Leq - equivalent continuous sound level
Noise 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 milkman clattering around for several minutes. Then the general noises of the day occur, before relative peace returns in the late evening.

One of the general noise sources 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 noise or equivalent continuous noise level meter comes in. This meter faithfully follow all the fluctuations, stores them in it's short-term memory and at the end of the measurement calculates an 'average' value called the Leq value. When we say average, this is not a simple arithmetic average because we are measuring in decibels which logarithmic values. So our meter converts the dB values to 'real numbers', adds them all up then divides by the number of samples and finally converts this true energy average back to dBs.

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 noise 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.

Leq is also used in the assessment of noise dose 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 and applies to all ISO and British Standard measurements. In some Countries, for example the USA OSHA Standards use a 5 dB exchange rate.

LAT   the time averaged 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.

In earlier times the exponential levels were averaged over time and therefore the results were dependent of the meter settings and could not be combined to get the overall result. See also exponential averaging and linear averaging