The app below show what happens when two waves ([color=#1551b5]blue[/color] and [color=#c51414]red[/color]) with different frequencies ([math]f_1[/math] and [math]f_2[/math]) arrive at the same place at the same time. The sum of the two waves, supposed to be in phase at [math](x=0, t=0)[/math], is shown in [color=#0a971e]dark green[/color] 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 [b]beating[/b]. The frequency of the resulting wave is the average of [math]f_1[/math] and [math]f_2[/math] (hence very similar to both), while the amplitude is modulated with time (or space).