1326 Chapter 34
some trouble in a system that includes background
music (the music may be interpreted as ambient noise or
may prevent measurement of the noise), and they
usually cannot adjust the paging level during a page.
34.6.3.7 Thermal Layers of Air
Sound travels faster in warmer air. By itself, this fact
has little significance for the sound system designer.
Often, however, a layer of warm air will lie above or
below a layer of cool air, and the difference in the speed
of sound in these two layers can cause the sound to
curve upward or downward toward the cool layer as
shown in Fig. 34-86. This can be a help or a hindrance,
depending on whether the cool layer is on the top or the
bottom of the warm layer.
For example, in the early morning on a golf course,
when the sun first comes up and begins to warm the air,
the earth maintains a relatively cool layer near the
ground. Thus, a sound wave will curve toward the
ground, effectively hugging the ground, and can travel a
great distance with seemingly little attenuation. Golfers
at relatively great distances from each other can speak
and be understood almost as if they were just a few feet
apart. The same effect occurs over a quiet lake, even in
the afternoon sun, since the lake will maintain a cool
layer of air all day long. A wind, of course, will mix the
layers of air and add noise so that the effect of thermal
layers is lost.
In the early evening, when the sun begins to go
down, the opposite situation occurs on a hot parking lot.
The cool layer is now on top with a warm layer, main-
tained by the parking lot, on the bottom. The sound
effectively curves upward toward the cool air and sound
attenuation near the ground is effectively increased.
These phenomena can cause a paging system to
work erratically, depending on the time of day.
Large-area paging systems, over an airport runway, for
example, are sometimes designed to overcome the
changes caused by thermal layers. Elaborate systems
can be designed to measure the temperature near the
ground and above the ground and adjust the electrical
input to each component of a vertical array of horns (or
even mechanically re-aim the horns) to compensate for
the effective curving upward or downward of the sound.
In many smaller systems, however, the only necessary
action is to test the system for satisfactory operation
under worst-case conditions, which will probably be in
the late afternoon on a hot day.
34.6.3.8 The Effect of WindSound travels faster in the direction of the wind. Wind
above the ground tends to be faster moving than wind
near the ground. These factors combine to make the
sound waves bend downward for a listener who is
downwind from a sound source. Similarly, the sound
waves will bend upward for a listener who is upwind
from a sound source. Because this situation can vary
unpredictably, wind can cause unpredictable changes in
sound level and quality.
For outdoor concert systems with two stacks of loud-
speakers (one on either side of the stage), a crosswind
can cause very perceivable changes in phase cancella-
tions at a given listener’s position. In addition, because
high-frequency horns are often very directional at
higher frequencies, a small change in the direction of
the sound can dramatically change the apparent
high-frequency response to a listener.
While there are no permanent solutions to these
problems, outdoor theater designers should be aware of
them and should choose a site with as little wind as
possible. Sound system designers may want to consider
a distributed system approach to move the sound source
closer to the listeners.Figure 34-86. Effect of thermal air layers on sound travel.
Courtesy Bosch/Electro-Voice.C. Cool air on the bottom causes sound to
curve toward the ground.A. Wind mixes layers of air losing effect
of thermal layers.B. Warm air on the bottom causes
sound to curve upward.Wind velocity
fast
Wind velocity
slowWind directionSound sourceSound sourceSound sourceCool airCool airWarm airWarm air