We think of sound as something we hear—something that makes noise. But in pure physics terms, sound is just a vibration going through matter.
The way a vibration “goes through” matter is in the form of a sound wave. When you think of sound waves, you probably think of something like this:
But that’s not how sound waves work. A wave like that is called a transverse wave, where each individual particle moves up and down to create a snake situation.
A sound wave is more like an earthworm situation:
Like an earthworm, sound moves by compressing and decompressing. This is called a longitudinal wave. A slinky can do both kinds of waves:
Sound starts with a vibration of some kind creating a longitudinal wave through matter. Check this out:
That’s what sound looks like—except picture an expanding ripple of spheres doing that. In this animation, the sound wave is being generated by that vibrating grey bar on the left. The bar might be your vocal chords, a guitar string, or a waterfall continually pounding down into the river below. By looking at the red dots, you can see that even though the wave moves in one direction, each individual particle only moves back and forth, mimicking the vibration of the gray bar.
So instead of a curvy snake wave, sound is a pressure wave, which causes each piece of the air to be at either higher-than-normal pressure or lower-than-normal pressure. So when you see a snake-like illustration of a sound wave, it’s referring to the measure of pressure, not the literal path of movement of the particles:
Sound waves can go through air, which is how we normally experience it. But it can also go through liquid2 or solid matter—much of the jolting that happens during an earthquake is the result of a huge sound wave whizzing through the earth (in that case, the movement of the fault is serving as the gray and red bars in the animations above).
How about the speed of sound? Well it depends on how quickly the pressure wave can move in a given medium. A medium that’s more fluid, like air, is highly compressible, so it takes longer for the wave to move, while water is far less compressible, so there’s less “give” to slow the wave down. It’s like two people holding an outstretched slinky between them—if one pushes their end toward the other person, the wave will take a little time to travel down the slinky before the other person feels it. But if the two people are holding a broomstick, when one pushes, the other feels it immediately, because the broomstick is much less compressible.
So it makes sense that the speed of sound in air (768 mph / 1,234 kmph under normal conditions) is about four times slower than the speed of sound in water, which itself is about four times slower than the speed of sound through a solid like iron.
Back to us and hearing. Ears are an evolutionary innovation that allows us to register sound waves in the air around us and process them as information—without ears, most sound waves would be imperceptible to a human with only the loudest sounds registering as a felt vibration on our skin. Ears give us a magical ability to sense even slight sound waves in a way so nuanced, it can usually tell us exactly where the sound is coming from and what the meaning of it is. And it enables us to talk. The most important kind of human communication happens when our brains send information to other brains through complex patterns of air pressure waves. Have you ever stopped and thought about how incredible that is?
I was about to move on, but sorry, I can’t get over this. The next time you’re talking to someone, I want you to stop and think about what’s happening. Your brain has a thought. It translates that thought into a pattern of pressure waves. Then your lungs send air out of your body, but as you do that, you vibrate your vocal chords in just the right way and you move your mouth and tongue into just the right shapes that by the time the air leaves you, it’s embedded with a pattern of high and low pressure areas. The code in that air then spreads out to all the air in the vicinity, a little bit of which ends up in your friend’s ear, where it passes by their eardrum. When it does, it vibrates their eardrum in such a way as to pass on not only the code, but exactly where in the room it came from and the particular tone of voice it came with. The eardrum’s vibrations are transmitted through three tiny bones and into a little sac of fluid, which then transmits the information into electrical impulses and sends them up the auditory nerve and into the brain, where the information is decoded. And all of that happens in an eighth of a second, without any effort from either of you. Talking is a miracle.
Anyway—
The ear can discern many qualities of a sound it hears, but two of the most fundamental are pitch and loudness.
Pitch
Pitch is all about wavelength—i.e. how far apart the pressure waves are:
The shorter the wavelength, the higher the pitch. Humans can hear frequencies as low as 20 Hz (which is a 56 ft /17 m long wave) and as high as 20,000 Hz (.7 in / 1.7 cm). As you age, you lose your ability to hear the highest pitches, so most of you probably hear nothing when you listen to the frequencies approaching 20,000 Hz (your dog will disagree). But you’ll have an easier time hearing the lowest part of the range.8 The reason you can feel low sounds, like low bass notes in music, is that the wavelength is so long that it actually takes 1/20th of a second for a full wave to pass your body (hence 20 Hz).