Interference (acoustic)
Physical superposition of sound waves in which vibrations can reinforce or cancel each other.
What it means
Interference is the physical superposition of sound waves. When two or more waves meet, their displacements add up: if the wave crests coincide, the sound is reinforced (constructive interference); if a crest meets a trough, it is weakened or cancelled out (destructive interference).
Musical use
In music, interference shows especially as beating: when two notes have nearly but not exactly the same frequency, a periodic swelling and fading of loudness arises, which musicians use when tuning instruments to match two notes exactly. Interference also plays a role in room acoustics, when sound waves reflect off walls and superpose with the direct sound, which can reinforce or weaken certain frequency ranges.
Interference should be distinguished from the mere superposition of several sounds into a chord, which is thought of musically rather than purely physically, and from resonance, in which a body capable of vibrating is itself set vibrating by an external stimulus.
Performance practice
Use beating as an interference effect when tuning: play two notes at once and listen for the swelling and fading; as the beating slows, you are approaching an exact match. When it disappears entirely, the notes are tuned pure.