1-6
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.
Q6. What are some examples of transverse waves?
Q7. What example of a longitudinal wave was given in the text?
MEDIUM
We have used the term medium in describing the motion of waves. Since medium is a term that is
used frequently in discussing propagation, it needs to be defined so you will understand what a medium is
and its application to propagation.
A MEDIUM is the vehicle through which the wave travels from one point to the next. The vehicle
that carries a wave can be just about anything. An example of a medium, already mentioned, is air. Air, as
defined by the dictionary, is the mixture of invisible, odorless, tasteless gases that surrounds the earth (the
atmosphere). Air is made up of molecules of various gases (and impurities). We will call these molecules
of air particles of air or simply particles.
Figure 1-6 will help you to understand how waves travel through air. The object producing the waves
is called the SOURCE—a bell in this illustration. The object responding to the waves is called a
DETECTOR or RECEIVER—in this case, the human ear. The medium is air, which is the means of
conveying the waves from the source to the detector. The source, detector, and medium are all necessary
for wave motion and wave propagation (except for electromagnetic waves which require no medium).
The waves shown in figure 1-6 are sound waves. As the bell is rung, the particles of air around the bell
are compressed and then expanded. This compression and expansion of particles of air set up a wave
motion in the air. As the waves are produced, they carry energy from particle to particle through the
medium (air) to the detector (ear).
