At school today we were looking at the different harmonics and how they set up in strings, open and closed pipes. One of the students asked a cool question. He could see the closed pipe and string examples where there was “something” at the end of the length to reflect the wave, but didn’t understand why the waves would set up in the open pipe since it was open at both ends. Why didn’t the wave “fall out” the ends of the pipe? Or set up only partly in the tube and partly in the air or whatever medium was outside? Well, the answer is to do with the fact that although in the Year 13 book we are using at my school, and in most texts, the wave is represented as transverse, it is actually a longitudinal wave. This means that it is compressing and rarefacting in the pipe. The pressure at the ends of the pipe come in to play because the wave is setting up between these two areas of pressure which kind of act like the ends of the string. This is a pretty easy to read explanation of what is going on here. Also there’s a whole course of Physics lectures from MIT on waves online here. video 9 is where he goes into fundementals, harmonics and relates it to musical instruments.