In my sophomore mechatronics class, the professor mentioned that when we design rotating devices (i.e. a fan), we should make sure the outside edges don't move faster than the speed of sound. I always wanted to ask him why but never got a chance. Since then the question has been bothering me and it comes back to me every now and then, I'm finally posting it! . What's wrong with parts moving faster than the speed of sound in a machine?