Sound Waves

Key Concepts

Frequency is a measure of how many times something moves back and forth. A swing, a pendulum, a leg of a walking person all have a frequency. All those things start at one place, move, and come back to the same position that they started. This moving and coming back is one vibration. The faster something vibrates, the more frequency that something has. Frequency is measured in Hertz (Hz).

Waves are the way energy moves from place to place. Sound moves from a mouth to an ear by waves. Light moves from a light bulb to a book page to your eyes by waves. Waves are everywhere. As you sit there reading this, you are surrounded by radio waves, television waves, cell phone waves, light waves, sound waves and more. (If you happen to be reading this in a boat or a bathtub, you're surrounded by water waves as well.) There are waves everywhere!

Our ears are very good antennas. They are very effective at picking up quiet, loud, high-pitched and low-pitched sounds. It is difficult for people to make microphones that are as sensitive as our ears. Our ears can pick up and tell the difference between sounds as low-pitched as 20 Hz and as high-pitched as 20,000 Hz. Some animals can hear things that are even higher or lower pitched than that. Our ears and brain are also very good at picking out the direction a sound is coming from.

Experiment & Video

Did you know you can make sound by slinging around objects? We're going to use everyday objects to build a harmonica that uses wind speed and resonance to make a buzzing sound.

Materials:

• tongue-depressor size popsicle stick
• approximately 3" x 1/4" rubber band
• 2 index cards
• 3 feet of string (or yarn)
• scissors
• tape or hot glue

Here's what you need to do:

What's Going On?

Do you remember where all waves come from? Vibrating particles. Waves come from vibrating particles and are made up of vibrating particles.

Here's rule one when it comes to waves.... the waves move, the particles don't. The wave moves from place to place. The wave carries the energy from place to place. The particles however, stay put. Here's a couple of examples to keep in mind.

If you've ever seen a crowd of people do the "wave" in the stands of a sporting event you may have noticed that the people only "vibrated" up and down. They did not move along the wave. The wave, however, moved through the stands.

Another example would be a duck floating on a wavy lake. The duck is moving up and down (vibrating) just like the water particles but he is not moving with the waves. The waves move but the particles don't. When I talk to you, the vibrating air molecules that made the sound in my mouth do not travel across the room into your ears. (Which is especially handy if I've just eaten an onion sandwich!) The energy from my mouth is moved, by waves, across the room.

Questions to Ask

1. Does the shape of the index card matter?

2. What happens if you change the number of rubber bands?

3. What if you use a different thickness rubber band?

4. What happens if you make the string longer or shorter?

5. can you make a double by stacking two together?

6. Can you get a second or third harmonic by swinging it around faster?

7. Why do you need the index card at all?