Noise Exposure – What is LEPd or LEX?

What is LEPd and why is it used?

LEPd (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 LEPd and that of Leq. Since it is normalised to 8 hours, the LEPd would equal the 8 hour Leq of a worker exposed for 8 hours. Find out the definition of Leq here
LEPd Equation

For example:

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

Note that LEPd is a logarithmic measurement as to dose which is a linear function.

The relationship between LEPd and Dose is given by the following equations:

Where ‘Log Length’ is the total logged period in hours.
LEPd Equation 2
Lep'd Equation
Each time the sound level increases, the LEPd increases. If the exposure time doubles, say 4 hours to 8 hours, the LEPd increases by 3dB.

Some noise monitoring instruments use the term Projected LEPd, 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.
Projected Lep'd Equation
A handy tip: The Projected LEPd = Leq unless the shift length is greater than 8 hours.