Aircraft noise propagation - the simplest case

Geometrical spreading, air absorption, and refraction are
the main wave phenomena that govern the propagation of aircraft noise. The
results of this study make possible the calculate noise at ground level in terms
of the sound exposure level, LAE. The model applies to an aircraft with a
nondirectional radiation pattern (e.g., helicopter, one-engine propeller) while
the atmosphere is at rest. The derived equations provide some insight into the
nature of noise propagation, that can be useful for teaching purposes.