1-14. 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 large arrow shows the
direction the wave travels. Radio waves, light waves, and heat waves are
examples of transverse waves.
1-15. 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
1-16. Waves that make up sound, such as those set up in the air by a
vibrating tuning fork, are longitudinal waves. When struck, the tuning fork
in figure 1-5 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 pressure is raised above that at other points in the surrounding
medium (air). Because air is elastic, the disturbance is transmitted in an
outward direction as a compression wave. When the tine returns and moves
in the inward direction, the air in front of the tine is rarefied (made less
dense or expanded) so that its pressure is lowered below that of the other
points in the surrounding air. The rarefied wave is propagated from the
tuning fork and follows the compressed wave through the medium (air).
Figure 1-5. Sound Propagation by a Tuning Fork