frequency of a sound from a source is held constant. The wavelength of the
sound will also remain constant. If both the source and the receiver of the
sound remain stationary, the receiver will 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.
1-46. 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.
1-47. 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,
1-48. 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 as follows:
The mediums through which they travel.
The velocities at which they travel.
SOUND--WHAT IS IT?
1-49. 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.
1-50. 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,