Details

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

References

[1] F. S. Crawford Jr., Waves, New York: McGraw-Hill, 1968.

[2] The Physics Classroom. "Interferences and Beats." (Nov 28, 2016) www.physicsclassroom.com/class/sound/Lesson-3/Interference-and-Beats.