Dr. Gadget's Science Machine
Sending your voice along a string
By Joe Laszlo
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| Dr. Gadget talks to 13-year-old Katrina Menor on a tin-can telephone. Sending sound along a string is this week's experiment.
Jeff Widener • The Honolulu Advertiser |
Aloha! Probably all of you have listened to music recorded on a CD. But have you ever wondered how sounds were first recorded?
You may have heard of the inventor Thomas Alva Edison. Among other things, he invented a way to record sound. The first "record" was a wax cylinder, not a disk. To record someone, the person stood in front of the large end of a huge, hollow cone, the "microphone." The person spoke or sang into it while the wax cylinder was moving. The sound energy moved a membrane suspended in the small end of the cone. The membrane had a needle attached to it. The vibrations of the person caused the membrane to vibrate, which in turn caused the needle to vibrate and cut a groove around the moving wax cylinder.
To hear the sound of the person, the process was reversed. The wax cylinder was rotated, and the needle in the groove vibrated as the cylinder moved, causing the membrane to vibrate. The cone became the "speaker," and acted like a mechanical amplifier. It caused more air to vibrate and make the sound louder. Pretty amazing when you think about it.
How can you move sound over a greater distance? Let's make a telephone. Here's what you'll need:
- two containers of the same size (try paper cups, to start)
- some string (about 20 feet; plain cotton string or even monofilament fishing line, or suji, will do)
- a toothpick
- Break the toothpick into two pieces.
- Make a small hole in the center of the bottom of each container, using a push pin.
- Starting from the outside of one cup, thread the end of the string into the hole. Reach into the cup and pull out the string.
- Tie that string's end to one piece of toothpick. Pull the string from the outside of the cup so that the toothpick lies against the inside of the bottom.
- Do the same thing to the other cup.
- Now you need someone to talk to. Get a friend and have him or her hold one cup while you hold the other. Go as far apart as your string will allow, and pull it a bit tight (but not so tight that it bends the bottom of the cup!). One of you talk and the other listen. Can you hear each other? Why?
The sound vibrations from your mouth are causing the bottom of the cup to vibrate also. These vibrations are traveling down the string to the other cup. They cause the bottom of the other cup to vibrate, and since the cup is like the cone we talked about earlier, it is able to cause the air in it to start vibrating, and go into the ear of the person listening.
If you can get two more friends to help, make another set of telephones. Then, at the center of each string, wrap one around the other. Then the four of you go off in different directions, pulling the strings with the same tension. One person talk and the other three listen. Can all of you hear what is being said?
You and your friends might try using containers other than paper cups. Plastic soda bottles and water bottles might work. Just cut off the top with scissors and you have a container. You might also use containers of different sizes on the same string. Or you might try different kinds of string, or even thin wire. Listen for a difference in the loudness of the sound.
Until the next time, a hui hou!
Dr. Gadget's Science Machine is written by Joe Laszlo, a retired science teacher.
