Hearing protection devices are often labeled with Noise Reduction Ratings, or NRR. For example, an earmuff might be listed as NRR 31, or a set of earplugs be advertised as NRR 25. So what do those NRR numbers mean, and what Noise Reduction Rating do you need for your workplace?
NRR and dB
Noise Reduction Ratings measure how much sound is blocked by earmuffs or earplugs. The loudness of sound is measured in decibels, or dB, so the NRR number tells you how many decibels the hearing protection device blocks.
So if the noise level at your workplace is 90 dB—about as loud as a lawnmower—using NRR 30 earmuffs would lower that to 60 dB, or the sound of a normal conversation. Right? Unfortunately, it’s not that simple.
The problem is that there are different ways of measuring decibels. Noise levels are often measured using what’s called the “A-weighting” of decibels, or dBA. A-weighting adjusts the decibel scale to account for how well humans hear different frequencies. Some very low or very high frequencies don’t sound very loud to human ears, while other frequencies are quite easy for us to hear. For example, a human might not hear a dog whistle at all, even though the dog whistle is actually very loud.
NRR ratings are not calculated using the dBA scale; they measure changes in regular dB. So although a pair of NRR 30 earmuffs might decrease the noise level by 30 dB, that does not equate to a 30 dBA reduction in sound. To make sure you’re getting a safe level of hearing protection, it’s recommended that you convert the NRR rating using this formula:
- Subtract 7 from the NRR number.
- Divide the result by 2.
The resulting number estimates how much protection is provided on the dBA scale. For NRR 30 earmuffs, subtracting 7 from 30 gives you 23, and 23 divided by 2 is 11.5. In the lawnmower example above, that means the earmuffs would lower the sound level from 90 dBA to 78.5 dBA. That might not look like a big reduction, until you consider the fact that decibel ratings use a logarithmic scale. On a logarithmic scale, a decrease of 10 decibels means the sound is 10 times quieter, or only 10% as loud.
What Noise Reduction Rating do you need?
OSHA sets limits for how much noise exposure is allowed in the workplace. The limits are based on both the noise level (measured in dBA) and the number of hours per day a worker is exposed to that noise level. The louder the dBA level, the less time a worker can spend in the environment.
The noise exposure limits are:
|OSHA’s Permissible Noise Exposures|
|dBA Sound Level||Hours Per Day|
Based on this chart, you can calculate how much hearing protection you need. For example, if someone is going to be in a 110 dBA environment for two hours, they need hearing protection that reduces the noise level to 100 dBA or less. (The NRR 30 earmuffs in our example above would work for this, since they would lower the noise level by 11.5 dBA.) If you’re going to be in a 97 dBA environment all day long, you’ll need hearing protection that lowers the noise level to below 90 dBA—or else you’ll have to quit working after just three hours.