method of coupling to a person under test and to the mechanical coupler; essential characteristics and datum level of the masking noise applied to the ear not under test.

Specifies the relations existing between the sound pressure levels and frequencies of pure sinusoidal continuous tones in several conditions. These relations are expressed as curves and represent a fundamental property of the human auditory system.