1. When two waves occupy the same position and are in phase, the individual amplitudes (or displacements) add together to produce a resultant wave of larger amplitude. This is called constructive interference or reinforcement . NOTE: This is similar to resonance BUT remember that resonance requires a system with a natural frequency (f 0 ) to be subjected to an external forcing frqeuency.

2. When two superimposed waves are in antiphase (  radians out of phase), the individual displacements (or amplitudes) still add BUT, as one displacement will be negative, the result is partial or total cancellation. This is called destructive interference or annulment .