Standing Wave Machine

This apparatus was shown to me by Don Rathjen during the 1996 San Francisco Exploratorium's Teacher' Institute summer program. The design was developed by Don Rathjen, Tien Huynh-Dinh, and Guillermo Trejo-Mejia.

Materials needed:

White, 1/2" PVC
3  12.5" long
2  3 1/4" long
2  2 1/4" long
6  1/2" x 1/2" 90 degree elbows
2  1/2" x 3/4" 90 degree elbows

1  2" x 15"x 3/4" pine board
5  2" long finishing nails
1  16" long piece of light string
2  small DC motors (Radio shack part 273-223, 1.5 - 3.0 VDC)
1  25 ohm rheostat (3 watt, wire wound variable resistor, Radio Shack part 271-265B)
2  alligator clips
4  pieces of insulated hookup wire, each 30" long
1  1/2" diameter eye hook
2  3/4" long, 1/4" diameter wood dowel
2  2" long pieces of strapping tape
2  2" long pieces of self stick Velcro (loops and hooks)
1  1/2" long piece of self stick Velcro (loops and hooks)
2  D-cell batteries
2  strong rubber bands

Using a hacksaw or band saw, trim the two 1/2" x 3/4" 90 degree elbows so that they look like the drawing below.

This is the original figure from the Exploratorium instructions.  To view at a larger scale in Netscape, click on image with right mouse button and select "View Image."  For MS Internet Explorer, click on image with right mouse button and select "Save Picture As."  If, for example, you save the picture as c:\swave.gif, then enter file:\\c:\swave.gif into the address box of MS Internet Explorer in order to see the enlarged version.

 

std_wave.gif (10131 bytes)

 

The following comments are directly from the original instructions:
bulletMany of the dimensions above are arbitrary -- this configuration works, but don't be afraid to experiment with design changes
bulletUse a hacksaw to cut the 3/4" end of the 1/2" x 3/4" 90 bend to hold the motors -- a detail of the cut is shown in the drawing -- use rubber bands to hold motor in cradle created by the cut. (Added suggestion: Be sure to connect motors so they turn in the same direction!
bulletYou can use a 5K ohm pot that is cheaper, but the 25 ohm pot gives better control -- you can also just eliminate the pot and run at one speed
bulletTo produce fundamental mode of wave, spread motor arms apart to increase tension in string until fundamental occurs
bullet"Pinch" string to adjust nodes
bulletTry strobing the string -- it's spectacular!

Gain a better understanding of standing waves

Don Rathjen developed a worksheet to help students understand why two waves, one traveling to the right and one to the left, would generate a standing wave with nodes and antinodes in fixed locations.  The worksheet has eight frames, each showing two waves.  One of the waves, identified by occasional dots, is traveling to the left while the other is traveling to the right.  In each frame the student draws the sum of the two waves.  Try it!  (Because the scale of a figure printed from a web browser can vary, better results could be obtained by viewing and printing the PDF version of this worksheet.)  For an animated drawing that shows two moving waves and their sum, see Tom Henderson's site.

Tom Henderson's "Physics Classroom" is a good source of more in-depth information on waves.
http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/soundtoc.html

Pictures

Here are a few pictures of the standing wave machine in action.  Meredith Lamb has posted some pictures from the USGS 1999 Open House in Golden, CO, that show the standing wave machine and as well as other hands-on exhibits.