Control of Noise at Work Regulations
Article Index
Control of Noise at Work Regulations
Daily Exposure
Hearing Protection Assessment
New Levels
Health Surveillance
Conclusions and Further Reading
Page 2 of 6

Daily Exposure - Leq 'v' Lex (Lep'd)

The main actions required by the law (both existing and new) are based on assessing a daily sound exposure figure (written as Lex or Lep'd). This is an often misunderstood, yet fundamental point, which if not observed, will invalidate any risk assessment. In fact, sound exposure is just like any other form of exposure, for example, dust, gas, radiation and even sunlight.

To take Sunlight as an example: if the average strength of the sun is measured over the course of a day (call it 8 hours), this will give a figure showing how strong the sun is. This does not, however say anything about how brown (or burnt) a person might get as this in turn depends on how long they stay outside. If the sunbather spends 4 hours in the sun, then they have had half of the available sunlight so it would be logical to take half of the number measured and use that as the exposure figure.

The same principle applies to sound exposure, where Leq is a measure of the level that exists at the workers ear position. This can be measured using a reasonably basic sound meter such as the Castle GA215 or GA113. The Lep'd (or lex) is then the proportion of that Leq relative to the time spent exposed. There is a formula to calculate this as it is not simple arithmetic, but the principle is the same. Alternatively, a dosemeter (Such as the Castle GA256) can be worn that will give the actual Lep'd (Lex) as it is accumulated.

The Difference between Pmax (Lpeak) and Lmax

RMS and Peak
Figure 1
The Noise regulations, both old and new set maximum Peak pressure levels that employees should be exposed to. Many people confuse Peak (written as Pmax, Lpeak or Ppeak) with Lmax (written as Lmax!), which is the maximum sound pressure level and is quite different.

To explain, if it is accepted that sound waves are somewhat like waves on the sea where the crests rise above normal sea level and the troughs dip below. To Look at one wave crest and measure how much water is in the wave, it would have to be levelled off into a box, making it lower. There would then be 2 measurements, one for the height of the wave and one for the level of water in the wave. For a sound wave, the height of the wave is the Peak and the level of the water is the rms. The more impulsive the noise, the bigger the difference between the Peak and the rms, so using the rms maximum (Lmax), instead of the Peak reading can give seriously erroneous readings. See figure 1
 
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