[Slight differences in noise stimulation and NI-TTS (noise-induced temporary threshold shift)].
This paper was attempted to study on the temporary threshold shift (TTS) at 4000cps by a 20-min noise exposure through a earphone to each of a subdivided tone, approximately 2db step, within the range of 79 to 88 phon of white noise. The change of auditory sensitivity was observed by means of Bekesy-type audiometer. Nine subjects tested were adult men with normal hearing who had well experienced the Bekesy audiometry. These results were as follows:1) Although the intensities of stimuli of tones were slightly different respectively, a noise intensity which produced a remarked rise of TTS and a prolonged recovery from it was found in all the subjects.2) A noise intensity level producing of such an elevated threshold was significantly defferent each other:No. 1 and No. 7 were found at 81db; No. 2 at 82db; No. 3, No. 5, No. 6 and No. 9 at 84db; No. 4 and No. 8 at 86db.3) Observing the recovery process over a duration of 20 min, the following was noted, that the elevated threshold after stimuli, being recovered to normal threshold, still showed the elevation in 7-9 min after stimuli were ceased. This was classified to class IV & V. These recovery types were more frequently found on 1 grade less intensity of tone than the fatiguing tone intensity producing the remarked threshold shift and prolonged recovery of it. And these recovery pattern dissappeared on the interruped threshold tone at a frequency of 4000cps.
Recovery of the Auditory Threshold after Strong Acoustic Stimulation
After an ear is exposed to an intense sound, the absolute threshold for many sounds is raised, usually temporarily. The manner in which the absolute threshold recovers to its normal value after such stimulation is the subject of these experiments. It is found that recovery from such auditory fatigue is not a simple monotonic process. Rather these experiments show that under certain conditions the threshold first recovers to an approximately normal value about 1 minute after the cessation of the exposure but then rises again to a higher value that reaches a maximum at about 2 minutes after the exposure. This diphasic recovery curve, with its characteristic “bounce,” is found when the exposure involves sound pressure levels between 100 and 120 db and durations of the order of several minutes.Recovery curves are shown for the auditory thresholds of clicks, bands of noise, and pure tones. Fatiguing stimuli include bands of noise and pure tones.Two subsidiary observations are also reported. First, in many reco...
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.