For energy radiating outward, the energy density diminishes as the reciprocal of the distance travelled squared.

For example, twice as far from the source of a disturbance, the energy density is 1/4 as much (1/2 squared) because it is distributed over a surface (4*pi*radius*radius) that is four times as large.

As amplitude is the square root of energy, the amplitude of a disturbance attenuates linearly with distance travelled.

Two other effects, absorption and dispersion, make the dimunition of peak amplitude and energy even faster.