TC 9-64 _________________________________________________________________________
used conventionally as the symbol for inductance, and "l" is used for
dimensional length; therefore, λ is used to indicate the length of waves.
1-22. Two waves may have the same wavelength, but the crest of one may
rise higher above the reference line than the crest of the other. Compare
wave 1 and wave 2 of figure 1-7 again. The height of a wave crest above the
reference line is called the amplitude of the wave. The amplitude of a wave
gives a relative indication of the amount of energy the wave transmits. A
continuous series of waves, such as A through Q, having the same amplitude
and wavelength, is called a train of waves or wave train.
Frequency and Time
1-23. Time is an important factor in wave studies. When a wave train passes
through a medium, a certain number of individual waves pass a given point
in a specific unit of time. For example, if a cork on a water wave rises and
falls once every second, the wave makes one complete up-and-down vibration
every second. The number of vibrations, or cycles, of a wave train in a unit of
time is called the frequency of the wave train and is measured in hertz. If 5
waves pass a point in one second, the frequency of the wave train is 5 cycles
per second. In figure 1-7, the frequency of both wave 1 and wave 2 is four
cycles per second (abbreviated as cps).
1-24. In 1967, in honor of the German physicist Heinrich Hertz, the term
hertz was designated for use in lieu of the term "cycle per second" when
referring to the frequency of radio waves. It may seem confusing that in one
place the term "cycle" is used to designate the positive and negative
alternations of a wave, but in another instance the term "hertz" is used to
designate what appears to be the same thing. The key is the time factor. The
term cycle refers to any sequence of events, such as the positive and negative
alternations, comprising one cycle of electrical current. The term hertz refers
to the number of occurrences that take place in one second.
1-25. The two types of wave motion, transverse and longitudinal, have many
of the same characteristics, such as frequency, amplitude, and wavelength.
Another important characteristic that these two types of wave motion share
is velocity. Velocity of propagation is the rate at which the disturbance
travels through the medium, or the velocity with which the crest of the wave
moves along. The velocity of the wave depends both on the type of wave (e.g.,
light, sound, or radio) and type of medium (e.g., air, water, or metal). If
longitudinal waves are plotted as a graph, they appear as transverse waves.
This fact is illustrated in figure 1-8. The frequency of a longitudinal wave,
like that of a transverse wave, is the number of complete cycles the wave
makes during a specific unit of time. The higher the frequency, the greater is
the number of compressions and expansions per unit of time.