Aram Glorig

Temporary Threshold Shift from Octave‐Band Noise: Applications to Damage‐Risk Criteria

The growth and recovery of TTS in normal observers following exposure to octave‐band noise is shown to follow the same course as that after broad‐band noise: both are linear in log time. Rate of growth varies with frequency of exposure band and test frequency, being greatest at 4 kc following exposure to 2400–4800 cps or 1200–2400 cps, less at lower test frequencies and octave bands. The time for total recovery apparently is a function of the initial TTS. The results support present damage‐risk criteria for continuous noise, which (1) suggest ear protection when octave‐band levels exceed 85 db SPL and (2) require it in levels above 95 db SPL.

Dysequilibrium of ageing (presbyastasis).

We reviewed clinical findings in 740 patients over age 65 who consulted the Otological Medical Group, Inc., during a one-year period for dizziness. A thorough neurotologic evaluation is indicated in every such case to determine the specific cause of dizziness. In 21 per cent of these patients, a specific cause of dizziness was found. In the remaining 79 per cent, the diagnosis of primary dysequilibrium of ageing (presbyastasis) was made. We classified dysequilibrium of ageing (presbyastasis) according to the character, time course, and precipitating factors of dizziness. Two clinical types were described: constant and episodic; episodic dizziness was subdivided into orthostatic, positional, and unclassified. The histological findings in the temporal bones of four cases with dysequilibrium of ageing were reviewed. Pathological changes other than those in the peripheral vestibular system seem to be responsible for dysequilibrium of ageing. In the present series, about three-fourths of the patients had a daily dose of nicotinic acid to produce flushing of the skin. In 16 per cent, the dizziness was minor, requiring no special treatment. In the remaining 9 per cent with incapacitating vertigo, a vasodilator regimen, antivertiginous drugs, and Cawthornes vestibular exercises were prescribed.

Exploratory Studies on Temporary Threshold Shift from Impulses

Some exploratory studies of temporary threshold shifts (TTS) induced by acoustic pulses are described. Attempts to determine the effect of pulse rate on TTS were only partially successful because at rates of 1 click/sec or more, residual activity of the protective reflex produced by one click reduced the effective intensity of the next click. The protection induced by this reflex was next examined by using, TTS from clicks as the dependent variable; a tone of 1000 cps at 103 db SPL, presented to the contralateral ear 105 msec before the click, reduced its effective intensity by about 10 db. The TTS at 4 kc was shown to increase linearly with exposure time; that is, the TTS from pulses is proportional to the number of pulses presented. The average TTS produced by pulses has a broad maximum at 4 kc, but this maximum may vary from 2 to 10 kc for different observers. Striking individual differences were observed throughout the experiments. Certain aspects of the distinction of pitch that is often correlated w...

Temporary Threshold Shift Produced by Intermittent Exposure to Noise

The TTS at 4 kc was measured 2 and 17 min after successive 12‐min exposures to broad‐band noise at 106 db SPL separated by 18 min of silence. The results indicate that the TTS existing at the beginning of a particular exposure can be treated as additional time of exposure. Thus, if the residual TTS has a value that would be produced by R min of exposure, then the total TTS at the end of an M‐min exposure will be given by solving the equation for growth of TTS with exposure time set equal to M+R.

Relation between Recovery from Temporary Threshold Shift and Duration of Exposure

Approximately 20 db of TTS at 3 and 4 kc, measured 2 min after cessation of the TTS‐producing stimulation, was produced in 14 normal listeners by three different exposures to 1200‐ to 2400‐cps noise: to 106 db SPL for 12 min, to 98 db for 27 min, and to 90 db for 117 min. Despite the fact that the TTS produced at other frequencies was different for the three exposures, the courses of recovery at 3 and 4 kc were indistinguishable. From these results and those of others, it is concluded that the course of recovery from TTS is uniquely determined by the value of TTS at 2 min recovery: i.e., by the TTS remaining after the first fast recovery process, the so‐called R‐1 recovery process, has run its course.

Magnetically Coupled Stimulation of the Ossicular Chain: Measures in Kangaroo Rat and Man

A ferrite core coil is used to drive a small magnet fastened to the ossicular chain in kangaroo rat and man. The cochlear potentials recorded in kangaroo rat to such a drive are the same as when acoustic stimuli are used. When magnetic coupling is used to drive the human ossicular chain, the subject hears sounds that are indistinguishable from sounds generated through earphones.

Hearing‐Conservation Programs

A brief review of noise‐exposure criteria is presented, including the relations of level, spectrum, and time. Criteria are recommended. Suggestions for initiating hearing‐conservation programs are made. Educational materials are presented and methods for their use are given. Noise control at the source is discussed and comparisons of personal protective devices are made.

Temporary Threshold Shift in a Changing Noise Level

When an individual is exposed first to a high‐level noise H for a time T and then to one of lower level L for time t, his temporary threshold shift (TTS) is equal to the numerical sum of (1) the TTS in db that would be produced by level L acting over a period of time T+t, and (2) the TTS in db that would remain after t min of recovery in quiet from TTSD, where TTSD is the difference between (a) the TTS at the end of the T‐min exposure to H and (b) the TTS that would have been produced by T min of exposure to L. It appears that this result rules out any simple explanation of TTS.

Susceptibility and Sex

Hearing surveys invariably indicate that women have more sensitive hearing than men, the difference increasing with age. This is generally attributed to the fact that women are, on the average, exposed to less noise and gunfire than are men. A possible alternative, however, is that women are less susceptible than men: that in the same noise environment, men will suffer more permanent loss than women. If this were so, one might expect the same to be true of temporary threshold shift (TTS). Therefore the effects of one hour of exposure to 100‐db 1200‐24OO‐cps noise were determined for two groups of normal‐hearing college students: 15 men and 15 women. There were no significant differences, either in growth or recovery of TTS at 3 and 4 kc, between the groups. These results strengthen the hypothesis that differential exposure, rather than differential susceptibility, is responsible for the observed differences in auditory sensitivity between men and women. (Research supported by research grant B‐1122 from the National Institutes of Neurological Diseases and Blindness, Public Health Service.)

Determination of the Normal Hearing Reference Zero

The present American normal hearing reference zero is considered by many investigators to be too high. The data from two recent studies on normal hearing provide a possible explanation. One of these studies, which was a survey type study, confirmed the present reference zero. The second study, a laboratory type study, confirmed the results of other laboratory studies that had led to the opinion that the present zero is too high.In our opinion, the difference between the results of these two studies and various other studies of the auditory threshold are due to the inherent difference in experimental technique in laboratory and survey type studies. In establishing normal hearing reference zero, it is no longer a question of resolving differences in various studies but a question of whether survey or laboratory data will be used.