This is an empirical law that calculates the variations in dBspl when distance from a sound source varies. If at d1 distance from a sound source we have a sound pressure level of dBspl1, at a distance d2 > d1 the resulting sound pressure would be the following:
Equation 3.4. Inverse distance law
Let's take a look at a practical application of this formula: if d1= 1m and at this distance from the sound source we measure dBspl1 = 100 at a distance d2 = 2m we'd have a sound pressure equal to:
Equation 3.5. Application of the inverse distance formula
From this simple example we obtain an easy, useful empirical rule: every time we move away from a sound source, doubling our distance from it, the sound pressure level decreases by 6 dBspl. Vice versa if we move closer to the sound source, halving our distance from it, we perceive an increase in sound pressure of approximately 6dBspl. This rule is called the inverse distance law.