Tutorial: What is Sound?

Sound is created by a vibrating object.  Sound travels as a wave through a medium, for example, a liquid (such as water), a solid (such as the seafloor), or a gas (such as air).  Therefore, sound does not exist in the vacuum of space.

A sound wave is an example of a compressional or longitudinal wave.  Below is an animation in which a hand applies pressure to the end of a simulated slinky to create a longitudinal wave. The particles in a longitudinal wave move parallel to the direction in which the wave is traveling.

There are places where the springs of the slinky compress and places where they expand. The compressions represent regions of high pressure; expansions represent regions of low pressure.

Although sound travels as a wave, the individual particles of the medium do not travel with the wave, but only vibrate back and forth centered on a spot called its equilibrium position, as shown below.

Sound is a longitudinal wave. Red dots and arrows illustrate individual particle motion. The black arrow illustrates the motion of a wave as the compression of particles moves through the medium. Image used with permission –  copyright: Dan Russell, Grad. Prog. Acoustics, Penn State, https://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html.