NRR Rating Guide

Overview

The following are the different ways that the Noise Reduction Rating (NRR) can be used. This document provides a brief summary and several examples of these evaluation methods. An employee may not be exposed to noise levels exceeding 90dBA as an 8 hour Time Weighted Average (TWA). A comprehensive hearing conservation program must be implemented when noise level exceeds 85dBA as an 8 hour TWA. The OSHA hearing conservation standard is 29 CFR 1910.95

OSHA NOISE AND HEARING CONSERVATION

Refer to the following links for OSHA guidance on a hearing conservation.
Noise Hearing Conservation Overview
Noise Hearing Conservation Standards
Regulations (Standards – 29 CFR) – Table of Contents

• Use the following formulas to estimate the attenuation afforded to a noise-exposed employee in a work environment by a hearing protection device.

A common method used for single protection (either muffs or plugs) is as follows

1. Determine the laboratory-based 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 C-weighted TWA workplace noise level, as follows:

Estimated Exposure (dBA) = TWA (dBC) – NRR

In a working environment with a noise level of 110, an earplug with an NRR of 33 gives an Attenuated Noise Level of 77.

If C-weighted noise level data is not available, A-weighted 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, NRR=33 dB
Estimated Exposure = 110 – (33-7) = 84 dBA

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:

Estimated Exposure (dBA) = TWA (dBC) – [NRR x 50%], or
Estimated Exposure (dBA) = TWA (dBA) – [(NRR – 7) x 50%]

When noise is measured and attenuation estimated using OSHA C Weighted: Noise Level – 50% of NRR = Attenuated Noise Level

Example: 97-33/2 = 80.5

When noise is measured and attenuation estimated using OSHA A Weighted: Noise Level – (NRR – 7dBA)/2 = Attenuated Noise Level

Example: 97-(33 – 7)/2 = 87.5

Moldex Solutions: Moldex manufactures a wide range of hearing protection devices, from our very popular SparkPlugs disposable foam earplugs to our Glide twist to fit push in earplugs, for additional information and to request samples, please see below.
Hearing Protection
Foam Earplugs
Reusable Earplugs
Hearing Bands
Earmuffs
Moldex PlugStations

Caution: Use this information only as a guide to the different ways NRR data is utilized in the field. These should not be used by the end-user to indicate the actual amount of attenuation attained by an end-user in a specific situation.

WARNING: The information contained in this Tech Brief is dated and was accurate to the best of Moldex’s knowledge, on the date above. It is not meant to be comprehensive, nor is it intended to be used in place of the warning/use instructions that accompany Moldex respirators. Outside of the USA, check with all applicable and local government regulations.

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