http://www.ck12.org Chapter 11. Vibrations and Sound
Like simple harmonic motion, sound waves have a number of basic properties. TheFigure11.1 shows the
wavelengthandamplitudeof a physical wave. We will use such wave representations for both longitudinal and
transverse waves. The Greek letter “lambda”(λ)is used to represent wavelength. The wavelength of a wave is the
distance between two successive crests or troughs, or, for that matter, any two identical points on the wave.
The amplitude, asFigure11.1 shows, is the maximum distance the wave extends beyond the equilibrium line (0).
It is measured either from the equilibrium to the crest or peak (highest point) of the wave, or from equilibrium to the
trough (lowest point) of the wave. The distance from crest to trough is twice the amplitude( 2 A). The maximum is
the highest peak above the equilibrium line, occurring at positions 0.50 m and 2.5 m, as shown inFigure11.1. The
minimum is the lowest point below equilibrium, occurring at positions 1.5 m and 3.5 m, inFigure11.1. The period
T, as with simple harmonic motion, is the time required for each vibrating point to complete a full cycle.
FIGURE 11.1
Wave Interference
Waves can interfere with each other so that their resulting amplitude can be either greater or smaller than the
amplitude of either wave. If two waves overlap crest on crest and trough on trough, they are said to be completely in
phase and the amplitude of the combined wave will be twice that of either wave(A+A= 2 A). The same is true if two
waves are of different amplitudes. As long as the crest (or trough) of both waves overlap, their combined amplitude
will equal the sum of the maximum amplitudes. This is called completelyconstructive interference,Figure11.2
A. Conversely, if two identical waves are completely out of phase,their resultant amplitude is zero(A+−A= 0 ).
This is called completelydestructive interference,as shown inFigure11.2 B. Waves may produce various degrees
of constructive or destructive interference, as shown inFigure11.2 C. The process of adding two or more waves
together is calledsuperposition. The superposition of two or more waves is the sum of their respective amplitudes
at their correspondingx−values.