Development of a new standard laboratory protocol for estimating the field attenuation of hearing protection devices. Part III. The validity of using subject-fit data.

The mandate of ASA Working Group S12/WG11 has been to develop "laboratory and/or field procedure(s) that yield useful estimates of field performance" of hearing protection devices (HPDs). A real-ear attenuation at threshold procedure was selected, devised, tested via an interlaboratory study, and incorporated into a draft standard that was approved in 1997 [J. D. Royster et at., "Development of a new standard laboratory protocol for estimating the field attenuation of hearing protection devices. Part I. Research of Working Group 11, Accredited Standards Committee S12, Noise," J. Acoust. Soc. Am. 99, 1506-1526 (1996); ANSI S12.6-1997, "American National Standard Methods for Measuring Real-Ear Attenuation of Hearing Protectors" (American National Standards Institute, New York, 1997)]. The real-world estimation procedure utilizes a subject-fit methodology with listeners who are audiometrically proficient, but inexperienced in the use of HPDs. A key factor in the decision to utilize the subject-fit method was an evaluation of the representativeness of the laboratory data vis-à-vis attenuation values achieved by workers in practice. Twenty-two field studies were reviewed to develop a data base for comparison purposes. Results indicated that laboratory subject-fit attenuation values were typically equivalent to or greater than the field attenuation values, and yielded a better estimate of those values than did experimenter-fit or experimenter-supervised fit types of results. Recent data which are discussed in the paper, but which were not available at the time of the original analyses, confirm the findings.

Communication survey of CH-47D crewmembers.

U.S. Army CH-47D cargo helicopter crewmembers are exposed to high levels of noise on a routine basis, yet must accomplish demanding missions that rely heavily on clear communication. This study looked at the auditory influences affecting communication by 17 crewmembers of a CH-47D Army Reserve unit. Results revealed (1) noise levels that exceed those of other Army rotary-winged aircraft, (2) different exposure levels for aviators and crewchiefs, (3) an overall preference for hearing "critical or diagnostic" aircraft sounds rather than wearing double hearing protection, (4) complaints of post-flight tinnitus or muffled hearing, (5) helmet discomfort among the majority of crewmembers, (6) a high incidence of hearing loss, (7) difficulties in adjusting radio volume controls for aviators, and (8) the effects of eyeglasses or protective masks on hearing and communications.

Insert hearing protector effects.

The purposes of this investigation were two-fold: (1) to determine whether differences between measured ear canal diameter and actual earplug size affect sound pressure levels in the ear canal during exposure to high levels of noise; and (2) to determine if individuals would select the insert earplugs that conform to their measured ear canal diameter if the decision were left to them. The results indicate that: (1) very small differences between measured ear canal diameter and actual earplug size affect considerably the sound pressure levels in the ear canal during exposure to high levels of noise; (2) individuals typically select earplugs that are smaller than their measured ear canal dimensions when the decision is left to them; (3) the most skilled technician cannot always determine ear canal sizes due to limitations of available measuring instruments.

Threshold shifts in chinchillas exposed to octave bands of noise centered at 63 and 1000 Hz for three days(a).

Audiograms were obtained on eight binaural chinchillas trained on a shuttlebox avoidance procedure. Four of the animals were exposed to three successive levels of an octave band of noise centered at 63 Hz: 100 dB SPL (74 dBA), 110 dB SPL (84 dBA), and 120 dB SPL (94 dBA). The other four animals were also exposed to three successive levels of an octave band of noise centered at 1000 Hz: 75 dB SPL (75 dBA), 85 dB SPL (85 dBA), and 95 dB SPL (95 dBA). All exposure durations were 75 h. Little threshold shift (TS) resulted from the lower two exposure levels of the 63-Hz noise band. At the 120-dB exposure level, maximum TS of 43 dB occurred at 2000 Hz. Permanent threshold shifts (PTSs) of 16 dB at 2000 Hz and 11 dB at 1400 Hz were found. Exposure to the three levels of the 1000-Hz noise band produced TSs of 20, 45, and 61 dB at 1400 Hz. The 95-dB exposure level resulted in PTSs of 6 dB at 1400 Hz and 9 dB AT 2000 Hz. The major results were (1) high-frequency hearing loss to a low-frequency noise and (2) that noise bands matched within 1 dBA were not equally hazardous as dictated by damage-risk criteria. The 63-Hz noise band produced nearly twice as much PTS as the 1000-Hz noise band.