Written by Travis M. Moore
Last edited Sep-2019
Sound originates from a vibrating object, which pushes and pulls the air molecules around it back and forth as it vibrates. Note in this scenario there is (1) an object, (2) some source of energy that set the object into motion, and (3) air molecules. If one of these factors is missing, sound cannot exist.
The classic example of a simple sound is a vibrating tuning fork. The tines (arms) of the fork are struck against a solid object, transferring energy to the tines, and setting them into vibration:
Note that there are only two of the three required factors in this example so far. The tines can vibrate, but unless there are molecules around it for the tines to displace, there is no sound. This is a fundamental principle: sound must always travel through something. The "something" is referred to as a medium. Thus far we have been using air molecules as the medium, but there are others. For example, if the tines were to vibrate under water, now the water molecules make up the medium through which sound would travel.
The tagline from the 1979 film "Alien" was: "In space, no one can hear you scream." Creepy. But also accurate. Because outer space is a vacuum, there is no medium through which sound can travel. It's the same scenario as the tuning fork vibrating in nothingness in Figure 1. In space, your vocal folds may vibrate to produce a scream, but no one would be able to hear it because there is no medium to carry the sound to your ears.
Let's take a look at what happens to the surrounding molecules of the air when they are displaced by the vibrating tuning fork tines:
As the tines of the tuning fork move outward (away from each other), the air molecules are pushed away from the fork. Conversely, when the tines move inward (toward each other) the air molecules are pulled along with them. When the molecules are pushed together, it creates an increase in air pressure which is referred to as condensation. During condensation, the air molecules are packed closely together. When the air molecules are pulled apart, there are fewer molecules occupying the same space, which creates a relative decrease in air pressure. This decrease is referred to as rarefaction. So in Figure 2, the darker bands show condensation (dots closely spaced) and the lighter bands show rarefaction (fewer dots over roughly the same amount of space).
It is also worth noting that the tines are moving in a left-right fashion, and the air molecules are also moving in a left-right fashion. When molecules are displaced in the same direction as (parallel to) the vibrations of the sound source, the wave is called a longitudinal wave. Throughout this module, you can assume that all waves are longitudinal.
So far we've only considered the air molecules immediately surrounding the tuning fork tines. But recall Newton's first law of thermodynamics: energy cannot be created or destroyed. The energy passed to the molecules from the movement of the tines isn't simply destroyed; it has to go somewhere! The energy from the displaced molecules near the tuning fork is transferred to neighboring air molecules, which bump into the air molecules near them, and on and on until there is no more energy. It's a lot like a row of dominoes falling over.
The dominoes analogy isn't perfect, but it captures another fundamental point of sound traveling through a medium. It's fun to watch the wave of energy move down a row of dominoes as they tip over. Serious domino aficionados have connected over 10,000 dominos in a row, meaning the wave of energy transfer starts at one point, but ends at a point quite a distance away. However, despite the long distance traveled by the wave, the individual dominoes themselves don't travel very far. The same principle is at work when a sound wave travels through a medium. (Sound moving through a medium is referred to as sound propagation.) Take a look at Figure 3 to get an idea of how the air molecules simply move back and forth, yet still transfer a wave of energy across a distance.
Here is an interesting example of "molecules" (people) giving the illusion of a traveling wave, depsite each individual person staying in the same location: