Refraction

At the turn of the twentieth century, Lord Rayleigh was puzzled because some
powerful sound sources, such as cannon fire, could be heard only short distances
sometimes and very great distances at other times. He calculated that if
all the power from a siren driven by 600 horsepower was converted into sound energy
and spread uniformly over a hemisphere, the sound should be audible to a distance of
166,000 miles, more than six times the circumference of the earth. However, such sound
propagation is never experienced and a maximum range of a few miles is typical.
When dealing with sound propagation, particularly outdoors, refraction plays a
large role. Refraction is a change in the direction of sound propagation that occurs when
there is a change in the transmission medium. In particular, the change in the medium
changes the speed of sound propagation, and therefore the sound bends.
There are many reasons why Rayleigh’s estimate was wrong, and why sound is not
heard over great distances. For one thing, refraction in the atmosphere will profoundly
affect the propagation of sound over distance. In addition, the efficiency of sound radiators
is usually quite low; not much of that 600 horsepower would actually be radiated
as sound. Energy is also lost as wavefronts drag across the rough surface of the earth.
Another loss is dissipation in the atmosphere, particularly affecting high frequencies.
Early experiments by Rayleigh and others accelerated research on the effects of temperature
and wind gradients on the transmission of sound. This chapter will help to
advance the understanding of refraction effects