other. The principle of superposition tells us that waves cannot affect one another: one
wave cannot alter the direction, frequency, wavelength, or amplitude of another wave.
Destructive Interference
Suppose one of the experimenters yanks the string downward, while the other pulls up by
exactly the same amount. In this case, the total displacement when the pulses meet will
be zero: this is called destructive interference. Don’t be fooled by the name, though:
neither wave is destroyed by this interference. After they pass by one another, they will
continue just as they did before they met.
Constructive Interference
On the other hand, if both experimenters send upward pulses down the string, the total
displacement when they meet will be a pulse that’s twice as big. This is called
constructive interference.
Beats
You may have noticed the phenomenon of interference when hearing two musical notes
of slightly different pitch played simultaneously. You will hear a sort of “wa-wa-wa”
sound, which results from repeated cycles of constructive interference, followed by
destructive interference between the two waves. Each “wa” sound is called a beat, and
the number of beats per second is given by the difference in frequency between the two
interfering sound waves: