What are beats in tuning? Why do they happen?
Video about sound waves/what sound is: https://www.youtube.com/watch?v=hfzCLClVO8g
Image of frequency relationships of an octave, fifth, etc crossing (scroll down)
http://legacy.earlham.edu/~tobeyfo/musictheory/Book1/FFH1_CH1/1M_RatiosCommas1.html
