Fast and Easy Science Fair Projects: Cellular Can

Transmitting sound by vibrating materials

Purpose: To improve the sound of the traditional homemade "tin-can" toy phone.

Overview: Sound is formed by an object moving back and forth, or vibrating. These vibrations move molecules in the air by first compressing them and then causing them to spread apart. In order for us to hear something vibrating, the object must be quivering with enough force for our ears to detect it (loudness). It must also be vibrating between about 16 and 20 thousand times per second, which is the frequency response of the human ear.

     You can often feel the vibrations that are making a sound. Lightly touch a string on a guitar, or a harp, after it has been plucked. Lay a piece of paper on top of the speaker in a car and turn up the volume on the car radio.

     A popular toy kids often make is a "telephone," put together using two metal cans and a string pulled tight between them. A small hole is punched in the bottom of each empty can through which a piece of string tied and knotted. When the spring is pulled tight, speaking into one causes the can bottom to vibrate. These vibrations then travel along the taut string and vibrate the bottom of the other can, converting the vibrations back into sound. The vibrations of your voice are therefore "transmitted" to the person on the other end of the toy-telephone system, and he or she can hear you.

     Can you improve on this toy, making the sound either clearer or louder?

Hypothesis: Using larger cans instead if the usual smaller "soup" cans will improve the sound transmission of a homemade string-can telephone system. (We are defining "improve" to mean either louder or clearer sound.)

You need:

• an adult
• 2 small metal (soup) cans
• 2 large metal (juice) cans
• hammer
• nail
• a friend

Procedure: Make a set of toy "can telephones" using a piece of string and two small empty cans with one lid removed; use cans that are the size soup usually comes in. Have an adult check the can rims to be sure there are no sharp edges that might hurt you. Also, ask the adult to punch a small home in the middle of the bottom of each can, using a hammer and nail. Push one end of the string into the can through each hole far enough to be able to tie a knot in string inside the can. Knot it several times, making a knot big enough so it won't pull out through the hole. Pull the string tight and hold a "secret" conversation with your friend on the other end.

     Now make a set of can telephones using two larger cans, the kind that fruit juice might come in. Is there a difference in sound quality or volume between the telephones using the larger cans?

Results and Conclusion: Write down the results of your experiment. Come to a conclusion as to whether or not your hypothesis was correct.

Something more: 1. Can you think of any other kinds of materials of use instead of string that might work better? Replace the string with monofilament line (fishing line). How about thick monofilament line compared to thin line (different "pound test" strengths).

2. Try using cans that have short sides, such as ones that pineapple or tuna fish come packed in to improve sound.