Sound is a form of energy. We have learned that sound energy is produced by a body vibrating at
audible frequencies feeding into a sound medium, through which
the sound energy is propagated at a definite velocity, and in which the sound energy is detected by
setting the ear-drum into vibration and hence producing the proper sensation in the brain. The velocity
of sound in air is approximately 1120 feet per second at ordinary room temperatures.
For purposes of discussion, except where noted otherwise, we shall consider, as in Chapter 1, that our
sound source is a point source; that is, one from which the sound energy is emitted equally in all
directions. We can consider this spherical wave as being made up of many sound rays, one traveling in
each direction, and all coming from the same point, as shown in Figure 104. According to the law of
diminishing intensity, the greater the distance the wave has traveled the less intense is the sound, and
consequently of lesser sound loudness.
Figure 105 shows why the loudness of sound diminishes with distance. At A a certain amount of sound
energy is concentrated in a fairly small area. As the sound waves move along to B the area of cross section within which this sound
energy is confined increases so that the same amount of sound energy is spread over a greater area,
reducing the loudness of the sound.
Most of the loudspeakers actually used in theatres today are very directional and are not point sources,
as pointed out in the previous chapter on loud speakers. For practical purposes the energy which they
radiate may be considered as being confined within a 60' angle in the direction of the long axis of the
baffle mouth (laterally) as shown in Figure 106, and within a 30' angle in the direction of the short axis of
the baffle mouth as shown in Figure 107.
157. Some of the Properties of Sound Waves With Reference to Acoustics.-Sound waves display the
properties of reflection, absorption, transmission, dispersion, and interference. We shall take up most of
these properties in their application to auditoriums in what is to follow.
When a sound wave strikes the boundary between two different mediums, its energy is partially
reflected, absorbed, and transmitted as shown by Figure 108. For the purpose of illustration the sound
is shown as a concentrated beam striking a wall. The arrow points indicate the direction of travel. The
lines representing the sound beam are shown of different width to give an idea of the variations of
loudness. When the beam of sound waves strikes the wall, part of it is transmitted
as illustrated by the arrow penetrating the wall, part is reflected, and part is absorbed. The
sound energy absorbed is in reality transformed into heat energy and, therefore, no longer
exists as sound. The sum of the energies of the transmitted sound, absorbed sound, and
reflected sound is equal to the energy of the initial sound beam.