Rating of hearing protector performance for impulse noise.
Johnson DL; Patterson J Jr.
Proceedings: 1992 hearing conservation conference, April 1-4, 1992. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, task order 91-37982, 1992 Apr; :103-106
The validity of rating the performance of hearing protector devices (HPDs) for impulse noise was investigated. Subjects were exposed to noise levels as high as 100 impulses of 187 decibels (dB) at 1 minute intervals, while wearing two versions of a RACAL earmuff capable of being fitted under a military helmet. The two versions differed only in that one had eight 2.3 millimeter diameter tubes piercing the seal so as to simulate air leaks that would result from a poorly fitted muff. Attenuation curves showed that the muff with the holes actually amplified sound in the range of 160 to 250 Hertz. Base waveforms measured outside and inside the two types of HPD showed two peaks; the standard earmuff reduced first peak pressure to about 12dB, while the modified one reduced it to only 8dB. Of 57 subjects, one had early temporary threshold shift (TTS) when wearing the leaky earmuff. Unprotected exposures that resulted in comparable TTS were sought. Unprotected exposures at 159dB produced slightly greater TTS. Using peak level reduction, effective attenuation based on TTS for the modified muff was 28 to 45dB, while that for the unmodified muff was 140 to 150dB. Measuring the unweighted peak under the protector dramatically underestimated the protection given. Use of A-weighted energy reduction showed exposure under the modified muff to be about 118dB, while that under the unmodified muff was 110dB. Attenuation by the unmodified and modified muffs were 23dB and 15dB, respectively, while noise reduction rating (NRR) values calculated were 24 and 9, respectively. The authors conclude that NRR in its present form does not work for evaluating the performance of HPDs.
Audiometry; Ear-protectors; Hearing-disorders; Military-personnel; Noise-induced-hearing-loss; Occupational-exposure; Noise-measurement; Impulse-noise; Sensory-disorders
Proceedings: 1992 hearing conservation conference, April 1-4, 1992