Daniel L. Johnson

Field studies: Industrial exposures

The databases and models for the prediction of noise-induced permanent threshold shift (NIPTS) from industrial noise exposures are reviewed. Models available in 1973, compared with later models and data, are shown still to be reasonable. The effect of hearing conservation procedures on the acquisition of new data is discussed. Because of the impact of hearing conservation, new research focus is recommended in three areas: sex differences, newly hired individuals exposed for the first time in occupational noise, and the contribution of nonoccupational noise through the use of questionnaires and dosimetry.

Effects of gunfire on hearing level for selected individuals of the Inter‐Industry Noise Study

Using the information available from Inter-Industry Noise Study, the hearing levels of 68 pairs of workers, matched for sex, age, and exposure level, were evaluated. For each pair, one of the participants had indicated exposure to nonoccupational gunfire during the previous year. Differences in mean hearing levels between the male participants exposed to gunfire and those not exposed were clearly apparent and varied between 9 to 16 dB for the frequencies at 3000, 4000, and 6000 Hz. There were no significant differences in female hearing threshold levels. These changes in male hearing are equivalent to the effect of an occupational exposure of 89 dB for 20 years. However the results do not indicate that there are any synergistic effects between gun exposures during a participants recreational time and noise exposure during his working hours.

Description of the measurement of an individual's continuous sound exposure during a 31-day period.

A dosimeter based on the equal energy rule with range from 55 to 110 dB was used to monitor continuously the noise exposure of one subject. The subject, who is the second author of this report, works in a relatively quiet laboratory area. The daily A-weighted average sound level was measured for each 24-h period [L(eq(24))] beginning at approximately 8:00 a.m. During the day the noise exposure associated with specific activities was noted. The 31-day average sound level was 76 dB while the daily L(eq)(24)) values varied from a low of 59 dB to a high of 83 dB. The type of activity that contributed most of the subjects total sound dose was night-time parties at private homes or nightclubs. Five such parties accounted for 42% of his total sound dose. One exposure at an automobile hobby shop and three outings at a bowling alley contributed another 27% to the total sound dose. If these outside recreational activities had been avoided, the 31-day average sound level would have been reduced to 71 dB. The problems involved in monitoring a 24-h noise exposure as well as future plans for more elaborate studies are discussed.

The effect of reduced headband force on the attenuation of muff‐type protectors

The headband force of five different protectors was decreased in one‐half pound increments until the force was less than one pound. Using a dummy head, the attenuation was measured for each value of headband force. The results indicated that four of the muffs were relatively insensitive to a decrease in headband force and loss of 50% of the original force resulted in less than a 3‐dB reduction in the value of the Noise Reduction Rating (NRR). One muff, however, was very sensitive and a 50% loss in headband force resulted in roughly a 50% loss in attenuation. To verify the dummy head results, attenuation was measured at selected forces on three human subjects. Although the attenuations measured were somewhat less than those measured for the dummy heads, the effect of changing headband force produced similar results. While all protectors were unaffected by a reduction of one‐half pound in force, the effect of greater reductions varied dramatically with the make and model of the protector. Apparently, manufacturers need to set guidelines tailored to their specific models.

The typical noise: first step in the development of a short procedure for estimating performance of hearing protectors.

An exact measurement of the effectiveness of a hearing protector requires the determination of how well it works in the specific noise in which it is to be worn. As a practical matter, though, people do not generally remain in a single noise spectrum throughout a working career or even throughout a working day, so it does not matter so much that most measurers are not likely to be equipped to perform spectrum analyses. Other techniques for judging earplugs and earmuffs are obviously necessary. Previous research has tried to find the average attenuation given by a particular device in several (or many) noises, or it has tried to deal with the attenuation given in noises given with particular C-minus-A values. In this paper, a procedure is developed for compressing these multiple-spectra calculations into a single-spectrum computation that proves to be at least as accurate as the more complex methods.

Audibility of infrasound

The distortion of pure tones (1–16 Hz) caused by the nonlinearities of the middle ear was calculated. It is shown that the slope of the audibility curves for infrasound of Yeowart and Evans could be predicted, thus implying that infrasound might not be heard in the normal sense, but only heard as distortion after being transduced through the middle ear. To verify this result, subjects were exposed simultaneously with the 1–10‐Hz stimuli to a low‐frequency masking noise (10–100 Hz). This noise was shown to mask pure tones of infrasound of 1–10 Hz even when the SPLs of these tones were 15–25 dB above the masking noise overall sound‐pressure level. Clearly, this result implies that the pure tones of infrasound below 10 Hz are not heard in the same manner as tones above 16 Hz. The implications of these results to the importance of the infrasound components of any broad‐band noise and to the auditory effects of infrasound are discussed.

