Lex (Lepd) - Daily Personal Noise Exposure

Lex (or Lepd as it is sometimes referred to) is a measure of exposure and depends not only on the sound pressure level but also on the duration of the noise exposure. Lex 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 dependant on shift duration. Note the similarity between this definition and that of Leq. Since Lex is normalised to 8 hours, the Lex would equal the 8 hour Leq of a worker exposed for 8 hours.
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Example:

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

Note that Lex is a logarithmic measurement as to dose which is a linear function. Hence dose can be added arithmetically but since Lex is logarithmic, values cannot be added arithmetically.

The relationship between Lex and dose is given by the following equations:
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Where time is the duration of a shift, expressed in hours.

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Each time the sound level increases, the Lex increases. If the exposure time doubles, say 4 hours to 8 hours, the Lex increases by 3dB.

Some instruments use the term Projected Lex, this is used if the exposure duration of the worker is not 8 hours to calculate an expected Lex for that shift.
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Where duration is the total length of the shift expressed in hours.

 
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