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.

TRANSVERSE WAVES
To explain transverse waves, we will again use our example of water waves. Figure 1-3 is a cross section diagram of waves viewed from the side. Notice that the waves are a succession of crests and troughs. The wavelength (one 360 degree cycle) is the distance from the crest of one wave to the crest of the next, or between any two similar points on adjacent waves. The amplitude of a transverse wave is half the distance measured vertically from the crest to the trough. Water waves are known as transverse waves because the motion of the water is up and down, or at right angles to the direction in which the waves are traveling. You can see this by observing a cork bobbing up and down on water as the waves pass by; the cork moves very little in a sideways direction. In figure 1-4, the small arrows show the up-and-down direction the cork moves as the transverse wave is set in motion. The direction the wave travels is shown by the large arrow. Radio waves, light waves, and heat waves are examples of transverse waves.

LONGITUDINAL WAVES
In the previous discussion, we listed radio waves, light waves, and heat waves as examples of transverse waves, but we did not mention sound waves. Why? Simply because sound waves are LONGITUDINAL WAVES. Unlike transverse waves, which travel at right angles to the direction of propagation, sound waves travel back and forth in the same direction as the wave motion. Therefore, longitudinal waves are waves in which the disturbance takes place in the direction of propagation. Longitudinal waves are sometimes called COMPRESSION WAVES. Waves that make up sound, such as those set up in the air by a vibrating tuning fork, are longitudinal waves. In figure 1-5, the tuning fork, when struck, sets up vibrations. As the tine moves in an outward direction, the air immediately in front of it is compressed (made more dense) so that its momentary

Aaron