Interference of Two Coherent Sources
Signals from two identical sources, like stereo speakers, interfere constructively where crests meet crests and destructively where crests meet troughs.
The same thing happens for any two “coherent” sources of any kind of wave. “Coherent” means the sources must have a constant phase difference over many periods of vibration.
Notes:
In the diagram at left, the blue circles represent the crests of waves moving away from the source at the center, and the red circles represent the troughs.
The black curves labeled “constructive” are drawn through the points where waves from the two sources are “in phase,” that is, where crests from one source coincide with crests from the other; the green curves labeled “destructive” are drawn through points where the phase difference between waves from the two sources are p radians (180o) out of phase, so that crests from one signal coincide with troughs from the other.
If the two sources were stereo speakers producing harmonic tones in phase, a listener placed somewhere along the green curve would hear virtually nothing, and a listener on the black curves would hear a louder-than-normal sound.