Noise Reduction Rating (NRR) is a measurement of the effectiveness of hearing protection such as ear plugs or ear muffs, measured in decibels (dB). The higher the NRR number, the more effective the hearing protection provided.
Hearing loss is irreversible so it is important to understand the harm you are being exposed to from your work and activities. The table below highlights common noises and their decibel level. It is important to note that every 10 dB doubles the audible noise level.
| Noise level (dB) | Examples | Comparison | Hearing Damage |
| 10 | Breathing | None | |
| 20 | whisper, rustling leaves | None | |
| 30 | quiet rural area | None | |
| 40 | library, computer, light rain | None | |
| 50 | refrigerator, light traffic | None | |
| 60 | conversation, office, background music | None | |
| 70 | vacuum cleaner, toilet flush, city traffic | None | |
| 80 | garbage disposal, noisy restaurant, average factory, freight train | 2x 70dB | 8 hours @ 85 dB |
| 90 | diesel truck, snow blower, welder | 4x 70 dB | 4 hours |
| 100 | lawn mower, jack hammer, garbage truck | 8x 70 dB | 1 hour |
| 110 | rock concert, helicopter | 16x 70 dB | 15 minutes |
| 120 | thunder, chainsaw, oxygen torch, | 32x 70 dB | Immediate |
| 140 | aircraft carrier deck | Immediate | |
| 150 | Jet taking off 25 meters away, pistol shot | Immediate | |
| 160 | shotgun | Immediate |
While some levels, such as 140 dB, are never safe, some noise levels are safe in moderation. This table highlights safe exposure limits for different noise levels.
| Noise Level | Hearing Loss After: |
| 85 dB | 8 hours |
| 90 dB | 4 hours |
| 100 dB | 1 hour |
| 110 dB | 15 minutes |
| 120 dB + | immediate |
How does NRR change the effective noise and exposure level?
While it would make sense to just subtract the NRR rating from the decibel level of the environment you are in, but it is not quite that simple. The figure to represent your actual exposure level while using ear protection is called applied decibel level or dBA, you use the following formula:
Environment dB level – ((NRR – 7)/2) source
To apply this, if you were using a machine that had a noise level of 100 dB, and you were wearing ear plugs with a NRR of 31 dB, your level of noise exposure would be 88 dB and you’re actually reducing your surrounding noise level by 12 dB. Below is the math used to figure this out:
100 – ((31-7)/2) = 88 dB
It is important to note that these calculations are based off of laboratory data. Oftentimes, the real world data will not match laboratory results. The reason for this is usually improper fit of the ear plugs.
How does using ear plugs and ear muffs impact NRR?
If you are in a really loud environment, doubling up your hearing protection can be a good way to improve your hearing protection. To calculate the NRR of using ear plugs and ear muffs, simply take the highest NRR rating (dB) of either the ear plugs or ear muffs and add another 5 dB.
For example if you have ear muffs with a 22 dB NRR and ear plugs with a 30 dB NRR, your combined NRR would be 35 dB.