When a guitarist plucks a string to start the vibration, it not only vibrates the string, but it also vibrates the entire box of the guitar. This is called a forced vibration, which means that the motion of the original source vibration is also causing another object to vibrate (the box of the guitar). Since the box is larger than the string, it amplifies the vibration and makes it louder.

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You need:

• Table
• Spoon (or whatever is handy)
• Partner

1. Sit at a table.

2. Have your partner sit at the other end of the table.

3. Have your partner very lightly scratch the table with the spoon.

4. Listen to see if you can hear it.

5. While your partner is scratching, put your ear on the table. Do you hear a difference in the sound?

6. Switch roles so your partner gets a chance to try.

Did you notice how you could hear the soft spoon-scratching sound (I love a good alliteration) quite clearly when your head was on the table? The sound waves moved quickly through the table so they lost little of the loudness and quality of the original sound. When sound travels through the air, the sound energy gets dispersed (spread out) much more than through the table, so the sound does not travel as far nor as clearly. This next one is an oldie but a goodie!

A Couple Cups of Conversation

1. Using the scissors or a nail poke a hole in the middle of the bottom of both cups. Get an adult to help you with this. Since this isn’t biology, no bleeding allowed!

2. Thread an end of the string through the hole in the bottom of the cup and tie a big knot in it to keep it from sliding through the hole.

3. Do the same thing with the other cup so that when you are done you have a cup attached to both ends of the string.

4. Take one of the cups for yourself and hand the other cup to your partner. Walk apart from one another until the string is fairly taut.

5. Have your partner hold the cup up to his or her ear while you whisper into your cup.

6. Can your partner hear you? If not, see if you can stretch the string a little more.

7. Switch roles and try again.

The string being a solid and having tightly packed molecules allows the sound wave to move quickly and clearly through it. You can talk very quietly in one cup and yet your partner can still hear you fairly well.

Tips & Tricks

You can try different types of cups (foam, plastic, metal (like tin foil), paper…) and also change the sizes of the cups – is bigger or smaller better?  You can also change the connection between the cups – have you tried yarn, wool, string, nylon fishing line, rope, clothesline, or a braided combination?  You can also stick a slinky in place of the string of ‘space phones’.

Click here to go to next lesson on Vibrations and Speakers.

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