US national and many international hearing conservation programs (HCPs) have adopted a noise exposure criterion of 85 dBA for a time-weighted average of 8 hours, with a 3 dB per doubling exchange rate (safe exposure duration was cut in half for each 3 dB increase in noise level). Military personnel who work in extreme noise environments require high attenuation from hearing protection in order to complete a normal duty day without risk of permanent hearing loss. Improvements to both hearing protection and noise dose monitoring have been consistently recommended and pursued as a means to reduce risk for noise-induced hearing loss.
The goal of adequately protecting the hearing of military personnel in high noise environments could not be achieved without considering the bone/tissue conduction flanking pathways of noise, in addition to the air-conducted pathways through the ear canals. It was important to understand the combined effect of sound energy transmission pathways when attempting to calculate an individual’s true noise dose. Since temporary threshold shifts (TTS) is an auditory response to noise dose, it was used in this study to account for the total effect of noise exposure on the auditory system.
TTS studies in humans represent the only ethical means to accurately investigate the effects of noise on human hearing. While the risk of permanent hearing damage was not nonexistent, data from previous human studies at the same noise levels and exposure times used in this study indicated a less than 1% risk for permanent hearing loss. Test subjects were introduced to a brief duration of noise (5 minutes at 97 dB overall sound pressure level (OASPL), 10.5% of daily allowable noise dose) to ensure that subjects were comfortable with simply being in the noise environment, as well as to eliminate any subject with an unusually susceptible auditory system.
The first component of the experiment was conducted in a reverberant chamber — a specialized hearing test facility in which a subject’s behavioral hearing thresholds were assessed using Békésy tracking in a diffuse sound field. Subjects wore in-ear dosimetry earplugs for occluded ear sessions. Each in-ear dosimetry earplug had an integrated microphone for use in determining noise dose. Each subject’s predicted dose level was calculated to ensure subject safety. The subject’s passive noise attenuation was calculated for that day’s fit across seven octave band frequencies. The subject’s fit remained unaltered between the measurements and the noise exposure. The noise level in the room was determined using the octave band method. Based on the subject’s auditory responses to open ear sessions, it was determined if the subject should be exposed at 25%, 50%, or 100% noise dose from the individual susceptibility to threshold changes.
Pre- and post-noise exposure DPOAE measurements were collected for left and right ears at 3000, 3250, 4000, and 6000 Hz. Initially, data were collected at 91 dB as well as 97 dB; however, it was determined that 94 dB would be the only level used for determining the calibration factor. The maximum open ear TTS data at 94 dB for each subject were used to calculate a second-order polynomial function to display the individual TTS growth. The maximum occluded TTS was entered into the polynomial function to determine the effective noise dose for each individual subject.
Based on the subjects’ maximum TTS results from this study, exposed at 94 dBA in the single hearing protective device condition, the in-ear noise dosimeter substantially overestimated the noise dose by an average of 11 dB. Preliminary findings indicate that human subject data are extremely important in developing and calibrating the effective noise dose for any type of noise dosimeter, but particularly so for in-ear dosimetry. Additional studies should include investigating the dosimeter calibration under double hearing protection, investigating middle ear impedance using laser Doppler vibrometry, evaluating subjective and objective measures of stress during noise exposures, and examining TTS responses from noise generated inside the ear canal as opposed to outside the hearing protector.
This work was done by Hilary L. Gallagher and Richard L. McKinley of the Air Force Research Laboratory, Melissa A. Theis of Oak Ridge Institute for Science and Education, and Valerie S. Bjorn of the Naval Air Systems Command, Arnold Air Force Base. AFRL-0232