Hearing protection devices, such as earplugs and earmuffs, safeguard the inner ear from damaging sound levels. To compare the effectiveness of these devices, the United States uses a standardized measurement called the Noise Reduction Rating (NRR). This single-number metric, expressed in decibels (dB), indicates the device’s potential to lower the wearer’s noise exposure in a controlled environment. Understanding the NRR is the first step in selecting appropriate protection for any noisy setting.
How Noise Reduction Ratings Are Calculated
The NRR is the official rating system for hearing protection devices in the U.S., established and regulated by the Environmental Protection Agency (EPA). This regulation, detailed in the Code of Federal Regulations (40 CFR Part 211, Subpart B), mandates specific testing and labeling procedures for all products. The NRR represents the difference between the noise level outside the ear and the estimated noise level that reaches the ear canal when the protector is worn.
To determine this value, manufacturers must submit their devices for laboratory testing following a standardized procedure known as Real-Ear Attenuation at Threshold (REAT). This test uses a panel of human subjects, typically ten individuals, who are fitted with the device by an expert tester. The subject’s hearing thresholds are measured at standardized sound frequencies, both with and without the hearing protector in place.
The calculation involves mean attenuation values and standard deviations across test subjects to account for individual fit variability. A mathematical correction is applied, including the subtraction of a “spectral uncertainty” value to make the rating applicable across a wide range of noise types. The final NRR value is a conservative estimate of the noise reduction achieved under ideal, laboratory conditions, where a higher number signifies greater potential protection.
The Current Highest NRR for Earplugs
The maximum Noise Reduction Rating that a single hearing protection device can achieve is 33 dB. This rating is typically found on disposable, slow-recovery foam earplugs, which are engineered to expand and fill the entire ear canal, creating a complete seal. This ceiling exists not due to manufacturing limitations but because of a fundamental physical constraint: bone conduction.
Sound can bypass the ear canal and eardrum entirely by vibrating the bones of the skull, which directly stimulates the inner ear (cochlea). Even with a perfect seal, noise still travels through the tissues and bone surrounding the ear. This physiological pathway limits the effective noise reduction of any device worn over or in the ear.
While some specialized foam earplugs may be marketed with a rating of 35 dB, the widely accepted and achievable maximum NRR remains 33 dB. This physiological limit means that creating a product rated significantly higher than 33 dB is physically impossible. When wearing both earplugs and earmuffs simultaneously (“double protection”), the combined NRR is estimated by adding 5 dB to the higher of the two individual NRR ratings.
Ensuring Real-World Protection
The NRR printed on packaging is a laboratory result that does not perfectly reflect the protection a user receives in a real-world environment. The most important factor affecting actual performance is how the user fits the device. A poor seal, even a small gap, can drastically reduce the effective noise reduction by allowing sound waves to leak into the ear canal.
For foam earplugs, which offer the highest ratings, the proper technique involves rolling the foam into a tight, crease-free cylinder before quickly inserting it deep into the ear canal. The plug must be held in place while the foam slowly expands to conform to the canal’s unique shape, ensuring a snug, air-tight fit. Improper or shallow insertion can easily lower the effective protection by 50% or more.
Because of the discrepancy between lab testing and field use, regulatory bodies like the Occupational Safety and Health Administration (OSHA) require employers to “de-rate” the manufacturer’s NRR when calculating an employee’s actual noise exposure. A common adjustment is to subtract 7 dB from the NRR and then divide the remainder by two before subtracting that value from the measured A-weighted noise level. For instance, an earplug with an NRR of 33 dB would be de-rated to 13 dB for compliance calculations.