When two waves overlap they don’t crash and destroy one another; instead, they pass through each other without interaction. The graphs below show two waves approaching, overlapping, and moving away. They continue to move at the same velocity throughout. The arrows show their motions. The dotted drawings show the individual waves while the solid drawings show the resultants.

The amplitudes of the two waves add together, producing either a larger wave if they are both positive or both negative. They produce a smaller wave if one is positive and the other negative. In fact, as shown (4), they can produce no amplitude at all. The large amplitudes are called constructive interference. The reduced amplitudes are called destructive interference.