Key concepts
Physics
Sound waves
Vibration
Hearing

Introduction
Have you ever tried making “walkie-talkies” using a long piece of string and two tin cans? If you have, you know that they work surprisingly well—at longer distances you can hear people better through the cans and string than you can through the air!

In this activity we're going to use the same concepts to build a personal bell, one that makes sounds that only you can hear!

Background
If you've ever been near a speaker with a loud bass (or heard a car drive by with the radio turned up), you may have experienced the “buzzing” feeling in your body caused by the loud noise. This isn't your imagination; the sounds we hear are actually vibrations traveling through the air—or through other materials, as we'll observe in this activity.

Most of the sounds we hear come to us through the air. When your friend calls your name, your friend's vocal chords cause vibrations in the air, which travel through it as a sound wave and arrive at your ears. Sound waves, however, can travel through other materials, too, and in fact many materials are much better than air at transmitting sound! You can experience this for yourself by gently tapping a metal fork or spoon against a countertop, and listening to the sound. Next, put your ear to the countertop and tap the counter again with the fork or spoon. The sound should be much louder, because the counter is better than air at transmitting the sound vibrations caused by the tapping!

The difference in how well a material can transmit sound is determined by the material's density, or how closely packed the molecules (that make up the material) are to one another. Imagine a row of dominoes. If the dominoes are far apart, one or two of them can fall over but the rest will remain standing. If the dominoes are close together, one domino falling over will bump into the next one, which will bump into the next one and the dominoes will fall down in a traveling wave. This is similar to how a sound wave travels; if the molecules are close to one another, they will bump into together more often and the vibration will move through them more efficiently. Solid objects such as metal desks and even string have molecules that are packed together much more closely than are the molecules in air.

Materials
String
One unpainted metal hanger
Scissors
Metal fork or spoon
An adult helper

Preparation

• With the help of an adult, cut two lengths of string, each about two feet long.
• Tie one end of each string to a different corner of the base of the metal hanger. When you hold the hanger up by the strings, the hook part should be pointing toward the ground.

Procedure

• Hold the hanger by the hook in one hand and use your other hand to tap the metal fork or spoon against the hanger. Notice the sound that it makes. How long does the sound last? Would you describe it as “sharp” or “dull”? What other words would you use to describe the sound?
• Gently place one corner of the hanger (where you tied one piece of string) to the small flap of skin just in front of your ear, closing off the ear canal. (You don't need to press hard!)
• Using the hand that isn't holding the hanger (or asking an adult to help you), gently tap the metal fork or spoon against the hanger again. Notice the sound this makes. Is the sound different when the hanger is pressed against your ear compared with when you were holding it in the first step? How long does it last? What words would you use to describe this sound?
• With the help of an adult, take one of the pieces of string tied to the hanger and wrap it around your index finger a few times. Wrap the other string around the index finger of your other hand.
• While you hold the hanger away from your body by the two strings, have your adult helper gently tap the hanger with a metal spoon or fork. Notice the sound this makes. Is the sound different than in the earlier steps? How long does it last? How would you describe it?
• Press your index fingers (with the hanger assembly attached) carefully on the small flaps of skin just in front of your ears, gently closing off the ear canals without putting your fingers into your ears. Allow the hanger assembly to swing freely from your fingers in front of your body, hook pointed toward the ground. Don't let the hanger or the string touch anything (except where the string is tied to your fingers).
• Have your adult helper gently tap the metal fork or spoon against the hanger. (Just tap once.) Notice the sound that this makes. Is the sound different when the hanger is floating in the air compared with when you were holding it in your hands during the first two steps? How long does it last? What words would you use to describe the sound?
• Gently swing the hanger so that it bangs lightly against something hard, such as the edge of a counter or table. Notice the sound that this makes. Is the sound different when the hanger is pressed against your ear compared with when you were holding it in the first step? How long does it last? What words would you use to describe this sound? Do you notice anything about the strings after you bang the hanger against something? Are they moving? What type of movement?
• Keeping your index fingers pressed on your ears, use your other fingers to grab the strings in your hands. Repeat the previous step, swinging the hanger into something hard. Notice the sound that this makes. Is the sound different when you're holding the string in your hand compared with when it is hanging from your fingers?
• Extra: With your index fingers still pressed against your ears, try banging the hanger against something hard, then grab the strings right in the middle of the sound. How does holding the strings change the sound you hear?
• Extra: Repeat the activity using another metal household item, such as a cooling rack, metal salad tongs or a butter knife. How does the sound change with the different items?

Observations and results
When the hanger was hanging freely from your fingers, did you notice that the sound produced by tapping against it had more resonance (a deep, full, vibrating quality of sound)? It should have sounded more like a bell or gong when it was hanging from the string compared with when you held it or pressed it against your ears and tapped it.

Why did you hear different things? As we discussed in the background, sound vibrations can travel more easily through some materials than others. When the hanger assembly was hanging freely in front of your body, tapping the hanger caused it to vibrate. These vibrations traveled up the string and into your fingers, then through them into your head. When you held the hanger away from you and tapped it, the vibrations traveled through air to get to your ears. From this activity you can tell that the string and your fingers are much better sound transmitters than the air around you!

When you were holding the hanger against your ear with your hand, the hanger couldn't vibrate as much (because you were holding it in your hand). Therefore, the sound was muted because fewer vibrations were produced. Holding the strings with your hands when you swung the hanger would have a similar affect. The strings couldn't vibrate as much, and therefore the sound waves were not transmitted as efficiently to your ears.

More to explore
Talk Through a String Telephone, from Scientific American
Outer Space, the Silent Frontier: An Experiment on Sound Waves, from Science Buddies
What Material Makes the Most Resonant Soundboard?, from Science Buddies
Amazing Resonance Experiment!, from Illusions Science
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies