The actual effectiveness of any individual hearing protector cannot be determined under workplace conditions. However,
OSHA's noise standards (29 CFR 1910.95(j)(2) and
29 CFR 1926.52(b))
require that personal hearing protection be worn to attenuate the occupational noise exposure of employees to within the limits shown in
Tables
G16,
G16a, and
D2,
respectively. Hearing protectors are evaluated under laboratory conditions specified by the American National Standards Institute in ANSI
S3.191974 (OSHA's experience and the published scientific literature indicate that laboratoryobtained real ear attenuation for hearing
protectors can seldom be achieved in the workplace).

Appendix B: Methods For Estimating the Adequacy of Hearing Protector Attenuation provides
information on how to determine the adequacy of hearing protector attenuation
using the noise reduction rating (NRR) of a given hearing protector.
 Use the following formulas to estimate the attenuation afforded to a noiseexposed employee in a work environment by muffs, plugs, or a
combination of both.
 A common method used for single protection (either
muffs or plugs) is as follows
1. Determine the laboratorybased noise attenuation provided by the HPD. This is referred to as the Noise Reduction Rating (NRR) and
is listed on the packaging.
2. Subtract the NRR from the Cweighted TWA workplace noise level, as follows:
Estimated Exposure (dBA) = TWA (dBC)  NRR
If Cweighted noise level data is not available, Aweighted data can be used by subtracting a 7 dB correction factor from the NRR, as
follows:
Estimated Exposure (dBA) = TWA (dBA)  (NRR  7)
Example:
TWA=100 dBA, muff NRR=19 dB
Estimated Exposure = 100  (197) = 88 dBA
 For dual protection (ear muffs and plugs are used simultaneously) use the following:
1. Determine the laboratorybased NRR for the higher rated protector (NRR_{h}).
2. Subtract 7 dB from NRR_{h} if using Aweighted sound level data.
3. Add 5 dB to the fieldadjusted NRR to account for the use of the second hearing protector.
4. Subtract the remainder from the TWA as follows:
Estimated Exposure (dBA) = TWA (dBC)  (NRR_{h} + 5) , or
Estimated Exposure (dBA) = TWA (dBA)  [(NRR_{h} 7) + 5]
Example:
TWA=110 dBA, plug NRR=29, and muff NRR=25 dB
Estimated Exposure = 110  [(29  7) + 5] = 83 dBA
 OSHA's experience and the published scientific literature have shown that laboratoryobtained real ear attenuation for HPDs can seldom be
achieved in the workplace. To adjust for workplace conditions, OSHA strongly recommends applying a 50% correction
factor when estimating field attenuation. This is especially important when considering whether engineering controls are to be
implemented. The equations above would then be modified as follows:
 Single Protection:
Estimated Exposure (dBA) = TWA (dBC)  [NRR x 50%], or
Estimated Exposure (dBA) = TWA (dBA)  [(NRR  7) x 50%]
 Dual Protection:
Estimated Exposure (dBA) = TWA (dBC)  [(NRR_{h} x 50%) + 5] , or
Estimated Exposure (dBA) = TWA (dBA)  {[(NRR_{h } 7) x 50%] + 5}
