MadSci Network: Physics

Re: how can i demonstrate sound wave transmission?

Date: Mon Feb 26 06:38:33 2001
Posted By: Bruno Putzeys, Staff, Electroacoustics and Analog Electronics, Philips ITCL
Area of science: Physics
ID: 981935248.Ph


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,


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