Hi, I am a little confused about some aspects of sound waves. I was hoping to just check to see if I am on the right track. I have a problem where a guy is moving towards the great wall of china, with a air horn, emitting a constant frequency sound of 500Hz. The sounds is going to hit the great wall and bounce back, introducing some interferance. The speed of sound is 350m/s. The persons is running towards the wall at 3/ms, and I have to find the frequency of amplitude beats he hears and the frequency of the sound he hears reflected. ok, so f' = (v/(v +/- Vs)*f He is moving towards the wall, so Vs is negative? This gives f' = (350/(347)) * 500 = 504.3Hz Is the frequency of beats then just the difference, ie (504.3-500)Hz for 4.3Hz beat Hz. Am I making any sense? thanks!