Lep’d (formerly Lex) is one of the most widely used measures of noise exposure, and it depends not only on the sound pressure level but also on the duration of the noise exposure. It is the measure of noise exposure used in the ‘Noise at Work Regulations 2005’ that is currently in force in the UK.
It is intended to be used to measure the daily exposure of a worker, and it is therefore dependent on shift duration.
It’s worth noting the similarity between Lep’d and that of Leq. Since Lep’d is normalised to 8 hours, the Lep’d would equal the 8 hour Leq of a worker exposed for 8 hours. Read the definition of Leq here
Note that Lep’d is a logarithmic measurement as to dose which is a linear function.
The relationship between Lep’d and Dose is given by the following equations:
Where ‘Log Length’ is the total logged period in hours. Each time the sound level increases, the Lep’d increases. If the exposure time doubles, say 4 hours to 8 hours, the Lep’d increases by 3dB.
Some noise monitoring instruments use the term Projected Lep’d, and this is used if the exposure duration of the worker is not 8 hours to calculate an expected result for their individual shift.
Where ‘Shift Length’ is the actual shift length in hours. A handy tip: The Projected Lep’d = Leq unless the shift length is greater than 8 hours.