The Speed of Sound on a String

Research Questions:

When you talk to a friend, your voices travel from your mouths through the air (a gas) to each other's ears. What happens when you connect your mouths and ears with a piece of string (a solid)?

Materials:

• Two paper or plastic cups (small yogurt containers are durable)
• Scissors
• String
• Masking tape
• A friend

Experimental Procedure:

1. Using the point of the scissors, poke a hole in the middle of the bottom of each cup. (Depending on your age, it may be better to have an adult do this step.)
2. Stand a few feet away from a friend and talk to each other in normal (not yelling) voices.
3. Keep moving apart until you can no longer hear each other.
4. Hold one of the cups up to your mouth and speak into it while your friend listens into the other. Now can you hear each other?
5. Next cut a length of string long enough to stretch the distance between you and your friend when you could no longer hear each other.
6. Poke the ends of the string through the holes in the bottoms of the cups (poke it upwards through the bottom) and tape the ends securely to these bottoms.
7. Hold onto one cup, have your friend hold onto the other, and walk away until the string is taut (pulled straight, not sagging).
8. Talk into one cup while your friend holds the other cup over her ear. (Remember to keep the string taut.) Switch. Now can you hear each other through the cups? (Remember to move the cups from your mouths to your ears and back depending on if you’re listening or speaking.) Why do you think attaching the cups with the string allows you to hear each other?
9. More fun with this project!: Try it with longer and longer pieces of string. How far apart can you get and still have the “phone” work? Try different widths/types of string. What happens if you use different sizes, shapes, materials of cups? What if someone grabs onto the string while you’re talking?

Terms/Concepts: sound travels, solid, gas