Beats are interference patterns between two sounds of slightly different frequencies. In this Demonstration, beats are produced by sound waves emitted by two tuning forks. You can vary the frequencies and listen to the sound waves. The frequency depends on the length of the tuning fork. You can propagate the waves in time using the "time" button.
The vibrations of the tuning forks are shown under the tuning fork graphics. The plots at the bottom show the combined and summed waves (beat pattern).
To show the beats, the time range is expanded; the combined wave range is shown in green in the beat pattern .
Snapshot 1: two tuning forks vibrating at the same frequency; the resulting sound obtained by summing the two waves shows a doubled amplitude but the same frequency
Snapshot 2: two tuning forks vibrating at slightly different frequencies; beats are generated and the amplitude value about zero is highlighted in green in the "beat pattern" graphic, because the waves in the "waves combined" graphic are almost out of phase
Snapshot 3: two tuning forks vibrating at slightly different frequencies; beats are generated and the maximum amplitude value is highlighted in green in the "beat pattern" graphic, because the waves in the "wave combined" graphic have nearly zero phase difference
 F. S. Crawford Jr., Waves, New York: McGraw-Hill, 1968.