Observations and results
Could you hear your partner better using the cups and tight string than if you were speaking to each other in the same volume over the air?
In this activity, your voice vibrated the air inside of the cup, which in turn made the bottom of the cup vibrate. These vibrations were transferred to the string and then into the bottom of your partner's cup, which made the air inside of his or her cup vibrate and become detectable sound. When the string goes slack, the vibrations dissipate more easily and get lost along the way. (Landline phones work on the same idea but they transfer the sound waves into an electrical signal, which can travel even farther over wires—and the landlines don't have to be kept taut.)
Sound, such as human speech, travels in incredibly small waves—incredibly fast (about 1,126 feet per second), which is why you couldn't see it or detect a delay while it traveled across the cups and string.
Have you ever noticed how things sound different underwater? Because water's molecules are packed together more closely than those in air, sound waves move more easily—faster and farther—under water. Whales and other marine animals that use sound to communicate under water take advantage of this fact. Scientists think whales can hear each other from hundreds (and maybe even thousands) of miles away—without even a string telephone!
Share your string telephone observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.
Cleanup
Untie or cut the string from the paperclips or toothpicks. Recycle or reuse what materials you can.
More to explore
"Ear Cells Actively Amplify Sound" from Scientific American
"(Don't) Pump Up the Volume: Sound Waves Silence Whales' Song" from Scientific American
"Sound" Activities from The NASA Sci Files
"Frequency, Wavelength and Pitch" overview from Connexions
Sounds All Around by Wendy Pfeffer, ages 4-8
Janice VanCleave's Physics for Every Kid: 101 easy experiments in motion, heat, light, machines and sound by Janice VanCleave, ages 9-12
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4 Comments
Add CommentI get a kick from the TV commercial showing people talking with cans over limp strings. They even made a new one with more limp strings. Doesn't inspire confidence it that company---or at least in its advertising agency.
Reply | Report Abuse | Link to thislarrywac
This is pretty basic. Now how about making a Cartesian Diver, but one that can maneuver horizontally and pick up things from the bottom. I made one once, but it lacked a "keel", in this case to keep it from rocking back and forth due to it's mode of propulsion. A conventional diver demonstrates the relative compressibility of air and water and invisible transmission of a signal. This one also uses a signal with an analogy with DC and AC of varying frequencies. Contact danrob at efn dot org for more details if it's too much for you.
Reply | Report Abuse | Link to thisAs a kid I tried to use an electric fence with on power on it, as the string for a tin can version of this, it worked okay for short distances. The farmer must have thought it was funny when he turned the power back on the line. My bother and mine phone system came to a shocking end.
Reply | Report Abuse | Link to thisoops had typo in the above: As a kid I tried to use an electric fence with NO power on it, as the string for a tin can version of this, it worked okay for short distances. The farmer must have thought it was funny when he turned the power back on the line. My bother and mine phone system came to a shocking end.
Reply | Report Abuse | Link to this