Hearing protection devices (HPDs), such as earplugs and earmuffs, are designed to reduce the intensity of sound reaching the inner ear. Because exposure to high noise levels can cause permanent hearing damage, a standardized system is necessary to compare the effectiveness of different devices. This rating system allows consumers and occupational safety professionals to select the appropriate protection level for a given noisy environment. The primary standardized measure used in the United States to gauge this effectiveness is called the Noise Reduction Rating (NRR).
The Noise Reduction Rating System
The Noise Reduction Rating (NRR) is a single-number metric expressed in decibels (dB), representing the theoretical maximum sound level reduction a device can provide. This system is mandated and regulated by the U.S. Environmental Protection Agency (EPA) for hearing protection products sold in the United States. The NRR typically ranges from 0 to about 33, serving as a uniform benchmark. A higher NRR value signifies a greater potential to diminish the intensity of environmental noise.
The NRR is determined under highly controlled, idealized laboratory conditions. The published rating is a calculated value based on the measured attenuation across a range of sound frequencies. This number is not a guarantee of the protection a user will experience in a real-world setting. Instead, it offers a baseline for comparison between different styles and brands of hearing protectors.
Determining the Reduction Rating
The process for determining a device’s NRR involves a standardized laboratory test known as Real-Ear Attenuation at Threshold (REAT). This procedure measures the difference in a subject’s hearing threshold with and without the hearing protection device in place. Test subjects are exposed to specific sound levels across nine octave-band frequencies, typically from 125 Hz to 8000 Hz.
Trained laboratory technicians fit the HPD onto each subject to ensure an optimal, acoustically sealed fit. This expert-fit protocol is designed to achieve the best possible performance from the device. The attenuation data gathered from multiple subjects and repeated tests are averaged and statistically processed to arrive at the single NRR number. Because the fit is optimized by a professional, the resulting NRR is often an optimistic representation of the protection level achieved by a casual user.
Applying the Rating in Practice
The NRR rarely reflects the actual noise reduction achieved by an average person in the workplace or at home. Factors like improper insertion, movement, or poor maintenance can significantly diminish a device’s effectiveness. For this reason, safety professionals recommend “derating” the NRR to estimate a more realistic level of protection.
One common method for derating involves subtracting 7 dB from the NRR and then dividing the remainder by two. For example, a hearing protector with an NRR of 30 dB would be derated to 11.5 dB. The 7 dB subtraction accounts for the difference between the C-weighted scale used in NRR testing and the A-weighted scale often used to measure workplace noise exposure.
The Occupational Safety and Health Administration (OSHA) requires a derating of 50%, or dividing the NRR in half, when evaluating whether engineering controls are necessary. The National Institute for Occupational Safety and Health (NIOSH) recommends variable derating factors based on the type of device. NIOSH suggests reducing the NRR by 75% for earmuffs and 50% for most earplugs for a more accurate field estimate. Proper fit remains the most significant factor, meaning even a derated NRR is useless if the device is not worn correctly.
Other Measurement Systems
While the NRR is the standard in the United States, other rating systems are used internationally. The most prominent alternative is the Single Number Rating (SNR), which is the standard used throughout the European Union and many other countries. Like the NRR, the SNR is expressed in decibels and indicates the average noise reduction provided by the device.
The SNR is derived from different testing standards and calculation methods than the NRR, meaning the values are not directly interchangeable. Typically, an SNR value for a given device will be a few decibels higher than its corresponding NRR. The European standard also includes the HML rating, which provides three separate attenuation values for High, Medium, and Low-frequency sounds. This provides users with a more detailed assessment of how the protector performs against different types of noise.