This video shows a spectrogram (top) for a waveform (bottom) created by adding two harmonics with nearly equal frequencies. The beating is evident in the waveform. The spectrogram shows repeated dark bands, alternating with colored bands centered on a single frequency. Listen closely for how the volume of sound alternates between louder and softer---and how that exactly matches with the positions of high amplitude (louder) and low amplitude (softer) portions of the waveform--- and also matches with brighter red bands (louder) and dark bands (softer) portions of the spectrogram. If you count the number of lobes in the waveform from 0 to 1 seconds, you should be able to check that the beating rate is 4 beats per second. (NOTE: Setting the quality level to 720 HD is best.)