An Exploration of Vibration, Sound, and Music
A wave is a disturbance that transports
energy from one place to another without the transfer of matter. After a
wave passes through a medium, there are no residual effects; the medium
remains unchanged. For example, if you throw a stone in a pond, a
circular wave will spread out from the point of impact. If the wave
encounters an object floating in the water, the object will briefly bob
up and down. However, once the wave has passed, the object, and the
water that buoys it up, will be left undisturbed.
We encounter waves everyday. Some are apparent; others go unnoticed. The
room in which you are sitting is being criss-crossed by all sorts of
waves. These include light waves, radio waves, and sound waves. While we
have receptors for light and sound, our bodies are not capable of
sensing radio waves directly. Some waves, such as sound and water waves,
require a material medium. On the other hand, light and other forms of
electromagnetic radiation can travel through the vacuum of space.
A fascinating feature of waves is that two of them, traveling in
opposite directions, can pass right through each other and emerge with
their original identities. However, while the pulses overlap, the height
at any point is simply the sum of the displacements due to each pulse
itself. If the pulses are on the same side of the medium they add; if
they are on opposite sides, they subtract. This is called interference.
The importance of wave phenomena in everyday life cannot be overstated.
It is estimated that human beings receive over 90% of their information
from light and sound. The experiments that follow will allow you to gain
first hand experience with the properties of waves in general and sound
Key Concept: A wave is a disturbance
that travels through a medium. Waves are characterized by wavelength,
frequency, and amplitude. Waves reflect when they encounter a barrier or
A rope or a long
spring may be used to demonstrate many properties of waves. Hold one end
of the stretched medium in your hand while your partner holds the other
end. Now move one end up and down at different rates (frequencies). What
happens to the wavelength as you increase the frequency? Decrease the
Does the tension in the medium have any observable effect on the speed
of a wave? To find out, send a sharp pulse down the medium when the
medium is under various degrees of tension. What do you observe?
Send another sharp pulse down the medium. This time watch carefully as
the pulse reaches the fixed end. Does the pulse reflect? If so, is the
reflected pulse the same as the incident pulse or is it upside down? Do
the incident and reflected pulses travel at the same speed?
Helical Spring "Snaky"
Click here for information on Helical Springs (Snaky) or Super Springy!
Way of Waves
The overlapping, or superposition, of two waves produces reinforcement
in some instances and cancellation in others.
You can witness wave interference on a phone
cord, rope, or Slinky. After producing one pulse on the medium, generate
a second shortly thereafter. Watch carefully as the two pulses meet and
pass through one another. How would you describe the medium when the two
pulses overlapped? Did the pulses produce a larger or smaller resultant
pulse? What procedure must you follow to produce constructive
interference (a larger net pulse)? Destructive interference (a smaller
Key Concept: Standing waves are
formed when two sets of identical waves pass through a medium in
Move the end of the medium of choice (rope, phone cord, Slinky) up and
down at the right frequency to create a full wave. When this has been
accomplished you will notice that the center of the medium appears to
stand still. This stationary point is called a node. At a node, the
destructive interference of the incident and reflected waves is total.
On either side of the node are regions of maximum displacement called
antinodes. Have someone gently pinch the node with their fingers. What happens?
Increase the frequency of the up and down motion of your hand until two
nodes appear on the medium. How many antinodes are there now? How many
wavelengths do you observe? What happens when you continue to increase
the frequency of the waves? Can you obtain three nodes, four nodes,
etc.? Describe what happens to the wavelength as the frequency is
Click here for information on the hand held standing wave kit
Are You Chicken?
Key Concept: A surface set in motion
by a vibrating string amplifies sound.
Using a toothpick, puncture a small hole in
the center of the base of a paper or plastic cup. Pull a ˝ meter, or so,
length of string through the hole. With the cup turned upside down, tie
the string around the toothpick (see figure).
Rub a little rosin on your thumb and index
finger. Using a jerking motion, pull down on the string while gently
pinching it between the thumb and index finger. Describe what you hear
as the string moves between the fingers.
What is the source of the sound you hear? Why is the sound so loud? To
answer this question, it may be helpful to pull on the string when it is
not connected to the cup.
What's it sound like?
Key Concept: Longitudinal standing
waves may be produced in metal rods.
Hold the aluminum rod with your fingertips
at its center. (Note: The center may be located by balancing the rod in
your hand in the region between the thumb and index finger as in the
figure above.) Place some rosin on the tips of the thumb and index
finger of the other hand. Grip one end of the rod between the
rosin-covered fingertips and stroke the length of the rod between the
end and center. With a little practice, you should be able to produce a
piercing, high-pitched sound. If you are unable to get the rod to sing,
tap the end of the rod with a mallet or hammer or tap it on a hard
While the rod is “singing,” bring a ping
pong ball suspended on a string in contact with an end of the rod.
Describe what happens. Can you explain why this occurs? What did
stroking or tapping the rod do to it? Grab the singing rod at a point
off center. What happens? Why?
Click here for information on Singing Rods
The Bells of St. Weber
Key Concept: The grate’s small
surface area prevents efficient energy transfer to the air at low
frequencies. The strings provide a direct pathway for sound transmission
at a wide range of frequencies.
Pick up the grate from a barbecue grill by
the strings. ( If you don't have a clean grill grating, coat-hanger or
metal utensils work too!) With the grate hanging by the strings, knock it against the
side of a table. Describe the sound you hear.
How does the sound produced by the vibrating grate reach your ears?
Now, with your index fingers, place the ends of the strings on the
little flap of flesh that protrudes over the opening of each ear.
Allowing the grate to hang freely from the strings, again swing the
grate into the side of a table. Describe the sound you hear with the
strings pressed against your ears. How is the sound reaching your ears?
How do you explain the difference in sound quality?
In the next issue of CoolStuff…
your students have performed the first set of wave and sound activities,
you know how simple wave and acoustics experiments can fascinate and
delight. Such hands-on explorations form a sound (pardon the pun!)
conceptual foundation that can be drawn upon when more abstract concepts
are introduced. In the next issue of CoolStuff we will offer up an
additional seven activities that will allow your students to further
probe the science of sound.
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