Another interesting demonstration similar to the Chladni sand figures showing
nodes and antinodes is the existence of standing waves in a resonating
confined air/gas column.
To perform the demonstration you would need a long tube containing an
illuminating gas. I use an 8 foot, 4 inch diameter galvanized gutter tube
that is drilled in a row with 1/32 inch holes at 8 inch intervals and
connected to a natural gas source through an adjustable plunger. Cover the
other end of the tube with a thin layer of aluminum foil or parchment paper
and secure it in place. Turn the gas on to fill the tube for about 30
seconds and light the escaping gas along the top of the tube. Remember the
tube gets fairly hot to the touch. Play some music close to the aluminum
foil and slide the plunger in and out at the other end to attain resonance.
The small gas flames will appear to dance to the music. Where the nodes
occur in the vibrating gas the molecules are not moving. At these points the
pressure is high and the flames are tallest. Halfway between the anti-nodes'
region where the molecules vibrate back and forth with large amplitudes, the
flames are low. Bernoulli's principle is largely responsible for the
pressure differences. Where the velocity of the molecules is high, the
pressure is low, and where the velocity is low the pressure is high.
The normal safety precautions need to be considered. Do the demonstration in
a well ventilated room. I had some students with asthma complain of the gas
smell.
It is fun to do and the students watch with fascination.
Greg Darakjian
Anaheim High School
811 W. Lincoln Ave
Anaheim, CA 92805
Tel: 714-999-3717