Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
of this process are beyond the scope of this book. For more information, see (Davis and
Patronis, 2006) and (McCarthy, 2009). Effects of Temperature
In addition to the physical obstructions with which sound interacts, the air through which sound
travels can have an effect on the listener’s experience.
As discussed in Chapter 2, the speed of sound increases with higher air temperatures. It
seems fairly simple to say that if you can measure the temperature in the air you’re working in,
you should be able to figure out the speed of sound in that space. In actual practice, however, air
temperature is rarely uniform throughout an acoustic space. When sound is played outdoors, in
particular, the wave front encounters varying temperatures as it propagates through the air.
Consider the scenario where the sun has been shining down on the ground all day. The sun
warms up the ground. When the sun sets at the end of the day (which is usually when you start
an outdoor performance), the air cools down. The ground is still warm, however, and affects the
temperature of the air near the ground. The result is a temperature gradient that gets warmer the
closer you get to the ground. When a sound wave front tries to propagate through this
temperature gradient, the portion of the wave front that is closer to the ground travels faster than
the portion that is higher up in the air. This causes the wave front to curve upwards towards the
cooler air. Usually, the listeners are sitting on the ground, and therefore the sound is traveling
away from them. The result is a quieter sound for those listeners. So if you spent the afternoon
setting your sound system volume to a comfortable listening level, when the performance begins
at sun down, you’ll have to increase the volume to maintain those levels because the sound is
being refracted up towards the cooler air.
Figure 4.35 shows a diagram representing this refraction. Recall that sound is a
longitudinal wave where the air pressure amplitude increases and decreases, vibrating the air
molecules back and forth in the same direction in which the energy is propagating. The vertical
lines represent the wave fronts of the air pressure propagation. Because the sound travels faster
in warmer air, the propagation of the air pressure is faster as you get closer to the ground. This
means that the wave fronts closer to the ground are ahead of those farther from the ground,
causing the sound wave to refract upwards.
Figure 4.35 Sound refracted toward cooler air
A similar thing can happen indoors in a movie theater or other live performance hall.
Usually, sound levels are set when the space is empty prior to an audience arriving. When an
audience arrives and fills all the seats, things suddenly get a lot quieter, as any sound engineer
will tell you. Most attribute this to sound absorption in the sense that a human body absorbs
sound much better than an empty chair. Absorption does play a role, but it doesn’t entirely
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