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.
