The HML (High, Medium, Low) method estimates the sound level at the ear and, unlike the NRR method, takes some account of the frequency content of the noise. This method is a little more complicated than the NRR method, but more convenient than the Octave Band method, which is recommended for noise with a high tonal content.
In order to use the HML method you need the following:
Almost all sound level meters come fitted with A-Weighting as standard, as it is the most common type of measurement. For HML hearing protector calculations check the meter must also have C-Weighting fitted.
First calculate the PNR::
If (LC - LA) > 2dB then
The assumed sound level at the ear is then calculated as:
L'A = LA - PNR
L'A is the effective A-weighted sound level at the ear
LA is the measured A-weighted sound level
LC is the measured C-weighted sound level
PNR is the predicted noise level reduction
H, M and L are the High, Medium and Low values for the hearing protector
Some regulations, such as those used in the UK, recommend adding 4dB to the final result to account for real-world factors, such as the poor fitting of protectors.
Aim to reduce the 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.
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.
NoiseMeters provides a range of online applications for calculating noise exposures along with other noise related calculators. These applications are free to use.
You can use our HML hearing protection calculator to carry out the above calculations and produce a simple report. All you need are the A-weighted and C-weighted sound level measurements and the HML figures from the hearing protector specifications.
We have had calls in the past from customers having difficulty calculating the sound level at the ear using the NRR 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 occupational noise will clearly display the A-weighted Sound Level and the C-weighted or Z-weighted Peak together as these are the first measurements you use when making an assessment, so the mistake is very easy to make.