What is NRR or Noise Reduction Rating?

The Noise Reduction Rating (NRR) is a laboratory-derived single-number rating designed to characterize a hearing protector's noise reduction capabilities. To quantify hearing protector performance, the EPA specifies laboratory test protocols that call for a trained experimenter to fit individual subjects with appropriately sized hearing protectors. Noise attenuation (reduction) values are calculated by comparing the subject's hearing threshold results with and without the hearing protector in accordance with ANSI Standard S3.19 test procedures. Test results are reported for individual octave band test frequencies 125-8000 Hz and then calculated into a single-number overall Noise Reduction Rating.

The NRR is intended to be used to estimate the amount of protection provided by the hearing protector. The EPA originally estimated that the level of noise entering a person's ear, when the hearing protector is well-fitted and worn as directed, is approximated by the difference between the environmental noise level and the NRR. Read further for lessons learned over the years about applying the NRR.

Hearing protector packaging labels must list the single-number NRR, product name, and manufacturer information (see example primary NRR label). A secondary label must also be included that provides further detail regarding the average amount of noise reduction across the required ANSI test frequencies (125 to 8000 Hz) and instructions on how to apply the NRR. example: EPA format for primary NRR label

Regulatory matters

When OSHA promulgated its Hearing Conservation Amendment (29 CFR 1910.95) for general industry, it incorporated the existing Environmental Protection Agency (EPA) Noise Reduction Rating (NRR) system for assessing performance of hearing protectors. The EPA developed this laboratory-based system for quantifying noise reduction in the late 1970s and codified it in 40 CFR Part 211, Subpart B. Although numerous changes and improvements have been recommended over the years, the original NRR testing and labeling requirements are still in force today.

Soon after the Hearing Conservation Amendment went into effect, OSHA issued a controversial noise control guideline intended to clarify employer responsibilities for reducing noise given the new Amendment. This compliance directive, CPL 02-02-035, Appendix A, instructs companies to take into account certain factors such as hearing protection, shifts in hearing, cost of controls, etc. when comparing the relative effectiveness of hearing protectors and engineering and/or administrative controls. In this situation, OSHA instructs employers to "apply a safety factor of 50 percent" to the NRR. This directive does not apply to hearing protector requirements under the Hearing Conservation Amendment (OSHA 1910.95 Appendix B; Technical Manual Section III: Chapter 5, Appendix F. Noise Reduction Rating).

When MSHA introduced its latest Occupational Noise Exposure Standard in 1999, Part 62, it did not limit mining operators to use of the EPA's NRR system. Instead, MSHA took a much more flexible approach, allowing hearing protectors to be assessed according to "a scientifically accepted indicator of noise reduction value."

Other Rating Systems

Although safety professionals in the United States are most familiar with the NRR, there are many different rating systems used in other parts of the world. As example, Canada allows a number of hearing protector ratings including a classification system that divides hearing protectors into Classes A, B or C, determined by how much noise reduction is provided in a specified laboratory setting (CSA Z94.2-14 Hearing Protection Devices). Class A hearing protectors offer the highest protection and may be used in 8-hour time-weighted average noise exposures up to 105 dBA; Class B protectors are rated for average noise exposures up to 95 dBA, and Class C up to 90 dBA. In addition, the suffix 'L' for "low frequency" is added to class A, B, or C when a hearing protector provides at least 20 dB of attenuation at 125 Hz.

NRR Method

To estimate worker's protected noise exposure using the laboratory-derived Noise Reduction Rating (NRR):

1. Determine the employee's workplace noise exposure in dBA TWA*
2. Calculate the estimated noise reduction as follows:

o Start with the manufacturer's NRR for this hearing protector (NRR)
o Subtract 7 dB (if using A-weighted noise exposure values*)
o Divide in half to estimate "real world" performance (required if reviewing feasibility of engineering controls

3. Subtract the noise reduction estimate from the employee's workplace noise exposure to approximate protected exposure (target 85 dBA TWA or below):

*Note: If workplace noise exposure readings are available in C-weighted form (dBC TWA), there is no need to deduct 7 dB when estimating protection using the product's NRR.

Understanding an NRR label

This example shows the "primary" EPA-required NRR label, which must be placed prominently on the packaging of all hearing protectors sold in the US. In addition, the EPA requires a "secondary" label which provides further detail regarding the average amount of noise reduction across a broad range of test frequencies (125 to 8000 Hz), and instructions on how to apply the NRR.

In the above example, the NRR is a single-number value intended to represent the overall amount of protection provided if the device is worn correctly and consistently. Generally, the higher the NRR, the more protection could be provided if the user has a proper fit and wears the device correctly whenever exposed to loud noise. Although personal preferences are important, generally, extremely loud activities such as using a chainsaw may call for a higher NRR than listening to music in a club, for example. Choose lower NRR products for lower noise exposures, so that the individual is not "overprotected" in the environment.

Remember that features such as comfort and ease of use are just as essential to success in protecting hearing. A device that is uncomfortable or difficult to use won't do much good if the user is reluctant to wear it or if it doesn't stay put for the duration of the noisy activity.

