In our article about NRR 33 hearing protection for hunters, we mentioned that "an increase of 10 dB in NRR (moving from 0 to 10 NRR) will reduce the sound you hear by about half." Today, let's take a look at how this works in a little more detail.
First and foremost, steady state noise such as you would find in a factory is very different from impulse noise such as muzzle blasts.
Steady State Noise (Continuous)
As a basic rule of thumb, you can subtract the NRR rating from the noise level you are trying to get protection from when dealing with steady state noise. Let's look at some examples. The article's comment about 10 dB protection reducing sound at the ear by half is true -- but that half is measured on a log scale. That means the 10 dB would be simply subtracted from the maximum. For example, at a blast of 160 dB, a 10 dB protector would reduce the volume at the ear by half, to about 150 dB (not nearly enough protection in a steady state noise environment at that level). At a 95 dB average sound level, the same 10 dB protector would reduce the noise by half, to 85 dB (a safe level of protection). The logarithmic scale can be very confusing, because the numbers it measures are non-linear. Just remember that each 10 dB increase on the dB scale doubles the sound volume, and each 10 dB decrease on the dB scale halves the sound volume. Again, with steady state noise such as you might find in a factory, subtracting the hearing protector's NRR rating from the total dB of sound in your environment is a quick and simple way to figure out how much protection you are getting from noise.
Impulse Noises (Blast Sound)
Impulse noises such as muzzle blasts are very different from steady state noises. First, they tend to be much louder than any steady state noises you will ever encounter (unless you work around jet engines or rockets). Second, impulse noises last for just fractions of a second. Impulse noise has not been nearly as well studied as steady state noise, and there is not much in the way of agreement about how to measure the kind of protection you need. In general, most experienced shooters figure out what works for them through experience -- and unless you shoot a lot of large caliber rounds, most of the ear plugs and ear muffs on the market will give you adequate protection. When working with large caliber or magnum rounds and/or simply sending a lot of lead downrange, dual protection (ear plugs worn under a set of ear muffs) is generally recommended.
How to Find Protection for Impulse (Blast) Noises
Even understanding a bit more about how NRR works, trying to find the right hearing protection for shooting can still be frustrating. For more information, check out this great report showing study results on how many rounds you can safely expend per day from different kinds of weapons, at two different levels of hearing protection. Obviously more bullets fired equals more blast noise, which equals more risk of hearing damage and a need for more noise protection. Note that this report comes from a European company, and the European standard is SNR, rather than NRR. To convert from SNR to NRR, subtract 2 to 5 dB from the SNR ratings shown -- that will give you an approximation of the equivalent NRR. So for instance, the report tells us that you can fire a shotgun about 250 times in a day safely while wearing about NRR 20 hearing protection, while you can only fire a 7.5mm assault rifle about 30 times in a day safely using that same NRR 20 hearing protector. Note that this conversion from SNR to NRR is just an educated guess, as the two standards use completely different testing protocols. We do know that SNR is always higher than NRR for the same device, though.
So in short:
When it comes to steady state noise, you can estimate how much your hearing protection reduces noise by subtracting the NRR from the ambient noise level in dB's. When it comes to impulse noise, however, that rule of thumb just does not work. With blast noise, your best bet is to realistically consider 1) how loud your weapon is and 2) how frequently you will be firing it, and then to simply experiment with different hunting/shooting hearing protection solutions to find out what works for you.
Just remember that if you think the noise reaching your ears is too loud, it probably is!
Until next time, be safe and love your ears.
--Sarah Bergman, Web Developer
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