International Organization for Standardization

Acoustics - Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment - Second Edition
 N 1999

Annotation

This International Standard 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 handicap according to various formulae when the hearing threshold levels at commonly measured audiometric frequencies, or combinations of such frequencies, exceed a certain value.

NOTE 1 - This International Standard does not specify frequencies, frequency combinations or weighted combinations to be used for the evaluation of hearing handicap; nor does it specify a hearing threshold level ("fence") which must be exceeded for hearing handicap to exist. Quantitative selection of these parameters is left to the user. All sound pressure levels stated in this International Standard do not consider the effect of hearing protectors which would reduce the levels effective at the ear.

The measure of exposure to noise for a population at risk is the averaged A-weighted sound exposure (time-integrated squared sound pressure), EA,T, and the related equivalent continuous A-weighted sound pressure level, LAeq,T, over an average working day (assumed to be of 8 h duration), for a given number of years of exposure. This International Standard applies to audio frequency (less than approximately 10 kHz) noise which is steady, intermittent, fluctuating, irregular or impulsive in character. Use of this International Standard for instantaneous sound pressures exceeding 200 Pa (140 dB relative to 20 Pa) and for higher sound pressures should be recognized as extrapolation.

For the assessment of hearing impairment due to exposure to noise, formulae are presented to calculate the NIPTS for audiometric frequencies from 0,5 kHz to 6 kHz for 8 h per day daily A-weighted sound exposure of 364 Pa2.s to 1,15 x 105 Pa2.s (equivalent continuous A-weighted sound pressure level for a normal 8 h working day from 75 dB to 100 dB), and periods of exposure lasting from 0 to 40 years. Extrapolations to higher levels are not supported by quantitative data. The median values of NIPTS as well as the statistical distribution above and below the median value from the 0.05 to the 0,95 fractile are specified. The NIPTS data are the same for male and female populations.