Field verification of hearing protection
Answered by Brad Witt, audiology and regulatory affairs manager, Sperian Hearing Protection LLC, Smithfield, RI.
According to OSHA regulation 29 CFR 1910.95, the employer must evaluate hearing protector attenuation ("noise blocking") to ensure that workers are adequately protected from hazardous noise. The methods to be used are described in an appendix to OSHA's noise regulation, aptly titled "Methods for Estimating the Adequacy of Hearing Protection Attenuation." But therein lies the problem: All of the methods provide estimates of protection based upon population averages, not measurements of actual protection. The noise reduction rating that accompanies each hearing protector sold in the United States is simply a population estimate based upon 10 subjects tested in the acoustical lab.
Whether your workers actually achieve the published rating numbers in everyday usage with their own hearing protectors depends on many factors. For earmuffs, as long as the cushion makes a firm seal around the ear (no obstruction from thick hair or eyeglass frames), it is fairly easy to obtain the rated attenuation. But for earplugs, attenuation is highly dependent upon proper roll-down (for formable earplugs) and depth of insertion. Hidden differences in ear canal shape and structure can easily negate any protective effect from an otherwise adequate earplug. An earplug that is clearly in the ear canal might provide anywhere from 0 to 40 decibels of protection, but offers little visible indication whether the actual attenuation is closer to 0 or 40!
Several new tools now allow accurate measurement of real-world attenuation, so that workers (and employers) can verify how much protection is being provided by the earplugs. One such system uses a method in which the earplugs are fit with a microphone on either side. After the user inserts the earplug, a noise sample is played and the microphones calculate how much protection is provided by the earplug. Another system administers a short listening test to the user, both with and without earplugs inserted, and calculates real-world attenuation from the difference in the two tests.
Field studies of verification systems have shown some valuable results. In one study at eight different locations using a variety of earplugs, one-third of the tested workers achieved real-world attenuation that was equal to or higher than the rated attenuation for their respective earplug. The middle third of workers achieved attenuation within 5 dB below the NRR, and the bottom third had real-world attenuation anywhere from 5 to 30 dB below the NRR. When these results were correlated to a number of individual and program factors, only one factor was strongly related to high attenuation in the field: the number of one-on-one training sessions on fitting earplugs the worker had received. For workers who achieved low attenuation, the field studies also showed dramatic improvement in attenuation when the workers simply tried a different earplug, confirming the need for a variety of suitable hearing protectors at noisy worksites.
Although real-world verification of attenuation requires additional time for testing (beyond annual audiometric testing), the rewards to the employer are invaluable. Attenuation testing provides documentation of adequate fit, rather than just an estimation using the NRR. It allows newly hired workers to select the protector that truly provides the most comfortable protection for them.
For the noise-exposed worker, verification of attenuation provides a valuable feedback loop – an objective answer to the question, "Am I fitting this right?" And most importantly, it provides the proof that the hearing protection is doing its job, when it is worn properly.