Summary: Simple but clear explanation of why the beat frequency is f1-f2 rather than (f1-f2)/2. I chose to use text file to interpret the concepts.

https://blogs.ubc.ca/phys101/files/gravity_forms/1-72f8f8fec225ededbd95c074100f3d9f/2015/03/LO8-Chen-Chu.pages.zip