I was doing a physics experiment, and I encountered a question which I couldn't answer. The experiment was about using a radar technique to measure the speed of sound. The apparatus was a plastic tube 90cm long and the sound source was a speaker connected to a function generator. The generator was sending a square wave with a low frequency (a couple of Hz) so we were creating sound pulses and we were observing their echoes through a microphone which was connected to an oscilloscope. When the other end of the tube was closed we could clearly see the reflected waves on the oscilloscope screen. However even with an open tube we could see that the waves reflected off the open end. (and they returned with an opposite pressure) Why did this happen?
See for example: http://www.acs.psu.edu/drussell/Demos/reflect/reflect.html
Reflection from hard vs. soft boundary. Closed tube is like a hard boundary, open tube a soft boundary. In either case there is an impedance discontinuity and hence reflection, however the phase of the reflected wave differs depending on the nature of the discontinuity.