Constructive Interference
When two wave signals S1 and S2 have the same frequency, the same amplitude, and the same phase, the net effect is a signal (S1+ S2) that has twice the amplitude as either of the original signals.
Notes:
The graph shows the net effect of adding together two simple harmonic wave motions at points where they are in phase. When crests of one coincide with crests of the other, the net signal is like the individual signals, but with twice the amplitude.
It turns out that intensity is proportional to the square of the amplitude, so the intensity after constructive interference is four times larger. Now how many decibels is that?