As one moves further away from a sound source, the SPL decreases. Why?
Molecules lose energy often through heat.

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The actual energy loss through collisions is quite small. The main affect is pure geometrical. The farther away from the source the more surface area the sound has to go through . In open space the area goes like 4*pi*r^2, therefore the inverse square law. Think about the light energy- the closer you get to the Sun, the hotter it gets.