Noise-Induced Permanent Threshold Shift at 2000 cps and 4000 cps

Three samples of industrial workers were drawn from environments in which continuous, steady-state noise had average sound pressure levels in the octave bands from 150 to 4800 cps that ranged from 77 to 96 db. Each sample was sub-grouped according to length of time on job, yielding sub-samples whose mean times on job (mean exposure time for typical work years) ranged from less than one year to over 25 years. Median hearing levels were calculated for 2000 and 4000 cps for each sub-sample. Age effect (loss of auditory sensitivity due to aging) corrections were applied to the median hearing-level values appropriate to the mean ages of the sub-samples. The resulting values were defined as noise-induced permanent threshold shift (NIPTS). It is shown that: (1) there is a maximum NIPTS produced at 4000 cps which occurs within the initial 10 years of exposure to 8 hr/day exposures of continuous, steady-state noise of any intensity greater than 70–75 db in the 1200–2400 or 2400–4800 octave bands; (2) the maximum NIPTS values which are produced at 4000 cps are approximately equal to the noise-induced temporary threshold shift (NITTS) values predicted from the appropriate SPL's of each sample for 8 hr/day exposures; (3) there is no apparent maximum NIPTS occurring at 2000 cps within the exposure ranges studied; and (4) NIPTS for both 2000 and 4000 cps are monotonic functions of exposure time and SPL.