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Noise reduction ratings: outlining the process

December 1, 2006

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How are hearing protection devices tested?

Answered by Jeffrey S. Birkner, M.S., CIH, vice president, technical services, Moldex-Metric Inc., Culver City, CA.

EPA's Noise Control Act of 1972 (40 CFR 211) governs the testing and labeling of hearing protection devices. Devices covered under this act include disposable and reusable earplugs, bands, and earmuffs. Even though these products can be very different, all are tested in a similar fashion.

The Noise Control Act calls for a strict testing protocol that must be followed in accordance with a procedure developed by the American National Standards Institute's S3.19-1974. The procedure consists of test subjects – often called listeners – placed in a special noise-insulated booth, and exposed to varying noise frequencies and intensities. The listeners first are screened using hearing tests without the hearing protector to make sure they have the proper hearing threshold levels. This prescreening ensures all tests are conducted using listeners with similar hearing thresholds, and prevents hearing protection devices from earning favorable ratings by using individuals who have poor hearing to begin with, and vice versa.

When a listener passes the prescreening test, he or she is given a hearing protection device and fitting instructions. Once the hearing protector is fitted, the listener once again is exposed to varying noise frequencies and intensities. Each hearing protector must be tested on 10 listeners who have passed the prescreening portion of the test. After all data is collected, the noise reduction rating is calculated using a calculation set forth in the EPA regulation.

The noise reduction rating is a benchmark that indicates how much noise the hearing protector would be expected to reduce, if used properly. EPA requires manufacturers to list a hearing protector's NRR on the device's packaging.

Each manufacturer must have its products tested and labeled in accordance with the Noise Control Act. Even though all hearing protectors must be tested in the same way, the EPA regulation is less stringent as to who is allowed to test the product. Some manufacturers conduct the testing in their own laboratories; other manufacturers have NRRs determined in an independent laboratory.

EPA does not express an opinion on the use of in-house labs versus independent labs; however, products tested in an independent lab are not subject to the potential bias inherent with in-house testing. Biases can occur during any test procedure for any number of reasons, but the use of an independent laboratory puts all manufacturers on the same playing field by ensuring tests are conducted by personnel with no stake in the outcomes.

For users of hearing protection, independent testing provides greater credibility for NRRs than manufacturers' in-house testing. This is illustrated further by the numerous independent consumer reports that are produced for retail products, which consumers often rely on to aid their buying decisions.



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