When noise levels are particularly high, it is important to assess the performance of the hearing protectors that are being used. Three methods are available for calculating the sound level at the ear. This page covers the High - Medium - Low method, better known as the HML method . The other methods are Octave Band and NRR.
For the HML method you need to measure both the A-weighted and the C-weighted sound level of the noise in question. These figures are entered into the hearing protection calculator.
You also need the H - M - L figures for the hearing protector. These figures are related to the attenuation that the protector offers in the high, medium and low frequencies and are provided by the hearing protector manufacturer.
In this example the hearing protector's HML values are 30, 24 and 22. The A weighted Sound Level was measured at 96.0 dB(A) and the C weighted Sound Level was 99.0 dB(C). The resultant level at the ear is 72.3 dB.
Over Protection and Under Protection: You should usually aim to reduce the noise level at the ear to between 70 and 80 dB. If you reduce it below 70 dB then there is a good chance that you are over-protecting, making it difficult to hear speech and warning sounders. The worker is also less likely to wear such protection at all times.