wave interference
Interference is what you get when two waves overlap and their combined pattern shows a fixed layout of louder and quieter (or brighter and darker) spots. Drop two stones into a pond and where the ripple rings cross you see a lattice of choppy and calm patches. The question it answers is: when waves meet, why do some places reinforce and others cancel?
Interference is the result of superposition, at each point the waves add. Where they arrive in step (in phase) they reinforce, and where they arrive out of step (out of phase) they partly or wholly cancel. For two sources of the same frequency, what matters is the path difference, the extra distance one wave travels to reach a point. A path difference of a whole number of wavelengths gives reinforcement, while a difference of a half-wavelength (plus whole ones) gives cancellation.
Interference is everywhere: the coloured sheen of an oil film or a soap bubble, noise-cancelling headphones, and the bright-and-dark fringes of the famous double-slit experiment that revealed the wave nature of light. An honest note: a clear, steady interference pattern needs coherent sources, waves with a fixed, matching frequency and a steady phase relationship, otherwise the pattern shifts too fast to see.
Two loudspeakers play the same steady tone. Walking across the room, you pass through loud spots (crest meeting crest) and surprisingly quiet spots (crest meeting trough), a sound interference pattern you can actually hear.
Overlapping identical waves build a fixed pattern of reinforcement and cancellation.
Interference needs coherent waves (matched frequency, steady phase). Two unrelated sounds do not make a stable pattern.