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.
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
Yeast Alive! Watch Yeast Live and Breathe
What you'll need
• Fresh packet of baker's yeast (check the expiration date)
• Tablespoon of sugar
• Clear plastic bottle with a small opening (such as a water bottle)
• Small balloon
• Warm water