Alternative to Relying on the NRR

February 25, 2014

Measurement of Insert-type Hearing Protector Attenuation on the End-user: A Practical Alternative to Relying on the NRR

NRR Data

Hearing protectors are labeled with a Noise Reduction Rating (NRR) that is derived from laboratory measurements of the attenuation provided to trained and motivated human subjects in a ‘sterile’ environment. As intended, these measurements represent a best-fit condition. The literature indicates that the average attenuation provided to end-users of hearing protectors is often only a fraction of the labeled values. Because of this, various derating schemes are employed, including the 50% derating suggested in the OSHA inspector’s Technical Manual and the variable derating according to protector-type suggested in the 1998 NIOSH criteria document on occupational noise exposure.

De-rating the labeled NRR

De-rating the labeled NRR provides a better estimate of the average attenuation realized by a population of end-users. However, field measurements indicate that attenuation provided across end-users is highly variable (standard deviation > 10 dB), so many wearers will receive much greater attenuation than the average, and many will receive much less attenuation than the average. Therefore, if hearing protectors are selected according to a de-rated NRR, some individuals will still be under-protected and some will receive excessive attenuation, leading to potential communication problems. A solution to this problem is to base the selection process on objective data obtained by measuring the attenuation provided to each end-user of insert-type hearing protectors.

dB Blocker

In this article the Steel Industry experience is examined when it comes to individual hearing protector fit-testing.  In this industry Almost all HPD wearers at this plant wear insert-type devices. To learn more about the wide variability of attenuation provided by insert-type protectors a results please click here to download the entire study: DOWNLOAD PDF 



One Response to “Alternative to Relying on the NRR”

  1. Robyn Farrell

    I understand that custom fitted earplugs have the best obtainable NRR rating at 29, with the most comfort (and subsequent compliance). In order for them to be utilized by our heavy equipment company, we need to know if they’re CSA approved. Can you help by showing me literature of their CSA approval, and specifically with the DB filter?
    Thank you

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