Page 291 - The Master Handbook Of Acoustics

266 CHAPTER TWELVE

auditorium at times. With a normal heating and air-conditioning sys-
tem, great efforts are made to avoid large horizontal or vertical tem-
perature gradients. The goals of temperature uniformity and no
troublesome drafts have reduced sound refraction effects to inconse-
quential levels.
Consider the same gymnasium used as an auditorium but with less
sophisticated air conditioning. In this case a large ceiling-mounted
heater near the rear acts as a space heater. Working against gravity, the
unit produces copious hot air near the ceiling, relying on slow con-
vection currents to move some of the heat down to the audience level.
This reservoir of hot air near the ceiling and cooler air below can
have a minor effect on the transmission of sound from the sound sys-
tem 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
are also shifted. With a sound radiating system mounted high at one
end of the room, lengthwise sound paths would be curved downward.
Such downward curvature might actually improve audience coverage,
depending somewhat on the directivity of the radiating system.

Endnotes

1 Shockley, R.C., J. Northrop, P.G. Hansen, and C. Hartdegen, SOFAR Propagation Paths from
Australia to Bermuda, J. Acous. Soc. Am., 71, 51 (1982).
2 Heaney, K.D., W.A. Kuperman, and B.E. McDonald, Perth-Bermuda Sound Propagation
(1960): Adiabatic Mode Interpretation, J. Acous. Soc. Am., 90, 5 (Nov 1991) 2586-2594.
3 Spiesberger, John, Kent Metzger, and John A. Ferguson, Listening for Climatic Temperature
Changes in the Northeast Pacific 1983-1989, J. Acous. Soc. Am., 92, 1 (July 1992) 384-396.