Lep’d and Lex

Lep’d (formerly Lex) is a measure of exposure and depends not only on the sound pressure level but also on the duration of the noise exposure and it is the measure of noise exposure used in the ‘Noise at Work Regulations 1989’ currently in force in the UK.

It is intended to be used to measure the daily exposure of a worker, therefore it is dependent on shift duration. Note the similarity between this definition 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.
Lep'd Equation

  • Lep’d = 90dBA. This represents a continuous constant level exposure to a noise of 90dBA for 8 hours.
  • When the Lep’d = 90dBA, this represents a dose level of 100%, using a criterion level of 90dB and an exchange rate of 3dB.

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.
Lep'd Equation
Lep'd Equation
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 instruments use the term Projected Lep’d, this is used if the exposure duration of the worker is not 8 hours to calculate an expected result for that shift.

Where ‘Shift Length’ is the actual shift length in hours.
Projected Lep'd Equation
Note: Projected Lep’d = Leq unless the Shift Length > 8 Hours

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