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Reprinted by permission of Dick Berg, University of Maryland, for use on this website.

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The Quincke's Interference tubes can be used with someone speaking into it or what is probably easier is to use it with a wave generator and speaker so that you get a constant tone.  

One of the path lengths in the apparatus is approximately 50 cm longer than the other.   At around 350 Hz this apparatus will give a destructive interference.  By pinching off the long hose you can go from a node to an anti-node as seen on the oscilloscope.  At about 520 Hz you get constructive interference.  By pinching off the long hose here you will get a reduction on the size of the sine wave because of subtraction.