The effects of exposure of intense free‐field impulse noise on humans wearing hearing protection: Implications for new criteria

In a series of studies, human volunteers wearing earmuffs were exposed to high‐intensity free‐field impulse noise which simulated various military weapons noise. By detonating increasing amounts of explosive material, the exposure level was varied from one at which no effects were expected to a maximum level just below the threshold of injury to the lung and upper airway. The number of blasts was varied from 6 to 100. Temporary threshold shifts (TTS) measured 2 to 6 min after each exposure were used as the measure of effects on hearing. The highest level for each number of blasts at which 95% of the exposed population would not show a significant TTS was used to establish the maximum safe exposure levels as measured in the open and under the earmuffs. These maximum safe exposure levels exceed the exposure limits used in the United States and other countries. Also, these results suggest that the number‐intensity trading rule is more shallow than that used in most criteria. The exposures measured under the earmuff provide a good indication of the hazard. Implications of these results for impulse noise exposure criteria will be discussed.

Limits of exposure to high‐intensity impulse noise with a 1.5‐ms A duration

Over the past several years, the U.S. Army Medical Research and Development Command has sponsored a series of studies to determine the human tolerance limits of exposure to high‐intensity free‐field impulse noise. These studies have been conducted at the Blast Overpressure Test Site in Albuquerque, NM, by EG&G. The results of these studies for impulses with A‐durations of 0.8 and 2.9 ms have been reported previously. In the latest study, human volunteers have been exposed to intensities ranging from 173 to 193 dB SPL with an A duration of 1.5 ms. The number of impulses per exposure was varied from 6 to 100. Hearing protection was worn during all exposures. For 6 impulses and 120 impulses, the safe upper intensity limit was approximately 187 dB peak SPL. For 25 to 100 impulses, the limit was approximately 184 dB SPL. [Work supported by U.S. Army Medical Research & Development Command, Contract DAMD17‐88‐C‐8141.] In the conduct of research where humans were the subjects, the investigator(s) adhered to the p...

Maximum safe exposure levels for intense reverberant impulses in an enclosure.

Studies were undertaken to determine the maximum safe‐exposure levels in a reverberant wave environment like that produced by firing an antiarmor weapon from a small room. The approach was first, to establish a threshold of injury for organs other than the ear then to determine whether these levels were safe for the ear. Using sheep as an animal model, the threshold of nonauditory injury was found to be approximately 190 dB peak SPL (65 kPa) with a B duration longer than 200 ms for one impulse and approximately 187 dB peak SPL (46 kPa) for three impulses. Two groups of 40 animals were used to establish statistical confidence in the ‘‘no injury levels.’’ The auditory effects were investigated using human volunteers exposed to a progression of levels from 168 to 185 dB at the ear for one impulse and then, to two and three impulses at 183 dB. A temporary threshold shift (TTS), determined 2–4 min post exposure, was used as an indicator of auditory effect. The volunteers wore an earmuff modified to simulate a poor fit, during the exposures. No significant TTS in 59 volunteers was observed, indicating no effects on hearing up to the maximum safe‐exposure levels for other organs.

Hearing protector performance for impulse noise

Using Army volunteers, a series of studies were performed at the Blast Overpressure Test Site in New Mexico to establish safe exposure levels for impulse noise. The results of these studies demonstrated that for properly worn hearing protection, the limit of human exposure is set by the threshold of nonauditory damage. This nonauditory threshold can be depicted by a simple formula based on peak pressure, A‐duration, and number of impulses. Using this information, a standard for impulse noise will be proposed in which the attenuation of the hearing protective need not be measured or predicted. The protector need only to be shown to protect adequately. One method for evaluating this adequacy is based on a procedure that verifies that Temporary Threshold Shifts of Hearing are not occurring. Another method is to measure the total A‐weighted energy under the protection device or devices. The standard will also include a computer model that will provide the means for the user to predict the risk of hearing inju...