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hear the same frequency sound produced by the source. This is because the receiver is receiving the same
number of waves per second that the source is producing. Now, if either the source or the receiver or both
move toward the other, the receiver will perceive a higher frequency sound. This is because the receiver
will receive a greater number of sound waves per second and interpret the greater number of waves as a
higher frequency sound. Conversely, if the source and the receiver are moving apart, the receiver will
receive a smaller number of sound waves per second and will perceive a lower frequency sound. In both
cases, the frequency of the sound produced by the source will have remained constant.
For example, the frequency of the whistle on a fast-moving train sounds increasingly higher in pitch
as the train is approaching than when the train is departing. Although the whistle is generating sound
waves of a constant frequency, and though they travel through the air at the same velocity in all
directions, the distance between the approaching train and the listener is decreasing. As a result, each
wave has less distance to travel to reach the observer than the wave preceding it. Thus, the waves arrive
with decreasing intervals of time between them.
These apparent changes in frequency, called the Doppler effect, affect the operation of equipment
used to detect and measure wave energy. In dealing with electromagnetic wave propagation, the Doppler
principle is used in equipment such as radar, target detection, weapons control, navigation, and sonar.
Q15. The apparent change in frequency or pitch because of motion is explained by what effect?
SOUND WAVES
The study of sound is important because of the role sound plays in the depth finding equipment
(fathometer) and underwater detection equipment (sonar) used by the Navy.
As you know, sound travels through a medium by wave motion. Although sound waves and the
electromagnetic waves used in the propagation of radio and radar differ, both types of waves have many
of the same characteristics. Studying the principles of sound-wave motion will help you understand the
actions of both sound waves and the more complex radio and radar electromagnetic waves. The major
differences among sound waves, heat waves, and light waves are (1) their frequencies; (2) their types; the
mediums through which they travel; and the velocities at which they travel.
SOUND—WHAT IS IT?
The word SOUND is used in everyday speech to signify a variety of things. One definition of sound
is the sensation of hearing. Another definition refers to a stimulus that is capable of producing the
sensation of hearing. A third definition limits sound to what is actually heard by the human ear.
In the study of physics, sound is defined as a range of compression-wave frequencies to which the
human ear is sensitive. For the purpose of this chapter, however, we need to broaden the definition of
sound to include compression waves that are not always audible to the human ear. To distinguish
frequencies in the audible range from those outside that range, the words SONIC, ULTRASONIC, and
INFRASONIC are used. Sounds capable of being heard by the human ear are called SONICS. The normal
hearing range extends from about 20 to 20,000 hertz. However, to establish a standard sonic range, the
Navy has set an arbitrary upper limit for sonics at 10,000 hertz and a lower limit at 15 hertz. Even though
the average person can hear sounds above 10,000 hertz, it is standard practice to refer to sounds above
that frequency as ultrasonic. Sounds between 15 hertz and 10,000 hertz are called sonic, while sounds
below 15 hertz are known as infrasonic (formerly referred to as subsonic) sounds.
