Constructive Interference occurs when two troughs or two crests combine together as they travel in opposite directions towards each other, creating a greater combined wave and amplitude when they meet.

Destructive Interference occurs when a trough and a crest meet and combine to work against each other as they travel in opposite directions, which results in them momentarily cancelling out.

Standing Waves occur when the wavelength and the amplitude of the waves are the same as they approach one another, this creates a standing place of energy in the wave. The part of the wave that appears to not be moving is called a node/nodal point.

How do Noise-Cancelling Headphones Work?

Noise-cancelling headphones use destructive interference by taking the surronding sound waves and recreating the exact opposite type of wave allowing the crests and troughs of the sound waves to cancel out. Special material also helps cancel out any extra sound that may be heard.