Wave motion has certain definite characteristics; these
characteristics determine the loudness, frequency (or pitch) and tone of the
4. Loudness. Loudness (or amplitude) is
determined by the amount of difference in pressure between the maximum
compression and the maximum rarefaction. This corresponds in water waves to the
vertical height of the crest above the trough of the wave. (See Figures 4 and
5. Frequency. Any one of a series of variations, starting at one condition and returning
once to the same condition is called a "cycle."
If we should fix our attention at some point on the surface
of water in which waves exist, we would notice that at one particular point the
water will rise and fall at regular intervals. At the time at which the wave is
at its maximum height the water begins to drop, and continues until a trough is
formed, when it rises again to its maximum height. Therefore, if we notice all
the variations of height which one point on the surface of the water goes
through in the formation of a wave we will have witnessed a "cycle"
of wave motion.
The number of cycles
a wave goes through in a definite interval of time is called the
"frequency." Therefore the number of times the water rises, or falls,
at any point in one minute would be called the frequency of the waves per
minute, and we would express the frequency as a certain number of cycles per
In sound, the number of waves per minute is large, and it
is more convenient to speak of the frequency of sound waves as the number of
waves per second, or, more commonly, as the number of cycles per second. Thus,
a sound which is produced by 256 waves a second is called a sound of a
frequency of 256 cycles. When speaking of
sound, "cycles" always mean "cycles per second." Considered
from the standpoint of traveling waves, frequency is determined by the number
of complete waves passing a certain point in one second, and this, of course,
is equal to the number of vibrations per second generated at the source.
In the same way, when a racer goes once around the race
track and returns to the starting point, he completes a "lap," which,
in this case, is just another name for a "cycle."