Page 162 - Master Handbook of Acoustics

CHAPTER 8

Refraction

A t 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 features 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 advance the understanding of refraction effects.

The Nature of Refraction

The difference between absorption and reflection of sound is obvious, but sometimes there is
confusion between refraction and diffraction (and possibly diffusion, the subject of Chap. 9).

Refraction is the change in the direction of travel of sound because of differences in the velocity of
propagation. Diffraction is the change in the direction of travel of sound as it encounters physical
obstructions, edges, and openings. Of course, in practical situations, it is entirely possible for both
effects to simultaneously affect the same sound.
Figure 8-1 recalls a common observation of the apparent bending of a stick as one end is

immersed in water. This is an illustration of the refraction of light, caused by the different refractive
indices of air and water, and hence different speeds of propagation. The refraction of sound, which is
another wave phenomenon, is similar. In the case of water and air, the change in the refractive indices
is abrupt, as is the bending of light. Sound refraction can occur abruptly, or gradually, depending on
how the transmission mediums affect its speed.