“Good Vibrations Part II”
An Exploration of Vibration, Sound, and Music
the previous issue of CoolStuff ( CoolStuff 13 ) you were introduced to a number
of activities that reveal some of the unique properties of waves. In
this second installment on waves and sound we will present additional
demonstrations that not only show how sound waves are produced, but also
reveal how they may be recorded and reproduced.
Among the activities below are two that introduce students to analog
sound recording. Growing up in the era of digital recording, most
students are amazed to learn that sound can be recorded on an old
fashioned record and reproduced with nothing more than a needle and a
cardboard cone. However, some students may recognize this simple device
as a rudimentary Edison record player. Students of all ages never fail
to be amazed to learn that a single groove in a record may contain
information about a wide variety of musical instruments and singers and
that our ears are capable of sorting out the individual notes and voices
when a record is played.
Talkie Tapes can be thought of as linear records. However, instead of a
groove, the strip is covered with ridges and pits that cause an object,
such as a fingernail, to vibrate as it is dragged along the strip. As
with the Edison record player, amplification is achieved by attaching
the strip to an object capable of moving more air.
After completing their study of analog recording, students should be
encouraged to investigate the difference between analog and digital
recording processes. The Internet offers many sites that discuss the two
recording techniques. Oh, and one other thing: You might want to tell
students not to attempt to play their CDs with a pin! Believe me, more
than one student has tried it!
1. The Diving Tuning Fork
Key Concept: A vibrating object
possesses kinetic energy.
Strike the end of a tuning fork on a rubber
pad or on the heel of your shoe. Barely touch the surface of a cup of
water with the vibrating ends of the tuning fork.
What do you observe?
Do the vibrating ends of the tuning fork
How do you know?
Click here for information on Tuning Forks
Click here for information on Talkie Tapes
Talking Can Be A Drag!
Ridges embossed on the surface of this strip cause your thumbnail to
vibrate. The cup, with its larger surface area, amplifies the sound.
Hold the pointed end of the plastic talking
strip between the thumb and the index finger. Using your other hand, drag
your thumbnail along the ridges on the strip, moving from top to bottom.
Do you hear anything?
If you didn’t hear anything as you moved your thumbnail along the strip,
hold the pointed end of the strip against the end of a paper or
Styrofoam cup. What do you hear this time? Why was the sound louder when
the cup was used?
Try using other materials (for example, a piece of paper, a windowpane,
a blackboard, a balloon, your front teeth) to amplify the sound. List
the amplifying materials you test and describe their effectiveness as
How do you suppose the strip “talks” as you drag your thumbnail across
Can you think of any other device that
produces sound in a similar manner?
Music Box Marvel
Key Concept: The music box forces the
cup to vibrate. Because of its larger surface area, the cup causes more
air to move. This in turn results in a louder sound.
Teacher’s Note: Obtain the inside
mechanism from a mechanical music box. The mechanism should not have an
(any type of box or platform)
If necessary, gently wind the music box
mechanism. Listen to the tune.
Can you identify it? Can you even hear it?
Bring the mechanism in contact with a Styrofoam or paper cup, tabletop,
window, blackboard, etc. Is the sound louder when it is in contact with
a solid object?
Why do you suppose this is so?
Which object(s) makes the sound the loudest?
Which objects tend to be the least effective
Click here for information on the Music Box Mechanisms
1920's German Child's Victrola Record Player
Getting In The Groove
Key Concept: Grooves in a phonograph
record cause the needle to vibrate. The cardboard cone, with its large
surface area, amplifies these vibrations.
Form a cone out of a piece of poster board
or file folder. Tape the edge so that the cone will retain its shape.
After you place a straight pin through the tip of the cone, as is shown
below, you will have an Edison-style record player.
Cradle the cone in both hands as you lower
the straight pin into the groove of a spinning record.
should drag behind the base of the cone, with only the weight of the
cone holding it down.
What do you hear?
Describe the loudness and clarity of the
Explain how sound is produced with this
simple record player.
Since records may be foreign to many students, it may be instructive to
allow them to look at record grooves through a magnifying glass or
microscope. The recorded information appears as a squiggly line that
spirals in from the outer edge of the disk to the center. The nature of
the grooves reveals where the recorded sound is the loudest (the squiggle
is wide in loud passages) and where the sound has high frequency
components (the squiggles are close together). As the needle moves
through the grooves, these variations in the groove cause the needle to
vibrate, producing sound. Since the needle does not displace much air as
it vibrates, it’s necessary to attach the needle to the cardboard to
amplify the sound.
