Standards Development

Scope

This document specifies a method for calculating the expected hearing threshold levels of adult populations due to various levels and durations of noise exposure; it provides the basis for calculating the risk of hearing impairment according to various formulae when the hearing threshold levels at commonly measured audiometric frequencies, or combinations of such frequencies, exceed some certain values.

This document is based on statistical data and therefore cannot be applied to the prediction of the hearing loss of individual persons.

NOTE 1 This document does not specify frequencies, frequency combinations, or weighted combinations to be used for the evaluation of hearing impairment; nor does it specify a hearing threshold level (fence) which it is necessary to exceed for hearing impairment to exist. Quantitative selection of these parameters is left to the user. All sound pressure levels stated in this document do not consider the effect of hearing protectors which would reduce effective exposure levels and modify the frequency spectrum at the ear. Because there is evidence indicating a transition between metabolic exhaustion and mechanical damage in the inner ear, exposure to sound pressure exceeding 200 Pa (140 dB relative to 20 μPa) is not recommended.

The measure of exposure to noise for a population at risk is the equivalent continuous A-weighted sound pressure level normalized to an 8 h working day, or A-weighted noise exposure level, Lp,A,8 h or , for a number of years of exposure. This standard applies to noise at frequencies less than 10 kHz that is steady-state, intermittent, fluctuating, irregular, or complex with A-weighted noise exposure levels between 70 and 100 dB (daily A-weighted sound exposures between 0,115 × 103 and 115 × 103 Pa2s).

Formulae are presented to calculate the statistical distributions of hearing threshold levels at a range of audiometric frequencies due to exposure to noise as a function of level of noise exposure and duration of exposure (in years). Values of effect of noise, N, do not distinguish between male and female populations. The database for values of N in Table 1 was derived from people who typically started work around age 20 and whose noise exposures continued for many years; thus, this database is most accurate for populations of workers whose age minus years of exposure is about 20.

NOTE 2 Although the models of hearing loss are based on data assumed to stem primarily from populations exposed to occupational noise, they may be used, with some caution, for estimating the effects of comparable non-occupational exposures.

NOTE 3 The prediction method for Lp,A,8 h presented is based primarily on data collected with essentially broadband, steady-state, non-tonal noise. The prediction method for noise environments which include substantial impulsive/impact noise uses a kurtosis-adjusted noise exposure level normalized to an 8 h working day, .

To calculate hearing threshold levels and also to calculate the risk of acquiring hearing impairment due to noise exposure, it is necessary to make use of a database for age-associated hearing threshold levels for a comparable population. This document includes a highly screened otologically normal population (in accordance with ISO 7029) and examples of unscreened populations of five typical industrialized societies. The users of this document may choose a comparable population according to their particular requirements.

NOTE 4 All data and procedures presented in this document are based on deliberate simplifications of experimental data where the daily sound exposure duration did not exceed 12 h. The resulting approximations restrict the validity to the stated ranges of the variables, percentiles, sound exposure levels and frequency ranges.