Key concepts
Sound
Waves
Hearing

From National Science Education Standards: Transfer of energy

Introduction
Have you ever tried to have a conversation with someone so far away that you couldn't really hear each other? Without yelling, it's hard to have a conversation over long distances. So these days it's nice to be able to use telephones to talk with someone—whether he or she is 100 yards or 100 miles away.

Back before there were cell phones or even cordless phones, all telephones were hooked up to wires that helped to carry the sound of a person's voice (via an electric signal). And you can use the same concept to build your own telephone using just cups and some string. What message are you going to share over the string?

Background
When we talk, our vocal cords make molecules in the air vibrate. (You can feel the vibrations by holding your hand against your throat while you talk.) Those vibrating air molecules make other air molecules around them vibrate, and so on, which is how sound travels through the air. (Different pitches of sound move in waves that have different spacing between them—or "frequency.") Other sources of sound, such as guitar, violin or piano strings are good examples of how vibrations can generate sound.

Inside our ears are tiny sensitive hairs. They pick up the vibrations and transmit that information to our brains, which interpret it as sound. The brain interprets sounds as having different pitches, or tones, based on the frequency, or spacing, of the waves.

But the particles in air are spread out from one another more than particles in a liquid or solid. So sound vibrations tend to peter out before they travel very far. Having a soft connective material, such as cotton string—which has a higher density, or number of molecules in a given amount of space, than air—can help the sound waves move over a greater distance.

Materials
•   Two large paper cups (disposable plastic cups will also work)
•   Two paperclips or toothpicks
•   Length of cotton string or fishing line approximately 10 to 30 feet long
•   Quiet area

Preparation
•   Punch a small hole in center of the bottom of each cup (for plastic cups, you might need a nail or other sharp tool, so use caution when completing this step).
•   Thread one end of string through the bottom of each cup.
•   Place a paperclip or toothpick in the bottom of each cup and tie the loose end of the string around it (the clip or pick is just here to keep the string from slipping through the bottom of the cup).

Procedure
•   Give one cup to your conversation partner and hold one yourself.
•   Walk slowly apart until the string connecting the cups is straight and tight.
•   Put your cup over your ear and have your partner talk into his or her cup (keep the conversation relatively quiet if you are standing close to one another, but be sure to talk louder than a whisper).
•   Can you hear your partner talking?
•   Now you try talking into your cup and have your partner listen into his or her cup. Can he or she hear you?
•   Try letting the string go slack. Is the cup-and-string telephone still effective?
•   Now, keeping your voice at the same level and remaining the same distance apart, try talking to each other without using the cups. Can you hear as well?
•   Extra: If you have plenty of space, see how far apart you can get the cup-and-string telephone to work.
•   Extra: If you have a third person around, ask them to hold on to the center of the string with their hand. Will the sound still carry through? Why or why not?
•   Extra: If you have other materials (such as yarn, fishing line, nylon string, etc.) on hand, try them out. How do different materials change the quality of sound or how far the sound will travel?

Read on for observations, results and more resources.

4 Comments

1. 1. larrywac 10:50 AM 5/7/11

   I 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.
   
   larrywac

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2. 2. Daniel35 03:21 AM 5/8/11

   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.

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3. 3. jimmybo 11:01 PM 5/9/13

   As 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.

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4. 4. jimmybo 11:09 PM 5/9/13

   oops 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.

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