What is sound? Here is Molly Kaplan, gravity theorist and musician extraordinaire, introducing sound waves:

Using a Slinky, Molly demonstrates that waves can move in two different ways; up and down or back and forth. Waves that move up and down, such as waves in the ocean, are known as transverse waves. By contrast, waves that move back and forth along the Slinky are longitudinal waves. Sound waves are an example of longitudinal waves that come from a source and move through air. The reason we hear something is because the air around our ear drums vibrates! Just as Molly can make bigger or smaller waves with the Slinky, the volume of sound depends on the size of the wave. If Molly pushes hard enough, the Slinky wave will reach the end and bounce back; similarly, echo arises when a sound wave bounces off a wall.

In the second video, when Molly hits a tuning fork, it vibrates and creates sound waves in the air. This particular tuning fork is tuned to a frequency of 880 Hertz, meaning a wave hits your eardrum 880 times each second!

Molly then brings out a metal tube called a resonant cavity to describe resonance! When she brings the ringing tuning fork close to the resonant cavity, some of the waves enter the cavity. Because the tube is cleverly chosen to vibrate at the same frequency as the tuning fork, the sound waves bounce around in the cavity even after she brings the tuning fork away. Note that Molly doesn’t even have to touch the tuning fork to the cavity, because the waves travel through air.

Finally, Molly describes how the pitch of sound depends on the frequency of vibration. What we call pitch is actually the frequency of the air vibrations. The more times a sound wave hits your eardrum per second, the higher pitched it sounds. Molly shows that the tuning fork and sound machine both have the same pitch when tuned to 880 Hertz. The machine makes a higher pitch sound when the frequency is turned up, and a lower pitch sound when the frequency is lowered.