The **equivalent sound pressure level L**_{eq}** **for an eight hour day with a sound level varying as *L _{p}(t).*

You rarely know L_{p}(t) – the pattern of noise exposure over the day, but sometimes the data is available, but a censored set of the data is used, perhaps just to look at the equivalent sound pressure level from a particular machine.

For a series of noise measurements at regular intervals, the L_{eq} can be calculated in Excel with the following “**array formula**” – a powerful but little known feature in Excel.

=10*LOG10(AVERAGE(10^((your range)/10)))

When the range – say the cells D1 and D2 are selected and the **Shift+Control+Enter** keys are pressed simultaneously, the formula bar in Excel will show the formula enclosed in {}

If you just used the arithmetic average you would get 47.5, the wrong answer.

If you try to use an Excel VAB macro with that formula, it wont work as VBA does not have Log10() – you have to use Log(X)/log(10).