Destructive interference

When two waves meet which are in antiphase, their amplitudes add together to create a wave with a smaller amplitude.

This is because, at that point, one wave has a positive displacement, while one has a negative displacement - so they’ll add to make a smaller amplitude.

Thsi is called destructive interference.

If the phase difference between the two waves is or a multiple of and offset by , the waves are in phase. That means that the peaks of one wave line up exactly with the troughs of the other wave, and the troughs line up exactly with the peaks.

When this happens, if the waves are the same, they are in antiphase and so the trough is the negative of the peak. That means they will add together to make an amplitude of zero - they cancel out completely.