How Is Noise Reduction Rating Determined?

Learn how earplugs are evaluated to see how much noise they'll block out in a loud environment

What is Noise Reduction Rating and how is it determined?

If you work on a construction site or other areas with loud equipment, you and your co-workers probably use protective hearing devices. You may also see people use them at concerts or on firing ranges.

Hearing protection devices are important because they reduce sound exposure in unusually loud environments. This helps protect users' ears from long-term hearing loss, and are mandatory for most jobs in loud environments.

But how are these products tested, and what standards are used to evaluate them?

Well, all hearing protection devices are assigned a Noise Reduction Rating (NRR), and that rating is determined by thorough testing and application.

NRR Explained

The Noise Reduction Rating is a standardized rating to determine a product's effectiveness is reducing noise exposure. This is particularly important for people in environments with harmful decibels of noise. But what exactly constitutes a harmful noise environment?

OSHA (the Occupational Safety and Health Administration) defines anything over 90 decibels (dB) as harmful. For reference, that's the volume of a lawnmower, tools in a workshop, or the subway.

Anything using beyond 100dB is considered painful. This includes jackhammers on construction sites, airplane engines, and loud concerts. OSHA has developed a Permissible Noise Exposure chart to help determine what protections need to be in place for loud environments.


But what about workers who consistently work in these loud environments, like stage hands or jet engine technicians? That's where protective hearing equipment and the NRR become important.

An NRR tested product protects users against high decibel sound exposure by reducing the decibels that reach the user's eardrum. There is a wide range of protective hearing devices, with the highest rated sitting at 33dB.

However, this is where things get tricky. Just because a product is tested for 33dB does not mean it reduces noise exposure by that amount. To get the actual protection, users have to subtract seven from the NRR then divide by two. As below:


That means a 33dB rated device in a 90dB reduces the noise exposure to 77dB, not 57dB.

What about multiple layers of protection? Do you add the total NRRs together and then subtract seven and divide? According to OSHA, to determine the noise reduction users should add 5dB to the total protection of the higher device. So if you have earplugs rated for 27dB and earmuffs for 33dB, the total protection would be 28dB (10dB + [13+5]).

It may seem a little confusing at first, but once you wrap your head around the numbers it really isn't too complicated.

NRR Testing

How is the actual NRR determined, though? Well, all NRR rated hearing protection devices go through a series of tests to determine its official rating.

Lab tests are conducted on a minimum of 10 participants at least three times, amounting to at least 30 attenuation tests. In these tests, participants are exposed to noises of increasing decibels both with and without hearing protection.

After the tests are conducted, researchers adjust for various deviations in the tests. Not everyone's ears are tuned the same way, and these adjustments ensure the testing represents the majority of the population.

After the adjustments are made, researchers begin logarithmic additions to determine the decibel rating for the sounds. These additions are not straightforward, because sound is spectral. That means 80dB + 80dB does not equal 160dB. Researchers have to apply special logarithms to determine the actual decibel rating.

After the additions are made, there is an added correction factor of 3dB. This is meant to adjust for "spectral uncertainty" because the conditions in the lab are different from real-world noise exposure. This adjustment accounts for that difference.

With all these adjustments and additions in place, researchers can now determine an NRR that reflects the experience of 98 percent of the population. That means people with sensitive hearing will have a similar noise reduction experience as someone with average hearing.

Summing it Up

In short, the NRR is not an arbitrary number, but something that is heavily tested and verified. While no one can precisely measure the decibels reaching someone's eardrum, these tests ensure that users will get a consistent noise reduction experience from NRR tested products. The protective hearing devices are important for workers in loud work environments,  and are required by OSHA in most loud workplaces. You can also find the best earplugs for sleeping as well as learn more about high-tech vs. conventional protective hearing devices.

Resources - Occupational Safety & Health Administration, Industrial Safety & Hygiene News