Nrr Formula

Education
How to Use
the Noise
Reduction
Rating (NRR)

G

The NRR describes the average
sound level reduction (attenuation)
provided by a hearing protection
device (HPD) in a laboratorytest.
Since the NRR is based on
laboratory testing, it does not take
into account the loss of protection
that occurs when hearing protectors
are not fit properly or when they are
not worn for theentire time that the
wearer is exposed to noise
For most wearers, the NRR
identified on the current EPA label
(shown here) significantly
overestimates the protection of the
hearing protector inthe workplace.
This rating is based on an
“experimenter fit” method of
measuring HPD attenuation.

Hearing Protection

Using the EPA Noise Reduction
Rating (Experimenter Fit)

The NRR on theEPA label
shown to the right is based on the
average amount of attenuation
provided by an HPD when it is
worn by 10 different people during
a laboratory test. During this test,
the personconducting the test fits
the hearing protector on each
person. This “experimenter fit”
method results in ratings as high as
33 dB. Since research indicates
that these ratings overestimate theprotection that many wearers will
receive in the real world, 3M
RECOMMENDS REDUCING
THE NRR before attempting to
estimate the effectiveness of an
HPD as follows:
United States
Technical Service 1 800243-4630
Sales Assistance 1 800 896-4223
2000 Resource Guide

1. Subtract 7 dB from the NRR if
noise is measured on the Aweighted decibel scale (dBA).
(Skip this step if noise is measured
onthe C-weighted dB scale)
2. Divide the result of step 1 (NRR7) by 2. This is known as
“derating”.

in the real world. For more
information about the NRR (SF),
contact 3M Technical Service at
800243-4630.
Estimating Noise Reduction for
Individual Users

The labeled values of noise
reduction are
An Example of Reducing the NRR
based on
laboratory
8-hour TWA noise exposure: 93 dBA...