Page 282 - The Master Handbook Of Acoustics

CHAPTER
12

Refraction of Sound

bout the turn of this century Lord Rayleigh was puzzled because some
Avery powerful sound sources, such as cannon fire, could be heard
only short distances some times and very great distances at other
times. He set up a powerful siren that required 600 hp to maintain it.
He calculated that if all this power were converted into energy as
sound waves and spread uniformly over a hemisphere, how far could
it be heard? Knowing the minimum audible intensity (10 –16 watts per
sq cm), his calculations indicated that the sound should be audible to
a distance of 166,000 miles, more than 6 times the circumference of
the earth!
It is indeed fortunate that such sound propagation is never experi-
enced and that a range of a few miles is considered tops. There are
numerous reasons why sound is not heard over greater distances. For
one thing, the efficiency of sound radiators is usually quite low; not
much of that 600 hp was actually radiated as sound. Energy is also lost
as wavefronts drag across the rough surface of the earth. Another loss
is dissipation in the atmosphere, but this is known to be very small.
The result of such calculations and early experiments that fell far
short of expectations served only to accelerate research on the effects
of temperature and wind gradients on the transmission of sound.

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