W. Dixon Ward

Hazardous Exposure to Intermittent and Steady‐State Noise

The following document was prepared by NAS‐NRC CHABA Working Group 46. This group was asked to specify damage‐risk criteria for exposure to sound. The paper contains graphs of maximum sound‐pressure levels and durations of exposures that the Working Group believes would be tolerable and examples of the use of these graphs in addition to background information and a discussion of the rationale, assumptions, limitations, and general problems pertinent to the development and application of a damage‐risk criterion and related exposure contours.

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.

Temporary threshold shift and damage-risk criteria for intermittent noise exposures.

Twelve normal‐hearing students were exposed for up to 8 h to steady and intermittent noises that, according to the CHABA damage‐risk criteria (DRC) proposed in 1966, should produce an average TTS2 (temporary threshold shift measured 2 min after exposure) no greater than 10 dB at 1 kHz or below, 15 dB at 2 kHz, or 20 dB at 3 kHz or above. This objective appears to have been attained for single uninterrupted exposures and for intermittent exposures involving short (below 3–5 min) noise bursts and recovery periods. However, the cited limits of TTS2 tend to be exceeded when the noise bursts are quite long (10 min or more), owing to an erroneous previous assumption about the course of recovery during the quieter intervals between successive bursts. Modification of the DRC is therefore indicated. However, a more vexing finding is that intermittent exposures to high‐frequency high‐energy noise—either long bursts or short—often produce a delayed recovery; that is, more than 16 h of rest is required to restore the...

8 – Absolute Pitch

Publisher Summary This chapter provides an overview of absolute pitch. The ultimate in musical endowment is commonly regarded by musicians to be the possession of “absolute pitch” (AP), also called “perfect pitch” or “positive pitch.” It is also defined as the ability to identify the frequency or musical name of a specific tone, or, conversely, the ability to produce some designated frequency, frequency level, or musical pitch without comparing the tone with any objective reference tone. There are two major theories of why some persons have AP: (1) heredity, on the one hand, and some combination of learning, unlearning, and (2) imprinting (early) on the other. Therefore some possessors support the cause that AP is a special innate ability that one either inherits or not, that those who do inherit the trait demonstrates pitch-naming ability as soon as an appropriate situation arises, regardless of their early musical training, and that those who are not so genetically blessed can never attain the degree of excellence in identifying pitch displayed by the chosen few, no matter how much instruction they are given or how diligently they practice naming tones. Finally the development of AP depends on some more or less fortuitous set of circumstances whereby the individual is reinforced for trying to put labels on pitches.

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.

Effective quiet and moderate TTS: Implications for noise exposure standards

’’Effective quiet,’’ the highest SPL of a noise that will neither produce a significant temporary threshold shift (TTS) nor retard recovery from a TTS produced by a prior exposure to a higher level, is shown to be about 76 dB for octave bands of noise centered at 250 and 500 Hz, and around 68 dB for those centered at 1000, 2000, or 4000 Hz. On the other hand, a mean TTS2 (TTS 2 min after exposure) of no greater than 10 dB at all frequencies from 500 to 5600 Hz is produced by a broad‐band noise whose octave‐band spectrum falls off at −5 dB per octave (’’magenta’’ noise) and which has an A‐weighted level of 90 dBA. When the group mean TTS2 is 10 dB, less than 10% of normal ears will show a TTS2 of 20 dB. Therefore if a 20‐dB TTS2 is tolerable day after day with no adverse effects, the present industrial noise exposure limit of 8 h at 90 dBA would adequately protect more than 90% of exposed workers in noises with similarly falling spectra. Correction factors for unusual spectra should, however, be developed....

Temporary Threshold Shift in Males and Females

Various measurements of temporary threshold shift (TTS) from high‐intensity tones and noises were made on 24 male and 25 female young normal‐hearing adults. Significantly more TTS was produced in males by low‐frequency stimuli (below 1000 cps) and significantly less by high‐frequency stimuli (above 2800 cps). No differences between sexes in TTS from low intensities (40 dB SL), in auditory adaptation (perstimulatory fatigue at 1000 cps), in rate of recovery from a fixed value of TTS, or in TTS produced by impulse noise could be demonstrated. It is suggested that these results all imply that males and females do not differ in intrinsic fragility of sensory structures on the basilar membrane, but that women have more efficient middle‐ear muscles than men.

Recovery from High Values of Temporary Threshold Shift

Twelve normal observers were exposed to noise that produced at least 50 db of temporary threshold shift (TTS) measured 2 min after cessation of the noise. The TTS was measured at regular intervals until recovery was complete. Results indicate that while the recovery from these high values of TTS proceeds as a function of the logarithm of time during the first few hours, later recovery is instead linear in time.

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.