Hearing protection becomes necessary when noise levels exceed 85 decibels (dB), as prolonged exposure above this threshold can cause permanent hearing damage. Noise exposure is a complex measurement, which is why a standardized system is used to rate the effectiveness of protective devices. Understanding the maximum level of protection available requires moving beyond the number printed on the package to examine the real-world physics and the testing standards that govern these devices.
Understanding Noise Reduction Rating (NRR)
The Noise Reduction Rating (NRR) is the standard metric used in the United States to measure a hearing protector’s ability to reduce sound intensity. Expressed in decibels, the NRR is determined through a highly controlled laboratory test following protocols set by the Environmental Protection Agency (EPA). During testing, researchers measure the device’s attenuation, or sound reduction, across a range of frequencies.
The NRR value represents the theoretical maximum noise reduction achieved under ideal conditions, where the device is perfectly fitted by an expert. Both earplugs and earmuffs are assigned an NRR, providing a consistent way to compare the protective capabilities of different products.
The Maximum Certified NRR
For a single, passive hearing protection device, the highest certified NRR is typically 33. This rating is almost exclusively achieved by soft, disposable foam earplugs designed to fully expand and seal the ear canal. Earmuffs generally have a maximum NRR of about 31.
Passive protection devices cannot receive a higher rating due to the physical limitation of sound transmission known as bone conduction. Even if the ear canal is perfectly sealed, sound energy causes the skull and surrounding tissues to vibrate. This vibration bypasses the ear canal and transmits sound directly to the cochlea, the inner ear’s organ of hearing.
Bone conduction sets a hard limit on the total possible noise reduction. For most people, this limit is approximately 40 to 60 dB across different frequencies. Since a theoretically perfect earplug cannot block this sound, an NRR above 33 would be misleading for a single device.
Why NRR Doesn’t Equal Decibel Reduction
It is a common error to assume that an earplug with an NRR of 33 will reduce a 100 dB noise exposure to a safe 67 dB level. The NRR is calculated under artificial conditions with an expert fit, which rarely translates to real-world use. Because of this discrepancy, regulatory bodies recommend “derating” the NRR to estimate the actual protection received.
A common method for derating involves a formula that subtracts 7 from the NRR and then divides the remainder by two. For a device rated NRR 33, this calculation results in an estimated real-world reduction of 13 dB, not 33 dB. This adjusted number reflects the fact that improper insertion and fit drastically reduce the device’s effectiveness.
The primary factor in reduced performance is the fit. Earplugs must be correctly rolled and deeply inserted to create a complete acoustic seal. A poor seal allows sound to leak into the ear canal, rendering the high NRR irrelevant. The derating calculation is a necessary safety correction to account for the variability in user technique and fit.
Achieving Maximum Noise Protection
When noise levels are extremely high, such as above 100 dBA, users may require more protection than a single NRR 33 device can reliably provide. In these situations, the strategy is to use dual protection, which involves wearing earplugs and earmuffs simultaneously. This combination creates an additional barrier against noise.
The total protection is not additive; you cannot simply add the two NRR values together. Instead, the combined effective NRR is generally estimated by adding 5 dB to the NRR of the higher-rated device. For example, wearing an NRR 33 earplug and an NRR 31 earmuff together yields an approximate total protection equivalent to NRR 38.
This dual approach is the most effective way to achieve the highest possible sound reduction and approach the bone conduction limit. To maximize the combined effect, ensure the earplugs are properly inserted before the earmuffs are placed over them, creating a double seal.