The following are descriptions of demonstrations performed in the ISU Department
of Physics Demonstration Road Show. Teachers, especially those at schools
visited in this program, are encouraged to access this information. The discussions
provided with many of the descriptions parallel those provided in a typical
presentation to K-6 grade students. Only about 2/3 of the demonstrations
provided below can be incorporated into a typical 50 minute presentation.
Teachers are encouraged to select those demos most relevant to what they have
covered or will cover in their classes. This selection is usually done
during a meeting with an ISU Physics faculty member prior to a demonstration
SOUND AND WAVES
H and He Balloons
15" balloons filled with either H or He are popped with an 8" "torch" lighter.
This is used as an exercise in the scientific method. In science, we
make educated guesses based on observations, experiments are performed to
test those guesses, the guesses are modified if necessary, and the process
continues. Students are shown the balloons, and after noting that whatever
is in the balloons must be lighter than air, the students are asked what
might be in them. If mentioned, hot air can be discounted since there
is not an obvious heat source. Heat must be continuously be added to
a hot air balloon to keep it hot. Once choices are narrowed down to
H and He, the students are asked to come up with an experiment to tell the
difference. This results in the "flame test", since H is very flammable,
while He is not.
Laser Light Show
Music from a speaker vibrates a tight latex sheet with a mirror glues to
it. Laser light reflected off the mirror onto a wall or ceiling will
dance in cool patterns as the latex sheet vibrates. This is used to
show that sound is a vibration of the air. Air moving back and forth
from the sound makes the latex sheet move back and forth. This moves
the mirror and the laser beam that reflects off of the mirror.
Students are asked to make a wave by raising and lowering their hands after
their neighbor raises theirs. This kind of wave is popular at sporting
events. Sound waves are much like this. Sound will carry energy
from one side of a room to another, even though each air molecule only moves
back and forth a tiny bit.
Waves on Springs
Long springs are used to show the difference between transverse (up &
down) and longitudinal (back & forth) waves. Sound is a longitudinal
wave, and waves on strings are transverse. Water waves are both transverse
and longitudinal. Wave pulses on the springs will superimpose or add
as they pass through each other, but will be otherwise unaffected.
Standing waves, or harmonics, will be shown on the springs, and wavelengths
will be discussed. A wavelength is the distance it takes for the wave
to repeat itself. Long wavelengths mean slow vibrations and low pitches,
and short wavelengths mean fast vibrations and high pitches.
A giant torsion (twisting) wave machine is used to show that waves move faster
if there is a greater force pushing or pulling the stuff back to where it
was before the wave passed through. This is why sound travels through
stiff stuff like steel so fast. Also, the dependance of wave speed
on density will be discussed. Sound travels through water faster than
air because it is more dense.
Students will use short lengths of PVC pipe to make popping sounds.
Short pipes will make sound of short wavelength and high pitch, while long
pipes will make sound of long wavelength and low pitch. The students
will then play a simple tune with the pipes.
Resonance boxes are short wood boxes with tuning forks mounted on top.
Each box has a natural, or resonance, frequency. The length of each
box is chosen to match the frequency of sound made by the attached tuning
fork. If the tuning fork vibrates, it makes the air inside the box
vibrate as well, which makes the sound much louder than a tuning fork by
itself would make. If one resonance box vibrates, it will make a second
one vibrate as well through resonance. The sound from the first box
will make the air inside the second box vibrate, which will make the second
box and the attached tuning fork vibrate as well. Sometimes sound will
make several things in a room vibrate if the sound frequency matches a resonant
frequency of an object.
Hot Crossed Buns
Seventeen students will be directed to sit on whooppie cushions attached
to different lengths of PVC tubing to play the tune "Hot Crossed Buns".
The whooppie cushions make sound over a wide range of frequencies.
This is sometimes called "white" noise. The frequency that matches
the resonance frequency of each pipe will be the loudest.
A Bunsen burner is used to make white noise. Different lengths of tubing
are held above the burner. Sound matching the resonant frequency of
the tube will be amplified. Long tubes will resonate at lower pitch.
An aluminum rod stroked by fingers covered in resin will "sing" with a longitudinal
wave in the metal. Pinching to rod at different spots will change the
wavelength of the sound and the pitch.
A metal plate attached to a speaker will vibrate as the speaker makes sound.
Two-dimensional standing waves will be created in the plate. Waves
on a drumhead look like this. These will be made visible by sprinkling
sand on the plate. Where vibrations are large (antinodes) the sand
is flung off, but it will collect where the plate is still (nodes).
Singing Wine Glasses
Wine glasses will be made to vibrate by running a wet finger along the rims.
The pitch of each glass is determined by the amount of water in the glass.
With very little water, almost all of the glass vibrates, and the pitch is
low. With lots of water, much less of the glass vibrates, and the pitch
is high. In general, larger glasses will make lower notes, and smaller
glasses will make higher notes.
Break the Wine Glass
Sound inside a sealed box will make a wine glass vibrate at its resonant
frequency. The vibration is "slowed down" with the use of a strobe
light. If we can match the resonant frequency exactly, we should break
the glass. If not, just watching the large vibrations of a big glass
is pretty cool.
If two pure notes are very close together in frequency, they will alternately
constructively interfere and destructively interfere. This makes a
"beating" sound. You hear the average of the two notes, but the volume
oscillates, or changes. Musicians use this to tune their instruments.
The closer two notes are to each other, the slower the beating. Musicians
try to make their notes close enough to each other so that the beats are
far enough apart to not be very noticeable. The resonance boxes show
this very well. A weight moved slightly on one of the tuning forks
changes the frequency enough so that beats are heard when both resonance
boxes are struck.
Recipe of Sounds
Audioscope, a computer program, is used to analyze sound picked up by a microphone.
Sound vibration will be graphed vs. time. Pure notes will look like
sine waves, while most sounds or notes from instruments will look like several
waves added together. Different instruments and voices make different
wave patterns. In addition to the "pure" sound of a note, each instrument
will make many other related sounds, or "harmonics". The specific set
of harmonics and their differing loudness is what gives each instrument its
unique tone. We can easily tell our voices apart and the sound from
different instruments apart because of this. The program can also graph
sound intensity (loudness) vs frequency. Singers use programs like
this to help train their voices.
Doppler Nerf Ball
An electric buzzer inside a Nerf ball is used to demonstrate the Doppler
effect. If a sound source is moving towards you, the waves pass you
more frequently so you hear a higher pitch. If a sound source is moving
away you, the waves pass you less frequently so you hear a lower pitch.
Police officers use this same effect, but with infrared light, to check the
speeds of cars.
A speaker consists of a drumhead that is driven back and forth. The
sound wave sent forwards is exactly out-of-phase with the sound wave sent
backwards. Sound quickly spreads out, or diffracts. You can hear
someone speaking if they are facing away from you because of this.
The two waves made by a speaker will mostly cancel each other out unless
one of these waves is eliminated. Speakers are put in boxes to eliminate
the backwards wave. A speaker without a box is very faint.
The flame tube is a big metal tube with lots of little holes in a row on
the top. The tube is filled with propane, and speakers on both ends
are used to set up standing waves inside the tube. The sound wave can
easily be seen by how high the flames shoot up. Sound is a pressure
wave. Where pressure is highest (antinodes) the flame shoots up high,
and where the pressure is lowest (nodes) there is no flame at all.
When music is sent to the speakers this can be a real crowd-pleaser.