Beating
The app below show what happens when two waves (blue and red) with different frequencies ( and ) arrive at the same place at the same time. The sum of the two waves, supposed to be in phase at , is shown in dark green at the bottom of the screen.
Acting on the sliders, change the frequency of each wave and see what happens to their sum. The frequency of wave 1 can be changed from 1 to 10 in steps of 1, while the frequency of wave 2 can range from 1 to 10 in steps of 0.05. In particular see what happens when the two frequencies are close to each other: the phenomenon is called beating. The frequency of the resulting wave is the average of and (hence very similar to both), while the amplitude is modulated with time (or space).