The inverse square law of sound is a very important law that not many really understand. Sound waves propagate spherically and so distributed over and ever-increasing surface of diameter at the front surface of the wave. The inverse square law tells us that every doubling of the distance from the sound source in a free field situation the sound pressure level will diminish by 6 decibels.
Why many may not know this law is that it is hard to get a free field situation such as an explosion of a bomb in mid-air. Though not perfect for real life situations it is a very important guide to sound sources and how they diminish over distance.
As an example lets say a person shouts at 70dB at 3 feet away from you in a room, at 6 feet it will be 62dB and at 12 feet 56dB and at 24 feet 50dB. This example on works as a guide not taking into account reflections from any walls of a room and any other sound in the room at the time of shouting.