Most health and safety professionals responsible for hearing loss prevention rely heavily on hearing protection devices (HPD) as a vital part of hearing conservation programs. One essential aspect of HPD selection and use is the noise reduction rating (NRR). According to 40CFR211, any device offered for use as a hearing protector in the U.S. must be evaluated and labeled with its NRR.

But where do these numbers come from, and what do they mean to today’s hearing conservationist?

The NRR legacy

While HPD selection and administration are part of OSHA’s mandate under 29CFR1910.95, the actual evaluation of the devices has long fallen under the jurisdiction of the Environmental Protection Agency. The Office of Noise Abatement and Control (ONAC) within EPA has held statutory responsibility for HPD evaluation since the late 1970s. Unfortunately, ONAC has been inactive due to lack of funding since the early 1980s — thus there has been no ability to consider changes to the hearing protector rule. The evaluation process is frozen in time, requiring HPD manufacturers to evaluate their products using an ANSI standard from 1974 (ANSI S3.19-1974).

The good news is that, while the EPA is locked into the 1974 standard, the standards community itself has continued to review, revise and develop HPD evaluation standards. The most recent revision, ANSI S12.6-1997, reflects updates to laboratory evaluation standards. It includes an updated version of the lab-based, experimenter-fit procedure called Method A, where HPD is tested with significant experimenter interface. It also provides a subject-fit procedure called Method B, where interaction between the test subject and the experimenter is severely limited, providing a better indication of how HPD will perform in practice.

All of the current procedures use real ear attenuation at threshold (REAT) procedures, where the test subject is administered hearing tests with and without the HPD in place. The difference with HPD in and out, with some statistical corrections, provides the basis for the NRR.

Lab & field differences

A series of field studies compiled by Elliott Berger of AEARO Co. indicates that HPD do not perform in the field like they do in the lab. There are likely a number of reasons for this:

  • Effect of experimenter-fit HPD. Under the 1974 standard and Method A of the 1997 revision, HPD are inserted by the experimenter for optimal fit and attenuation with no regard for comfort.

  • Unrealistic conditions of the laboratory test chamber. Test subjects are required to sit still in a sound field for the relatively short duration of the tests. No consideration is given for workers who move, chew gum, talk and otherwise do the things normal to their jobs.

  • Ergonomics. Some HPD require significant manipulation before they can be used. They must be rolled down or otherwise prepared for insertion to the ear. Some workers may find this process difficult and may compromise the attenuation of the HPD in exchange for simplifying the insertion process.

  • Overprotection. NIOSH says that 90 percent of people exposed to noise in U.S. industry are exposed to less than 95 dB. If a worker who needs 10 dB of protection is provided with a device that, if used properly, provides 25 or more, how likely is it that they will compromise the HPD in order to hear better in the workplace?

    In essence, a higher NRR does not necessarily mean more protection in the field. The NRR is simply unreliable to determine the amount of protection provided by HPD in actual practice.

    Derating’s downside

    When evaluating HPD for sufficiency (i.e. is there enough protection to reduce the worker’s net exposure to 85 dB TWA or less?) using A-weighted noise measurements, OSHA simply corrects the NRR by 7 dB and does not derate. The 7 dB correction results from the NRR evaluation process being conducted using C-weighted noise. If C-weighted noise exposure measurements are available, the NRR can be used for this application at face value.

    However, OSHA has another application of the NRR. Under CPL2-2.35A, an employer may balance the cost and efficacy of a hearing conservation program against noise reduction via engineering controls. For this application, assuming the use of A-weighted noise measurements, OSHA requires the 7 dB correction to be applied first, then the remainder NRR be divided in half. In practice, the “50 percent derating” is commonly applied to all aspects of HPD administration, as managing NRR differently for two applications has proven complex and problematic.

    The unforeseen downside of derating is that HPD manufacturers are under pressure to improve the NRR of their devices to keep up with competition. If only allowed “credit” for half the NRR, most buyers will default to selecting the device with the highest possible rating. This can result in most HPD providing too much attenuation for most noise exposures if the devices are used properly in the field. If they are not used properly, they don’t perform in the field as they did in the lab, and the vicious cycle starts again.

    NIOSH weighed in on the HPD evaluation process in its 1998 Criteria Document. They suggest a variable derating system to account for differences among types of HPD, with a 25 percent derating for earmuffs (which are harder to misuse); 50 percent for formable/foam earplugs; and 70 percent for all other earplugs. This complex approach to managing an HPD program can be simplified by using the new NIOSH Hearing Protector Compendium Web site ( The compendium factors in the NIOSH derating system when selecting HPD by noise exposure.

    Industry’s response

    Many HPD manufacturers are addressing these issues by making new classes of products. Examples include:

  • New earplugs with relatively low attenuation and flat frequency response;

  • Custom molded earplugs that offer individual attenuation analysis and attenuation adjustments to match the HPD to noise exposure of each individual;

  • A high-volume headphone and software combination to measure the attenuation received by workers with the HPD they are currently using — a useful tool for HPD assessment and worker training.

    SIDEBAR: Keys to hearing loss prevention

    • Appropriate selection of HPD for the noise environment;
    • Consideration of worker ergonomics;
    • Fitting HPD to individuals;
    • Paying close attention to hearing conservation audiometric findings to detect trends of hearing loss early in the process.