Page 169 - Master Handbook of Acoustics

Refraction in Enclosed Spaces

Refraction has an important effect on sound outdoors and sound traveling great distances. Refraction
plays a much less significant role indoors. Consider a multiuse gymnasium that also serves as an
auditorium. With a normal heating and air-conditioning system, efforts are made to avoid large
horizontal or vertical temperature gradients. If there is temperature uniformity, and no troublesome
drafts, sound refraction effects should be reduced to inconsequential levels.

Consider the same gymnasium also used as an auditorium but with less sophisticated air
conditioning. Assume that a large heater is ceiling-mounted. The unit would produce hot air near the
ceiling and rely on slow convection currents to move some of the heat down to the audience level.
This reservoir of hot air near the ceiling and cooler air below could have a minor effect on the
transmission of sound from the sound system and on the acoustics of the space. The feedback point of

the sound system might shift. The standing waves of the room might change slightly as longitudinal,
and transverse sound paths are increased in length because of their curvature due to refraction. Flutter
echo paths may also shift. With a sound system mounted high at one end of the room, sound paths
could be curved downward. Such downward curvature might actually improve audience coverage,
depending somewhat on the directivity of the radiating system.

Refraction in the Ocean

In 1960, a team of oceanographers headed by Heaney performed an experiment to monitor the
propagation of underwater sound. Charges of 600 lb were discharged at various depths in the ocean
off Perth, Australia. Sounds from these discharges were detected near Bermuda, a distance of over

12,000 miles. Sound in seawater travels 4.3 times faster than in air, but it still took 3.71 hours for the
sound to make the trip.
Oceanic refraction played a significant role in this experiment. The depth of the ocean may be over
5,000 fathoms (30,000 ft). At about 700 fathoms (4,200 ft) an interesting effect occurs. The sound-
speed profile shown in Fig. 8-6A very approximately illustrates the principle. In the upper reaches of
the ocean, the speed of sound decreases with depth because temperature decreases. At greater depths

the pressure effect prevails, causing sound speed to increase with depth because of the increase in
density. The V-shaped profile changeover from one effect to the other occurs near the 700-fathom
(4,200-ft) depth.