Decibels - Combination of sound sources

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When several sound sources are combined, each of which generating a certain amount of dBspl, we can't simply sum up these values, but rather the following empirical formula must be used:

Equation 3.6. Combination of sound sources 

Combination of sound sources

Let's take a look at an example with two sound sources each having a 90dBspl intensity.

We would have:

Equation 3.7. Application of the formula for combined sound sources 

Application of the formula for combined sound sources

This equation also gives us a very important empirical rule, namely: if you sum up two identical sound sources you will have an increase of 3dBspl, and not simply double the value. So, if we have a P.A. system[4 ] which produces an acoustic pressure of 100dBspl, and we add another identical P.A. system to it, 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.

[4 ] P.A - Public Address. The loudspeaker system used for diffusing sound in a hall.

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