|MadSci Network: Physics|
Hi, The experiment you are attempting is called the "Kundt Tube". The Kundt tube is based on the notion of "standing waves", ie. waves of which the wavelength fits, for a fully open or a fully sealed tube, an integer number of times or for a half-sealed tube, an integer plus a half times in the length of the tube used. In your case, the tube is open on the input end and sealed on the other end, making it a half-sealed tube. The wave motion will then have a clearly discernible pattern of pressure maxima (where pressure variation is maximal and speed variation is minimal) and speed maxima (where pressure variation is minimal and speed variation is maximal). If the tube is made to contain dust particles of some sort, they follow he moving air at the maxima. However, the particles will not be able to follow the air completely because of friction. The net result is that the particles will shift away from the speed maxima and finally settle down on the speed minima, thus visualising the standing wave pattern. To do the experiment correctly, you need a pure sinewave sound source of sufficiently high frequency to make the wavelength short enough (which explains why a boom-box is not exactly it). Also, to make a perfect standing wave you will either need to have adjustable frequency or adjustable tube length (by replacing the fixed seal by a movable plunger). Thirdly, the powder in the tube must be light but be subject to relatively strong friction. Your styrofoam pellets are light but have little friction. Traditionally, cork dust is used for its relative insensitive to humidity (doesn't clog). The sound source need not be of excessive power. Kundt did his famous experiments with a tuning fork (the only pure sinewave source available at the time). Kind regards, Bruno
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