Audio Engineering

92 Chapter 3

hear it said, “ The wind blew the sound away. ” That is not so; it refracted away. Even a 50-
mph wind (and that’s a strong wind) cannot blow away something traveling 1130 ft/s:

1130 3600 1

770 45

ft

1s

s

1h

m

5280 ft

. 5 mi/h

770.45 mi/h is the velocity of sound at sea level at 72.5°F.

Wind velocities that vary with elevation can also cause “ bending ” of the sound velocity
plus or minus the wind velocity at each elevation.

R e fl ections from large boundaries, when delayed in time relative to the direct sound, can
be highly destructive of speech intelligibility. It is important to remember, however, that
a refl ection within a nondestructive time interval can be extremely useful. Refl ections
that are at or near (within 10 dB) equal amplitude and that are delayed more than 50 m s
require careful attention on the part of a sound system designer. Figure 3.5 shows how to
calculate probable levels from a refl ection. Figure 3.6 shows other infl uences. Calculation
of the time interval is found by:

1000

c

(DRDD ) Time interval (in ms) (3.13)

where c is the velocity of sound in ft/s or m/s, DR is the distance in feet or meters traveled
by the refl ection, and DD is the distance the direct sound traveled in feet or meters.

A large motor speedway used to make very effective use of ground refl ections on the coverage
of the grandstands behind the pit area. The very high temperature gradients encountered warp
the sound upward during the hot part of the day and in the cool of the morning, the ground
refl ection helps with the coverage of the near seating area. The directional devices are aimed
straight ahead along the ground rather than up at an angle, and when the temperature gradient
“ bends ” the sound upward, it’s still covering the audience area effectively ( Figure 3.4 ).

One caution about using ground refl ections in northern climes is that a heavy snowfall
can provide unbelievable attenuation, as the authors can attest after trying to demonstrate,
years ago, a high-level sound system the day after a blizzard in Minnesota.

3.10 Effect of a Space Heater on Flutter Echo ................................................................

The velocity of sound increases with an increase in temperature; therefore, the effect of
an increase in temperature with an increase in height is a downward bending of the sound