Is a higher NRR always better?

In the past, many companies mistakenly chose hearing protectors for their entire company based on one feature alone: The Noise Reduction Rating (NRR). Historically, conventional wisdom dictated that the more protection, the better, regardless of each employee's noise exposure, preference, and work requirements. It's long been known that too much noise exposure can cause permanent hearing loss. But now there is growing concern that too much noise attenuation (reduction) can be problematic, too.

According to OSHA, 80-90% of workplace noise exposures in general industry are under 95 dBA TWA. For these moderate noise environments, employees typically only need 10-15 dB of actual noise reduction to protect hearing. Overprotection occurs when a hearing protector reduces sounds to such low levels that it's difficult for the wearer to understand conversations and/or hear important sounds around them. An inability to communicate or detect warning signals can keep workers from performing their jobs safely and efficiently. Miscommunications or missed cues at best can lead to production errors and inefficiencies – at worst, accidents and injuries.

In the US, there is no formal recommendation for overprotection, but the National Institute for Occupational Safety and Health (NIOSH), has addressed the concern in various case studies and reports. In a 2005 hearing protector study, NIOSH stated: "Overprotection is a condition in which the worker receives more attenuation than necessary, and the worker's exposure is reduced to less than 70 dBA. Overprotection results in the reduction in verbal communication with other workers, the potential to miss safety signals, and reduced acoustic feedback from their machine or work process. Overprotected workers report being 'out of touch' with their environment or feeling isolated." The British Standards Institute and European Union jurisdictions recommend the ideal level of protection to be 70-80 dBA, while Canada promotes an ideal protected range of 75-80 dBA.

To learn more, check out this deep dive into Overprotection and potential solutions

Personal Attenuation Rating (PAR) - What does it mean and how is it derived?

Personal Attenuation Rating (PAR) is a measure of hearing protector attenuation for each worker as determined by Individual Fit Testing. Individual Fit Testing is conducted for each employee using his/her own hearing protector as normally worn. OSHA, NIOSH and many professional organizations have long recognized that the laboratory-derived NRR is not a good predictor of actual hearing protector performance in the real world. Individual Fit Testing is now a recommended best practice for hearing conservation programs.

In 2008, an alliance between OSHA, NIOSH and the National Hearing Conservation Association identified technologies designed to establish individualized hearing protection attenuation ratings for each worker. This group of experts recognized the limitations to relying on laboratory conditions and group statistics to predict an individual user's hearing protector performance in the field. "The consequence of this approach is that an individual user may actually receive more but usually less attenuation than is stated on the hearing protector label." Based on their review of research and emerging trends and technologies, the Alliance identified Individual Fit Testing as a recommended best practice for hearing conservation programs. Individual fit testing of hearing protectors is similar in concept to fit testing for respirators. Preferred methods produce a single number overall estimate of real-world attenuation for each worker; this measure is generally referred to as a Personal Attenuation Rating or PAR.

To best determine what is happening in the real world for each worker, conduct an Individual Fit Test to ensure that the chosen hearing protector is actually performing as intended. Similar to NRR calculations, the goal of the PAR calculation is to ensure the worker's protected noise exposure has been reduced to a "safe" level, in most cases considered 85 dBA TWA or below. The difference is that the PAR is specific to the worker, derived from individual testing, not laboratory estimates, and does not require A-scale or de-rating "corrections."

To learn more, check out this deep dive into Individual Fit Testing for Hearing Protectors

Why do OSHA formulas require us to subtract 7 dB from the Noise Reduction Rating (NRR)?

The 7dB reduction is a correction to be used with the employee's A-scale noise measurements. Most people are surprised to learn that the current NRR system was developed in the 1980s for use with C-scale workplace noise readings (the C-scale is a broad-spectrum frequency setting on sound level meters and dosimeters). Employee time-weighted-average noise exposures, however, must be calculated with an A-scale frequency setting on the sound measurement equipment (the A-scale is a weighted frequency setting more closely matched with human damage-risk criteria). The bottom line: if the employer has A-scale noise information, but not C-scale, then a 7 dB correction factor is required when using the NRR to assess hearing protection for the workplace.

For more information, check out our SoundBytes newsletter article, The NRR: What's in a Number?

Does OSHA require companies to divide the Noise Reduction Rating (NRR or NRR-7) in half?

It is a common misconception that OSHA requires employers to "de-rate" NRRs by 50% when assessing adequacy of the hearing protector for the worker's noise exposure. Instead, OSHA only requires applying a 50% "safety factor" when considering whether engineering controls are to be implemented. For determining compliance with basic hearing conservation program requirements, however, OSHA does not require de-rating the NRR (or NRR - 7 when the company uses A-scale noise measurements).

The background is that de-rating schemes were developed decades ago and generally are no longer deemed helpful. Instead, individual hearing protection fit testing is now considered best practice for verifying real world performance.

For more information, see the explanation provided in the OSHA Technical Manual Section III: Chapter 5, Appendix F: Noise Reduction Rating