When several sound sources are combined, each of which generates a certain amount of dBspl, it isn't possible to simply sum up these values, but we should rather use the following empirical formula:
Equation 3.6. Combination of sound sources
Let's take a look at an example with two sound sources each of 90dBspl.
We'd have:
Equation 3.7. Application of the formula for combined sound sources
From this equation too, we can obtain a very important empirical formula: if you sum up two identical sound sources you will have an increase of 3dBspl (not double the value!). So, if we have a P.A system [3 ] which produces an acoustic pressure of 100dBspl, and we add another identical P.A system we'd obtain an overall acoustic pressure of 103dBspl: to get to 106dBspl we'd have to add another two P.A's which would sum up to 4 systems, and so on.





