Background Essay: Sound and Solids: Visualizing Vibrations
Sound waves transport energy from one location to another in a chain reaction. An initiating event, such as the pluck of a guitar string or a knock on the door, disturbs nearby molecules and pushes them into each other, creating a region of higher density, called a compression, and leaving a region of lower density, called a rarefaction, in its wake. In wavelike fashion, the alternating compressions and rarefactions move outward in all directions through the medium (metal, wood, air, water, or whatever is transmitting the sound) as sound waves. Waves continue to form until the source of the disturbance stops making the vibrations that generate the waves.
A tuning fork is a steel instrument whose two prongs vibrate rapidly when struck, producing a clear tone of a fixed pitch. For this reason, it's used to help musicians tune their instruments. A tuning fork is also useful for providing visual proof that sound is produced by vibrations that travel in waves through a medium. Tap the fork against a solid object, like the heel of your shoe or a desktop. If you've struck it hard enough, you should hear sound coming from the prongs. Tap it again, and this time touch the prongs: You can feel them vibrating. Tap it one final time, but this time dip the ends of the prongs in a bowl of water. You couldn't see the air molecules bouncing off the vibrating fork because air molecules are too small to see. But you can see ripples in the water, which are caused by the same vibrations that produce sound.