Key Concept: As the air passes
through the tube, it strikes the ridges that line the tube. Increasing
the rate of twirling causes air to be drawn through the tube at a higher
speed. As air speeds up, the frequency of sound produced by the air’s
interaction with the tube walls also increases. However, since the
length of the air column determines the frequencies that are reinforced,
only certain rates of twirling will produce audible, sustainable sound.
Hold the large end of the plastic tube in
one hand and swing the other end over your head. Be certain that no one
is in the immediate area before you start swinging the tube. Start out
swinging the tube slowly, then speed up. You should hear higher and
higher pitches as you swing the tube faster and faster.
What do you think is producing the sounds you hear?
Why does increasing the rate of swings
increase the pitch of the sound produced?
Can you produce any pitch you wish or are
there only certain sounds that can be produced?
Now tear a sheet of paper into some small bits and place them on a
tabletop. Hold the stationary end of the tube over the bits of paper
while swinging the other end of the tube. Watch the paper fly!
Based on the movement of the bits of paper,
which way does the air flow through the tube?
Click here for information on
The Last Straw!
Key Concept: The vibration of air in
tubes is the mechanism for sound production in virtually all wind
instruments. The length of the air column determines the pitch.
Pinch together about ¾” of the end of a soda
straw. This may be accomplished by pulling the end of the straw through
clenched teeth. Using scissors, diagonally cut off the corners of the
flattened end (see figure on left).
Place the flattened end in your mouth and
blow gently. With a little practice, you will discover how to adjust
your lips and air pressure to allow the straw’s reeds to
vibrate correctly. When the reeds vibrate, a sound will be produced. The
device you have produced may be thought of as a “soda straw oboe,”
because like its namesake, it uses a double reed to produce sound.
Once you have produced a clear, loud sound, cut off successive pieces of
the open end of the straw.
What happens to the pitch of the sound as
the straw gets shorter?
How does a real oboe achieve changes in
Make another straw oboe, but this time cut small notches along the
length of the straw. The effective length of this device is changed by
covering and uncovering the holes. See if you can play a tune on this
Tuning Fork Interference
Key Concept: Because each of its
tines have a front and back surface, a vibrating tuning fork radiates
sound from a total of four surfaces. Sound from these surfaces
superimpose in the area surrounding the tines, producing an audible
Strike a tuning fork with a rubber hammer or
on the heel of your shoe. (Please do not strike the tuning fork on the
edge of the table.) Place the vibrating tuning fork near your ear.
Slowly rotate the vibrating tuning fork and note any variation in the
intensity (loudness) of the sound. Make certain that you rotate the
tuning fork through 360 degrees.
What do you hear?
Can you explain your observation?
The Domino Effect
Key Concept: Energy may be
transferred without the transfer of mass.
Arrange as many dominoes as possible in a
row. They should be placed on end and positioned so that when one
topples, it will cause the next domino in line to tip over. Describe the
disturbance as it passes through the dominoes.
Does it travel at a constant speed?
What do you suppose determines the speed at
which the disturbance travels?
Experiment with changing the spacing between
Does this affect the speed?
How is the domino model of wave propagation
similar to a wave on a spring or the surface of water?
How is it different?
Based on your knowledge of waves, do falling
dominoes correctly model wave behavior?
Paul Doherty used and improved on the Straw Oboe in the 2003 Iron
Science Teacher contest:
excellent site has animations of several different types of vibrations.
Be sure to scroll down for the Linear Quadrupole vibration that shows
the interference pattern around a tuning fork.
Russell's large collection of waves animations is listed here:
See past issues of CoolStuff or search by
specific topics in the new CoolStuff archives:
analog and digital recording:
detailed information on sound and recording, look at the Digital Audio
Primer here: http://www.teamcombooks.com/mp3handbook/11.htm
In the next issue of CoolStuff…
The next issue of CoolStuff will come to you
in September, when we will have more ideas for cool labs and
demonstrations for you to try. I hope you had a great school year and
that you will have a restful summer! If you are able, attend the
American Association of Physics Teachers summer meeting. You are certain
to come home with many new and exciting ways teach physics and physical
science. The theme of this year's meeting, which will be held in
Sacramento, CA July 1 - August 4, will be the physics of sports and the
human body. Visit
http://www.aapt.org for more information. And be sure to look for
the first issue of CoolStuff and the brand new Arbor Scientific catalog
in early September!
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