Noise Reduction Ratings

Question

Please discuss the current noise reduction rating (NRR) systems and their applicability to passive hearing protection devices.

Answer

''The larger the noise reduction rating (NRR), the better the hearing protector'' has been the mantra of many hearing conservationists, safety specialists and hearing protector manufacturers and distributors for the past 2 decades

The larger the noise reduction rating (NRR), the better the hearing protector'' has been the mantra of many hearing conservationists, safety specialists and hearing protector manufacturers and distributors for the past 2 decades. Only recently have hearing conservationists acknowledged the inadequacies of the NRR in predicting the actual attenuation afforded by the protector under real-world use conditions. Real-world studies reveal that individual attenuation varies from 5-95% of the manufacturer NRR depending on the particular protector used. Some individuals may receive less than 5 dB of attenuation in some instances, or stated another way; the same hearing protector may provide 4 dB of attenuation for one worker and 28 dB of protection for another worker in the same work environment.

Initially, the NRR was developed in the early 1970's as a single number rating system that would represent the optimum attenuation properties of hearing protectors when used with A or C-weighted sound level measurements. This rating scheme circumvented the need for cumbersome calculations of attenuation values at individual octave bands for each individual noise environment encountered. In 1979 the Environmental Protection Agency (EPA) subsequently adopted the single number rating system of the now-rescinded ANSI S3.19-1974 (American National Standards Institute, 1974) for hearing protection device manufacturer labeling rules. This same rating system is also incorporated in the Occupational Safety and Health Administration (OSHA) 29 CFR 1910.95 hearing conservation regulations for industry.

Subsequently, after years of study and refinement the NRR-Subject Fit (NRR-SF) protocol has been standardized in the recently revised ANSI S12.6-1997 ''Method B: Subject Fit'' testing. (This standard also was revised to include an updated ''Method A: Experimenter-Supervised Fit protocol). Manufacturers currently may voluntarily publish or provide NRR-SF data, however the EPA labeling rules have not been updated since the office was closed during the Reagan era. The method B values are currently referenced by the United States Armed Forces and referenced in the OSHA Technical manual at www.osha-slc.gov/dts/osta/otm/otm_iii/otm_iii_5.html

The following outline affords an opportunity for the reader to see the distinctions between the two NRR rating schemes;

NRR:

Represents optimum attenuation values

Laboratory based measurements which theoretically achieved by 98% of the subjects (2 standard deviations)

Experienced listeners used as subjects

Hearing protector fit by experimenter

Data represents capabilities of the hearing protection device used under ideal conditions

Applicable to C-weighted sound level measurements or requires a correction for use with A-weighted measurements

NRR-SF

Represents real-world minimum attenuation values

Laboratory based measurements (using 1 standard deviation) which is theoretically achieved by 84% of the actual wearers

Inexperienced listeners used as subjects

Hearing protector is fit by the subject referencing the manufacturer's written instructions only

Data approximates protection attained by groups of informed users in workplaces with representative well-managed and well-supervised occupational hearing conservation programs

Applicable directly to A-weighted sound level measurements without correction

As expected, the NRR-SF ratings are lower than traditional NRR values. Consequently, changing the perspectives and beliefs of hearing protector manufacturers, suppliers, sales personnel, purchasers, professionals and end-users will take a concerted effort at education and implementation of the NRR-SF. While we wait for the reopening of the EPA office and an update to the OSHA regulation, hearing conservationists must remember that the ultimate goal of the hearing protection device is to protect an individual wearer. This requires an approach that finds the critical balance between attenuation, comfort, ease of use, work equipment compatibility, safety and communication needs and does not rely solely on a single number value. Regardless of how it is derived.

BIO:
Deanna Meinke, M.A., FAAA
Associates in Acoustics, Inc.