Physics 312 Lecture: "Superposition of Waves and Standing Waves I." Apr 10, 1998

When two sinusoidal waves with identical frequencies and wavelengths interfere, the result is another wave with the same frequency and wavelength, but a maximum amplitude which depends on the phase difference between the input waves.
Constructive interference occurs when the phase difference between two waves is zero, or some multiple of pi radians
Destructive interference occurs when the phase difference between two waves is some multiple of pi/2 radians
Adding two sound waves which are out of phase may yield silence!
Standing waves occur when a wave reflects back and forth in a confined space and interferes with itself
Nodes are locations where a standing wave has zero amplitude
Antinodes are locations where a standing wave has maximum amplitude
Waves travelling on a string fixed at both ends are constrained to have specific wavelengths and frequencies

This lecture discusses material in Chapter 18 of Serway.