The Single Number Rating (or SNR) method is the easiest of the three recommended methods. This method should be accurate unless the noise is tonal or is dominated by low frequencies. In this case use either the HML or Octave Band method.
In order to estimate the sound level at the ear using the SNR method you need the following:
The calculation is performed as follows:
L'A = LC − SNR + 4dB
L'A is the effective A-weighted sound level at the ear
LC is the measured C-weighted sound level
SNR is the Single Number Rating for the hearing protector
4dB is added to allow for "real world" factors, such as badly fitted protectors. This is a recommendation of the HSE.
You should aim to reduce the sound level at the ear to between 70 and 80 dB(A).
Under-protection - If the level at the ear is still above 80 dB(A) then the protectors are not providing adequate cover for longer exposure times.
Over-protection - If the level at the ear is below 70 dB(A) then the worker is being over-protected. This can result in difficult communication and the inability to hear warning alarms. The hearing protection is also likely to be heavier and more uncomfortable then necessary.
Most measurements for noise at work use A-weighting. All action levels are also based on dB(A). Unfortunately the SNR method requires the C-weighted level, so you must use the dB(C) measurements and not dB(A).
You can use our online SNR hearing protection calculator to carry out this process and produce a simple report. Hearing Protection Calculator
We have had calls in the past from customers having difficulty calculating the sound level at the ear using the SNR or HML method. The reason is usually that they are using the C-Weighted Peak measurement, rather then the C-weighted Leq or Sound Level.
Most meters designed for Noise at Work will clearly display the A-weighted Sound Level and the C-weighted Peak together as these are the first measurements you use when making an assessment, so the mistake is very easy to make.