Science of Sound: 3. How Sound Travels
The Inverse Square Law states that the further away you are from the source of the sound, the lower the amplitude (pressure). That’s because the energy of a sound wave spreads out as it travels through the air, bumping into more and more molecules. Sound waves radiate in 3 dimensional space away from the sound source.
The greater the amount of sound energy (higher amplitude) the darker the color of the sphere. The further you are away from a sound source, the softer the sound due to the spread of energy which results in lower pressure (amplitude).