NRR stands for Noise Reduction Rating. It’s the US standard for measuring how effective ear protection equipment is in noise cancellation. It’s the American equivalent of the European SNR (Single Number Rating). Both of these numbers can be used for comparing ear defenders to see which reduces sounds more effectively.
The NRR is measured in decibels (dB), and refers to the average reduction in volume provided by the hearing protection equipment during a laboratory test.
It’s important to note that the NRR number is calculated under factory conditions, and that in practice the amount of noise reduction will usually be somewhat less. If a pair of ear defenders have an NRR of 27 dB, for example, that doesn’t mean they will reduce a 100 dB sound to 73 dB.
For this reason, it is common to apply a ‘derating’ to achieve a more accurate measure. You can do this by subtracting the result of applying this formula:
(NRR number – 7 ) / 2
For example, if the NRR number is 30 then the derating will be:
(30 – 7) / 2 = 11.5
So the estimated actual noise reduction in decibels will be 30 – 11.5 = 18.5.
You can usually increase the NRR by combining ear defenders with ear plugs.
To give you an idea for how loud different activities are, here are some common locations, activities and objects with their approximate noise levels in decibels:
- Quiet office: 40 dB
- Large office: 50 dB
- Noisy restaurant: 85 dB
- Factory machinery: 100 dB
- Leaf blower: 110 dB
- Football stadium: 117 dB
- Chain saw: 120 dB
- Small concert: 120 dB
- Plane taking off: 140 dB
- Large rock concert: 150 dB
- Rifle: 163 dB
- Shotgun: 170 dB
Combining ear defenders with ear plugs
Cooper Safety give an explanation of the effect of combining ear defenders with ear plugs on the total NRR here. They explain that instead of adding the two NRR ratings together, you add 5 dB to the item with the higher NRR. So if you have some ear defenders with an NRR of 27 dB and some ear plugs with an NRR of 30 dB, they would have an NRR of 35 dB when combined together.