This is called the inverse square law. It is often stated "The loudness
decreases 6 dB for each doubling of the distance".
Sometimes sound will reach a lot farther than the inverse square law
implies. This happens when the energy of the sound is focused somehow rather
than distributed evenly on an expanding sphere. Some common situations:
- Directional loudspeakers
- It is possible to build a speaker so that the sound is projected mostly in
one direction. In fact, it's difficult to build a speaker that doesn't do this
to some extent. Directionality is always more pronounced at high frequency than
low.
- Reflective support
- The back wall of a bandshell and the ceiling of a stage are designed to
bounce sound into the audience. The mountains around Bozeman Montana do the same
for train horns.
-
Atmospheric effects
- Since the speed of sound differs according to air temperature, sound
traveling along a boundary between warm and cold air tends to get bent. Well,
bent may not be the right word (refracted
is) but what happens is the sound is diverted into the cold
region. If you have a temperature inversion, with warm air above cold air, sound
will be refracted back to the ground, as people who live on a coastline will
tell you.