In the previous issue of CoolStuff 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!
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 possess energy?
How do you know?
Talking Can be a Drag!
Key Concept: 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 amplifiers.
How do you suppose the strip “talks” as you drag your thumbnail across the ridges?
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.
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 amplifiers?
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. The pin 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 sound.
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 sound pipe, 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?
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 ľ” 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 pitch?
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 oboe!
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 interference pattern.
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 dominoes.
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?