United StatesCanadaTechnical Service 1 800 243-4630Technical Service 1 800 267-4414Sales Assistance 1 800 896-4223Sales Assistance 1 800 265-1840 ext. 6137G122000 Resource GuideHearing ProtectionGEducationHow to Usethe NoiseReductionRating (NRR)The NRR describes the averagesound level reduction (attenuation)provided by a hearing protectiondevice (HPD) in a laboratory the NRR is based onlaboratory testing, it does not takeinto account the loss of protectionthat occurs when hearing protectorsare not fit properly or when they arenot worn for the entire time that thewearer is exposed to noiseFor most wearers, the NRRidentified on the current EPA label(shown here) significantlyoverestimates the protection of thehearing protector in the rating is based on an experimenter fit method ofmeasuring HPD the EPA Noise ReductionRating (Experimenter Fit)The NRR on the EPA labelshown to the right is based on theaverage amount of attenuationprovided by an HPD when it isworn by 10 different people duringa laboratory test. During this test,the person conducting the test fitsthe hearing protector on eachperson. This experimenter fit method results in ratings as high as33 dB. Since research indicatesthat these ratings overestimate theprotection that many wearers willreceive in the real world, 3MRECOMMENDS REDUCINGTHE NRR before attempting toestimate the effectiveness of anHPD as follows:1. Subtract 7 dB from the NRR ifnoise is measured on the A-weighted decibel scale (dBA).(Skip this step if noise is measuredon the C-weighted dB scale)2. Divide the result of step 1 (NRR-7) by 2. This is known as derating .A New Rating: NRR (SF)A new subject fit method ofmeasuring HPD attenuation will beused in the future to calculate adifferent rating; the NRR (SF). Thepeople (subjects) in this laboratorytest fit their own protector accordingto the manufacturer s instructionswithout the help of the personconducting the test. Compared tothe NRR shown on the current EPAlabel, the NRR (SF) is usually alower rating that may be closer to theperformance of the hearing protectorAn Example of Reducing the NRR8-hour TWA noise exposure: 93 dBANRR of hearing protectors: 29 dBSubtract 7 dB from the NRR: 29 dB - 7 dB = 22 dBDivide by 2: 22 2 = 11 dBSubtract 11 dB from the8-hour TWA noise exposure: 93 dBA - 11 dB = 82 dBDecide if 82 dB (known as the Protected Exposure )is below the PEL for noisein the real world. For moreinformation about the NRR (SF),contact 3M Technical Service at800 Noise Reduction forIndividual UsersThe labeled values of noisereduction arebased onlaboratorytests. It is notpossible touse these datato reliablypredict levelsof protectionachieved by agivenindividual ina ensureprotection, those wearing hearingprotectors for occupationalexposures must be enrolled in ahearing conservation program. Non-occupational users should have ahearing evaluation by an audiologist,physician, or other qualifiedprofessional, on a regular basis.