________________________________________________________________Wave Propagation
Figure 1-12. No Air, No Sound
1-53. Any object that moves rapidly back and forth, or vibrates, and thus
disturbs the medium around it may be considered a source for sound. Bells,
speakers, and stringed instruments are familiar sound sources.
1-54. The material through which sound waves travel is called the medium.
The density of the medium determines the ease, distance, and speed of sound
transmission. The higher the density of the medium, the slower sound travels
through it.
1-55. The detector acts as the receiver of the sound wave. Because it does not
surround the source of the sound wave, the detector absorbs only part of the
energy from the wave and sometimes requires an amplifier to boost the weak
signal.
1-56. As an illustration of what happens if one of these three elements is not
present, let us refer to our experiment in which a bell was placed in a jar
containing a vacuum. You could see the bell being struck, but you could hear
no sound because there was no medium to transmit sound from the bell to
you. Now let us look at another example in which the third element, the
detector, is missing. You see a source (such as an explosion) apparently
producing a sound, and you know the medium (air) is present, but you are too
far away to hear the noise. Thus, as far as you are concerned, there is no
detector and, therefore, no sound. We must assume, then, that sound can
exist only when a source transmits sound through a medium, which passes it
to a detector. Therefore, in the absence of any one of the basic elements
(source, medium, and detector) there can be no sound.
TERMS USED IN SOUND WAVES
1-57. Sound waves vary in length according to their frequency. A sound
having a long wavelength is heard at a low pitch (low frequency); one with a
short wavelength is heard at a high pitch (high frequency). A complete
wavelength is called a cycle. The distance from one point on a wave to the
1-17
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