The full audible spectrum
Fun physics experiment!!: Try opening two tabs each playing this same video (make sure they're both loaded). Try to have the audio synchronized (a small echo is ideal). When it gets to actually playing the sounds, the sounds will have a strobe-like quality. This is because of a concept called the beat frequency, periodic destructive interference, which is the difference between two frequencies. If they are synced better, the beat frequency will be lower. Try it out!!
Idea from karlsmuster