Octave band method
This one is the proper mutt's nuts way of working out how suitable hearing protection is for a particular noise risk and if you are paying for an external consultant to do your noise assessment for you then you should insist on this being part of it.
What is an octave band measurement?
Noise can either be measured as a 'lump of noise' where you measure the number of decibels for the noise as a whole, or you can break it down into individual frequencies and measure the noise level for each of those separately. Think of it is like a piano - you can put your arm sideways and bang every key at once and measure the total noise, or you could use a special meter which recognises the individual notes and measures the noise level for each on its own.
In an octave band measurement we don't measure individual notes as that would be just too many measurements to make in one go but instead we group them together into 'bands' which you could think of as chords, and then measure the noise level for each of those chords individually.
Relevance to hearing protection
This is all very nice and lovely, but what relevance does it have to hearing protection? Well, as mentioned elsewhere hearing protection offers differing levels of protection at different frequencies and this is where the octave band measurement comes in.
One of the classic and most widespread forms of hearing protector is the EAR Classic - the yellow foam plugs seen everywhere. These are a good protector and have been around for years to prove it. They come with an SNR of 28 so at their basic you can assume they knock 28dB off the noise levels the wearer is experiencing, but are they offering the same level of protection across all frequency bands?
And the answer to this is no, they are not, but this is not a fault and is entirely normal.
The important number is the 'APV' - assumed protection factor which accounts for variation in fit by the user. At the low end of the frequency range, 63Hz, the plugs are knocking around 16.9dB off the noise levels, reaching a peak at 4kHz of 38.1dB, then back to 34dB at 8kHz, so there are large changes between the low frequency bass noises and the top end high frequency stuff.
And this takes us nicely back to the point of the octave band measurement which allows a check to be made that the hearing protection is performing adequately well at the right frequencies. In the table above, it may be no good having these protectors for a loud noise source which was predominantly 1kHz or lower while the plugs are performing at their strongest at the 4kHz level. This would be the case for the blue example octave band noise assessment chart above for example which is largely low frequency noise. The Octave band noise measurement allows calculations to be done to confirm the hearing protection is not just strong enough overall, but is strong enough at the right frequencies.
Doing the calculation
Normal people do not do this by hand. Noise geeks may do it by hand for fun but these are not normal people. The software supplied with most higher-end noise meters will be able to do this for you, usually including a database of hearing protectors and their performance data for comparison. Otherwise there are several online tools where you can put the numbers in and the calculation is done for you.