What’s wrong with NRR? Choose A or B

March 31, 2011

Since 1974, hearing protection potential has been measured by testing hearing protection under a methodology described in the ANSI standard S3.19-1974.  In that standard, researchers set out to measure the optimal protection potential of a hearing protector. This test methodology has become the “Gold Standard” for hearing protection attenuation measurement. Since that time, researchers have discovered that the “potential” afford by a hearing protector is seldom achieved in actual use. “>The seminal research on this subject was a compendium of studies compiled in 1994 by Elliot Berger (AEARO), John Franks (NIOSH), and Fredrick Lindgren (University of Gothenburg).  In that research they determined from previous research that the average attenuation from a foam plug was 46% of it’s labeled NRR.  Likewise, a molded 3-flange plug afforded 17% of what its potential might be; muffs 65%.

These results are averages, however, and therefore not applicable to specific individuals.  How does a hearing conservationist apply this information to specific individuals? The answer is, you can’t.

Once the discrepancy between the NRR and what noise reduction wearers were actually achieving was well documented, conservationists struggled for a way to approximate hearing protector attenuation with little except the NRR to rely upon. OSHA and NIOSH, developed similar formulas to de-rate the NRR of a hearing protector in an attempt to provide hearing conservationists with a more realistic estimation of real world protection values. The OSHA formula for applying the NRR to determine the users exposure level (PEL), starts with the NRR, and first adjusts it for the different scales used to measure sound (The A scale for as the ear hears at moderate levels and the C scale for how loud noise is measured on a more linear frequency response).  If the dBA scale is used, the NRR is reduced by 7 before subtracting it from the TWA (Time Weighted Average) exposure level in dBA.  If the TWA was measured on the dBC scale, then the NRR is simply subtracted from the dBC TWA.  A field directive of OSHA  (OSHA Instruction CPL 2-2.20B, February 5. Field Technical Manual (pp. 4-1 – 4-15), Rockville MD: Government Institutes, Inc.) instructs that the NRR (or adjusted NRR-7) should be halved before subtraction from the TWA exposure.  “>NIOSH and OSHA differ on the order of these computations. NIOSH recommends dividing the NRR by 2 and then subtracting 7 if using a TWA measured in dBA.  OSHA prefers to subtract 7 and then divide by 2.

In practice, let us presume you have a hearing protector with an NRR of 33 and a TWA exposure measured in dBA.  For NIOSH’s purposes you would rate that protector as… 33/2 = 16.5-7 = 9.5dB.  OSHA would rate this protector as 33 – 7 = 26/2 = 13. As you can see, whether you use 9.5 or 13 for the Real World Reduction of this protector, you are a long way from the original, labeled NRR of 33 dB.  Hearing conservationists don’t like the de-rating approach because it speaks to averages, is not really reflective of true design differences between various protectors, and does not specify what the person they are trying to protect will achieve. Their goal is zero hearing loss. Even the most conservative estimation can still be wrong up to 5% of the time.  Regulators don’t like de-rating for the same reason – the lack of specificity.  Manufacturers don’t like it as it isn’t, by nature, accurate. It only approximates an average result. Since no one likes the de-rating approach developing a test methodology to yield more accurate estimates of real world attenuation was desired.

This realization spawned a search for an alternative standard; one that would predict the noise reduction a wearer might reasonably expect to get. The resultant standard, ANSI 12.6-1997 (R2008) contained a similar method to that of the original standard for hearing protector attenuation “potential” with some slight modifications. Also, and for the first time, ANSI 12.6 included a second standard for measuring the hearing protection a naive wearer, using the protector for the first time, with only the manufacturers printed instructions to rely upon might achieve.  These two methods for determining hearing protector attenuation (protectiveness) were labeled Method “A” and Method “B” and their resultant ratings NRRa and NRRsf (where the sf stands for Subject Fit).

Research into the applicability of the new NRRsf standard unequivocally demonstrated that it produce attenuation values that were very close to what people received when they first starting wearing a hearing protector.  As a result, NIOSH (National Institute of Occupational Safety and Health) and OSHA both supported accepting this as a measure of the “Real World” attenuation.  Now finally the hearing conservationist had a measure they could rely on for determining whether a given hearing protector would work in their facility. What could be better? As it turns out, lots. (to be continued in a future blog).


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