Hi everyone, A couple of question I was thinking about regarding sound: 1) A sound source is moving at the speed of sound, and because sound waves cannot move faster, the waves build up at the front (direction of movement). If there is also an observer at the source (traveling at the speed of sound), what will he hear? 2) A sound wave is sent from a stationary source. Together with sound wave we let an observer travel at speed of sound (let's say that the observers travels exactly on one of the crests of the wave). what will he hear? I hope the description is clear. I have my ideas but I would be happy to hear yours first. Thanks, R.