Webs Wires and Waves: The Science & Technology of Communication

String It Along: Make a Low-Tech Telephone

Activity 2 Overview
Youngsters will explore how sound travels by conducting a range of experiments with paper cup telephones.

Estimated Time and Age Level
Advance Preparation: 20 minutes
Activity: One 40-minute session
(Ages 8-12)

(Per Team)
2 paper cups
6 meters (20 feet) of waxed dental floss or colored fishing line
2 paper clips
paper and pencil

Divide the group into teams of two children each. Make sure that the area where youngsters will test their phones is fairly quiet.

Cut the floss or line into roughly 6-meter (20-foot) sections. If the children are younger than eight, you might want to provide assistance where appropriate as they assemble their paper cup telephones.

Set out the materials in a central location.

Set the stage for the activity by asking children how they think sound is traveling from your mouth to their ears. If (following from their experiences with Activity 1) they suggest that some kind of sound vibration is moving through the air, ask them to consider how sound vibrations could do this. Suggest that they explore how sound moves by conducting some simple experiments with a classic paper cup telephone.

To begin their experiments, children should create a "baseline" by standing a measured distance (six meters or 20 feet) from their partners and whispering into their cups. Can their partners distinguish the words? Ask them to keep track of their findings by recording their observations.

Children should begin making their telephones by using a pencil to poke a hole in the bottom of a cup. (They should stick the pencil right through.) Then they can tie one end of the floss or fishing line around a paper clip. The inner "loop" of the paper clip should then be inserted through the hole from the outside of the cup so that it is clipped securely to the circular bottom of the cup. Have children repeat this process using the other end of the line and the second cup.

Now youngsters should return to their "baseline" distance. Each child can take turns placing the cup against one ear while his or her partner whispers into the cup at the other end of the line, always making sure that the line remains taut. Youngsters should now be able to hear their partners' voices resonating in the cups against their ears. Ask them to describe their results and to follow up with their own questions (and possible explanations) about what happened.

For example:
How are the sound vibrations getting from one cup to the other? (They travel through the line.)

After the vibrations get to the cup at the end of the line, what might they be doing to the bottom of the cup? (They cause it to vibrate, moving the air inside the cup in the same pattern that was created by the original sound.)

Now challenge the youngsters to think of different experiments to try with their telephones. For example, they might find out whether or not their telephones will work when their lines are stretched through a closed door, or when their fingers are pressed against the bottom of the cup. Before they try an experiment, you might want to have them write down the procedure and the expected results. After trying the experiment, they can record their results and try to explain why or why not their prediction came true.

Bring students back to the original question. Ask them how they can use what they've learned with the paper cup telephone to explain how sound travels through air. If necessary, move the discussion along by giving children this hint:

When people talk to each other, the space between them is not empty; it's filled with air. Air is a substance just as the dental floss or fishing line between the cups is a substance. What does this suggest about how sound travels? (Sound needs a substance through which to travel.) Relate the talking-in-space anecdote, if you like.

To see how well your young scientists are understanding the properties of sound, challenge them to explore the properties of their paper cup telephones further by designing new experiments. If they get stuck, suggest the following:

Try using your telephone with a sagging line between the cups. Does it work? Why or why not? (It won't, because the pulling and pushing action of the diaphragm--the cup bottom--in response to your voice's sound waves will not transmit efficiently along a sagging line.) Might it work if you used copper wire instead of fishing line or dental floss?

Try using your telephone with someone holding the line in the middle. Does it work? If not, where are the vibrations going? (It doesn't work because the vibrations are partly diverted by the person holding the line, weakening the signal that reaches the "receiving" paper cup.)

Older students could use their paper cup telephones to explore another important quality of sound--that it travels at different speeds through different media. All they'll need to do is replace the line between their cups with one that's three times as long. Then ask them to stand as far as the line will allow, again making sure that the line is taut. As one partner puts the cup to one ear, the other can shout while holding the cup on the other end of the line about six inches in front of his or her mouth. The "receiver" should hear the shout first through the ear covered by the telephone and then--an instant later--through the other ear.

Leave it to them to hypothesize why the shout reached one ear before the other. (Sound travels faster through solids than through gases.)

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