A constructive wave id when two crests from opposite directions converge and the end result wave is the sum of the two crests.
A destructive wave is when a crest and a troph from opposite directions converge. the end result wave usually cancels out (no wave) or they act against each other.
How do noise-cancelling headphones use wave interference to eliminate unwanted sound?
A : The music going into the ear acts as a constructive wave. Another wave is created through the headphones that acts as the troph with the same amplitude, and the background noise acts as the crest that creates a destructive wave that cancels out any noise.
Reload document 
Open in new tab