Amplitude & Distance from the source.
Amplitude is based of the starting energy of the soundwave, but is overall measure of the size of the soundwaves, whereas distance is well, distance. Aka, how far it's traveled. So lets say you're standing near a plane engine or something. If you're standing right next to it, the sound isn't gonna have that far to travel before it reaches you, which means its gonna be really intense. And the amplitude, well, that's sorta determined by whatever's causing it.