waves would propagate rectarlinearly only if the speed of sound is
constant in space and there is no wind. Since the sound speed depend on
temperature which in particular varies with height, the sound speed is
almost never constant in space. Moreover, the air is often moved by the
wind that also carries the sound waves with different speeds. The additional
speed of the sound by the wind also depends on height as the wind velocity
increases with growing distance from the ground.
In a medium with variing
speed of sound the wave propagate no longer rectarlinearly. They are much
more refracted, i.e. they propagate along curved lines. This also happens
to the sound waves in the atmosphere.
Refraction is often visualised
by sound rays that descibe the direction of wave propagation like
If the sound speed decreases
with height, either because the temperature decreases with altitude and/or
the sound propagates against the wind (upwind propagation), the
sound waves are refracted upward. As a consequence, an acoustical shadow
is forming into which sound energy cannot penetrate directly. Merely by
and scattering sound energy can be shed
into a shadow. However, it remains noticeably less noisy in a shadow zone
than it would be expected from the distance form the source.
upward refraction (temperature lapse and/or upwind propagation)
If the sound speed increase
with height, either because the temperature increases with altitude (inversion)
and/or the sound propagates with the wind (downwind propagation), the
sound waves are refracted downward towards the ground. At the ground surface
the sound waves ate subject to reflection.
The reflected sound is again refracted downward. A possible consequence
is multiple reflection which is favourable to the sound propagation near
the surface over large distances.
downward refraction (temperature inversion and/or downwind propagation)