Sound is Vibration


Sound is vibration.  Waves of pressure move up and down a pipe, strings thrum, soundboards hum, drums beat.

When a vibration is fast enough, we hear it as a tone, rather than a series of beats. 

What is sound? Think of what happens when you’re at one end of a stretched “Slinky” toy and a friend is at the other. Rapidly move your end toward your friend, then return it to where it started. What you’ve just done is to create a “compression” or “condensation” wave that travels down the Slinky toward your friend. That’s exactly what happens with sounds in the air, except that what’s compressed are air molecules instead of the Slinky. The Figure below shows air molecules as green dots, which are closer together (more “dense”) In the areas of compression/condensation and less dense in between.

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Molecule: a group of at least two atoms held together by a chemical bond. Molecules make up most of the matter we know about, including the earth, the oceans, and the atmosphere.

Compression and Rarefaction: These terms refer to different parts of a sound wave in the air. Imagine a “Slinky” toy stretched between you and a friend. If you move your end rapidly toward your friend, you will compress the slinky at your end and create a compression wave that travels the length of the Slinky toward your friend. Another word for compression is condensation. If you observe the compression wave carefully, you’ll notice that it is followed by a region of expansion of the Slinky. This expansion is referred to as rarefaction.

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