I believe this question was asked in some form before, but I'm not clear on the answer. If a sound wave must equal air pressure when it exits a tube, why is it possible that at many points after the sound wave exits the pipe (is in open air) it does have greater pressure than the air (in the compression portion of the wave?
It does feel natural that as the air exits the pipe, it will revert to regular air pressure: I just don't understand why it is required, given that at many points after the pipe ends the wave does have high pressure. How does the air know that it's at the pipe's end? Why can't you have a wave leaving the pipe at a compression, and having pressure greater than air pressure?
Another way to state my question is like this: if a mechanism exists requiring a sound wave to be equal to air pressure on exiting a pipe, what removes that mechanism once the wave is further away from the pipe (in open air), allowing it to resume having pressures greater than air pressure?