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1. Constructive Interference
Constructive Interference is when a crest and another crest (or trough and trough) of the same amplitude combine energies to displace the medium. The energies are additive and after the interference, the combined wave would be twice as big as one of the waves.
2. Destructive Interference
Destructive Interference is when a crest and trough meet (“out of phase”) and the energies combine to work against each other. This causes the waves to cancel out and they have a moment in the middle where it becomes a straight lines with no waves.
3. Standing Waves
The same amplitude and wavelength interfering waves creates pattern that stay almost stationary (standing wave interference pattern). Depending on its frequency, there is a different number of nodal points. The first video has three nodes (2 at the end, 1 in the middle) and the second video has four nodes (2 at the ends, 2 in the equilibrium centres).
How do noise-cancelling headphones use wave interference to eliminate unwanted sound?
Noise-cancelling headphones happens by destructive interference to block the outside noise from entering your ears. The sound waves created by the headphones come into contact with the outside sound wave which cancels each other out. That way, no outside sound waves will come through.