To try this technique, take two large paper cups and punch a tiny hole in the center of the bottom of each with a sewing needle or similar. Take a piece perhaps feet [30 meters] of non-stretchable thread or kite string and thread each end through each hole.
Either knot or tape the string so it cannot go back through the hole when the string is stretched. Now with two people, have each one take one of the cups and spread apart until the string is tight. If one of you talks into one of the cups while the other listens, the second person should be able to hear what the first person says. Here is why it works. Imagine the bottom of the cup moving back and forth very quickly 1, times per second or more with the sound waves of the speaker's voice.
The vibrations travel through the string by pulling the string back and forth. Therefore, the bottom of the second cup should start to vibrate back and forth just like the bottom of the first cup is vibrating, producing sound waves. The second person can hear the sound waves and can therefore hear what the first person says. This is not much different from how a telephone works , except that electric current replaces the string in a telephone. In an old-style telephone, the person speaking vibrates a metal diaphragm.
The diaphragm's vibrations rapidly compress and uncompressed carbon granules, changing their resistance. A current passing through the granules is strengthened or lessened by the changing resistance. At the other end, the rapidly changing current runs through a speaker and causes its diaphragm to vibrate back and forth, so the second person hears the first person speaking.
See How Telephones Work for details. In a radio, the changing current from the microphone is used to modulate a radio transmitter. Figure 1. Step 1 of Part 1: Make a small hole in the bottom of a paper cup.
Figure 2. Step 2 of Part 1: Pull string through the cup and tie a knot. Figure 3. Step 3 of Part 1: Hold taut the string coming out of the cup. Figure 4. Step 6 of Part 1: Hold the string taut, as it comes directly out of the cup bottom. Alexander Graham Bell: Inventor of the telephone. Prediction : Before Part 2 of the activity, have students predict whether they think the string telephone will work better with a loose or taut string.
Record predictions on the board. Review their answers to gauge their mastery of the subject. Iterate the Designs : Have students think about modifying their final string telephone designs even more. What would they do to further improve their designs?
Would they change the materials for the cups or string? What might they use? Have students think about other features on telephones. What special features would they want to include on their string telephones?
Give students time to draw or describe their new design ideas. The string pulls out of the hole in the paper cups if students pull too hard. If this happens attach the string to a small paper clip to anchor it in the cup.
This does not affect the sound performance. Have students compare the loudness of sound with the distance of the cups. Give them a longer length of string with which to test, and have them decrease the length of string by half each time to see if distance affects the sound.
They should notice a difference, since sound waves lose strength with distance as learned in the associated lesson. Explain to students that engineers have helped sound travel great distances by designing devices that convert the sound waves into electricity and back. Have students try other modern telephone features on their string telephones. For example, does it work if they introduce "three-way calling" with three cups? However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.
Why Teach Engineering in K? Find more at TeachEngineering. Quick Look. Partial design process. The paper cup phone is a popular science experiment due to how well it illustrates the mechanics of sound transmission.
Sound requires a source to vibrate at an audible frequency generally between 20 Hz and 20 kHz. These vibrations travel through any solid, liquid or gaseous medium as longitudinal waves.
Though sound waves can travel through air, solid and liquid mediums transmit sound more effectively, due to their greater density. As the bottom of the cup vibrates, it transmits the vibrations into the taut string. The sound travels along the string as a longitudinal wave and ultimately vibrates the bottom of the receiving cup.
Because the sound travels through solid mediums — the cup and the string — it travels more effectively than through air, allowing the users to communicate across large distances with volumes that would be inaudible if spoken through air.
You must pull the string taut in order for the phone to work. Longitudinal waves travel through a medium by a process of compression and rarefaction, which in this case alters the tension of the string.
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