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I was doing a physics experiment, and i encountered a question which i couldnt 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?

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See for example:

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.

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WHich case is it exactly?Reflection from a high to low density?Why are the densities different? – user19955 Jan 17 '13 at 23:33
The Hard vs. Soft boundaries in that link are the better analogy. – user1631 Jan 17 '13 at 23:43
Sound wave propagating in tube will actually be slightly slower than sound wave in open space, so you could use the slow -> fast transition example. – user1631 Jan 17 '13 at 23:47

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