Constructive Interference:

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Constructive Interference is when a crest from one source meets a crest from another source.

Deconstructive Interference:

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Constructive interference is when a crest and a trough meet.

Standing Interference:

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When interfering waves have the same amplitude and wavelength, the resulting pattern remains nearly stationary.

How do noise cancelling headphones use wave interference to eliminate unwanted sound?

Sound is a pressure wave. A noise cancellation speaker emits a sound wave with the same amplitude but with inverted phase, so that the noise cancels out.

By Trevor Thirsk