Sound and Vibration Basics

Leq - Equivalent Continuous Sound Level - LAeq

Leq Definition

Leq is the preferred method to describe sound levels that vary over time, resulting in a single decibel value which takes into account the total sound energy over the period of time of interest..

Leq - equivalent continuous sound level : Sound levels often fluctuate over a wide range with time. For example in the middle of the night the level might go down as low as 30dB(A) with occasional passing vehicles of 70dB(A) or more. Later comes the dawn chorus followed by the general noises of the day before relative peace returns in the late evening.

Alternatively it may be a factory with different noise emissions throughout the day or week, with deliveries, intermittent compressors, and lots of varying noisy processes on top of the routine production noise levels.

How do you measure these noise levels and come up with an overall value?

This is where the Leq or equivalent continuous sound level meter comes in. This meter faithfully follows all the fluctuations, stores them in it's memory and at the end of the measurement calculates an 'average energy' or Leq value. When we say average, this is not a simple arithmetic average because we are measuring in decibels which are logarithmic values. So our meter converts the dB values to sound pressure levels, adds them all up then divides by the number of samples and finally converts this equivalent level back to decibels - dBs.

LAeq - It is common practice to measure noise levels using the A-weighting setting built into all sound level meters. In which case the term is properly known as LAeq and the results should say so - for example LAeq = 73 dB or Leq = 73 dBA

A good Leq sound level meter samples and 'captures' the noise levels 16 times a second which means over an hour it makes 16 x 60 x 60 = 57600 calculations, not difficult for a modern meter but quite an achievement a few years back.

Leq sound levels are logarithmic (dB) values and cannot be added directly. A doubling of sound level results in a measured increase of 3 dB, four identical sources in a room would increase the noise level by 6 dB and so on. This works both ways, say 10 similar machines in a room produce 100 dBA then removing one machine completely will only reduce the overall noise level to 0.5 dBA, you would need to silence or remove 50% of the machines to achieve a 3 dB reduction. See also our dB page for more details on adding and subtracting decibels - Leq Calculation.

Leq is also used in the assessment of noise dose or sound exposure in the workplace and the 3 dB 'doubling rule' applies to time and/or level. For example an Leq level of 85 dBA over 8 hours is currently assessed as 100% dose in the UK. Using the doubling rule then 85 dBA(8 hour) = 82 dBA(16 hour) or 88 dB is only acceptable for 4 hours a day. Similarly if 85 dBA = 100% dose then 88 dBA = 200% dose.

A note of caution - the Leq assumes an exchange rate of 3 dB for ISO and British Standard measurements. In some countries other rules may apply. for example the USA OSHA Standards use a 5 dB exchange rate.

LAT : the time averaged sound level, specified in the IEC 61672 Standard, is the same quantity. However the Leq term is in common use.

Short Leq

: the preferred modern method of storing sound levels and displaying the true time history of a noise event. The resulting 'time histories', typically measured in 1/8 second intervals may be used to calculate the 'overall' levels for any sub-period of the overall measurement time.