This animations shows the addition of two sine waves of slightly differering frequencies (f1 and f2). The individual sine waves are shown in the top two traces while the bottom trace shows the sum of the two waves.
Beats can be seen in the bottom trace.
When two waves of slightly different frequency overlap a phenomenon known as beats results. The effect is a kind of throbbing sensation, which can sometimes be heard when two musicians such as oboeists are playing together.