What is Noise Reduction Rating (NRR)?

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)ExamplesComparisonHearing Damage
10BreathingNone
20whisper, rustling leavesNone
30quiet rural areaNone
40library, computer, light rainNone
50refrigerator, light trafficNone
60conversation, office, background musicNone
70vacuum cleaner, toilet flush, city trafficNone
80garbage disposal, noisy restaurant, average factory, freight train2x 70dB8 hours @ 85 dB
90diesel truck, snow blower, welder4x 70 dB4 hours
100lawn mower, jack hammer, garbage truck8x 70 dB1 hour
110rock concert, helicopter16x 70 dB15 minutes
120thunder, chainsaw, oxygen torch, 32x 70 dBImmediate
140aircraft carrier deckImmediate
150Jet taking off 25 meters away, pistol shotImmediate
160shotgunImmediate

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 LevelHearing Loss After:
85 dB8 hours
90 dB4 hours
100 dB1 hour
110 dB15 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.

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