When noise levels are particularly high (for example, greater
than 90 dBA), 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 SNR.
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, including the 4dB real-world factor, is 76.3 dB.
Two results are shown:
Calculated Level
This is the level at the ear calculated to the
standard ISO 4869-2:1995.
Real World +4dB
In the real world there are factors that can reduce the effectiveness
of hearing protectors: imperfect fitting and the condition
of the protectors are two examples. To allow for this the HSE
recommends the addition of 4dB to the calculated level at the
ear.
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.
