Increase and decrease of sound level

A very important property of direct sound is that it becomes weaker as it travels away from the sound source, at a rate governed by the inverse-square law. For example, when the distance increases by a factor of two (doubles), the sound level decreases by a factor of four (the square of two). This results in a drop of 6 dB in sound pressure level (SPL), a substantial decrease. Likewise, when the distance to the direct sound source is divided by two (cut in half), the sound level increases by 6 dB.

(from Audio System Guide for Houses of Worship, Shure Educational Publication)