The Science of Sound

Sound is a form of energy.

Try this test. Put your index and middle finger on your neck. Say the word "Aah" as loud as you can. Then try it as soft as you can. You not only hear a sound, but you can feel a movement inside your throat. When you say, "Aah", your vocal cords vibrate. That means they move quickly back and forth. As your vocal cords vibrate they produce sound.

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Sound waves are compressional waves. Use a toy slinky. Stretch it out between two students. One student should grab several coils on the slinky. Then let go. You will see the group of coils go across to the other student. Then come back to the starting student. Make sure you notice how the coils stay together. This is what sound waves do.

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tuning fork #1 tuning fork #2 tuning fork #3

A sound wave moves through matter, such as air, just like a wave moves through a slinky. We have drawn three pictures of a tuning fork to help you visualize how air molecules might look around a tuning fork. Look at tuning fork #1. When the tuning fork is at rest, the fork is surrounded by molecules in the air. As a tuning fork's prongs move apart because of a vibration, the molecules ahead of it are crowded together. (See tuning fork #2). They look like they are being pushed together. They bump each other. As a tuning fork's prongs come back together, (see tuning fork #3) it leaves a region that has fewer molecules than usual. The region of a sound wave in which the molecules are crowded together is a compression. The region of a sound wave in which particles are spread apart is a rarefaction. As a tuning fork vibrates, it causes molecules in the air to move. The molecules bump into other molecules nearby, causing them to move. This process continues from molecule to molecule. The result is a series of compressions and rarefactions that make up sound waves. Unlike light waves, sound waves do not travel through a vacuum. They need matter to travel. That is why sound can travel through a wall.

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Sound