I was just wondering if anyone here had any experience with an open pipe system with a barrier imposed. Basically, say for a pipe open at both ends, at one end we place a barrier a distance away from the pipe. How would this affect the resonant frequency of the pipe? From experimentation, results I've gotten show that as the barrier distance decreased, the resonant frequency decreases as well. Likewise as the barrier distance increases, the resonant frequency increases until the point where the barrier has no effect where resonant frequency of pipe levels out. Does anyone have any idea about the theory behind this? The air particle movement, radiation impedance, free fields, some key terms that I could research (seeing as I'm stuck at the moment on how to explain this phenomena). Thanks in advance.