This post is part of Mini Week, where I’m posting a new mini post but not actually mini as it turns out every weekday this week.
I’ve always been a little confused about sound. So for “Tuesday’s” “mini” post, I decided to do something about that.
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:1
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:2
Like an earthworm, sound moves by compressing and decompressing. This is called a longitudinal wave. A slinky can do both kinds of waves:13
Sound starts with a vibration of some kind creating a longitudinal wave through matter. Check this out:4
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:5
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.6
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.
The ear can discern many qualities of a sound it hears, but two of the most fundamental are pitch and loudness.
Pitch is all about wavelength—i.e. how far apart the pressure waves are:7
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).34
The loudness5 of a sound we hear is determined by the amplitude of the pressure waves. In the animation above, the high and low-pitched sounds depicted have the same loudness, because the pressure curves at the bottom of the animation are the same size vertically. Louder sounds have a larger oscillation between the low and high pressure sections of the wave—i.e. loud sounds have higher high-pressure and lower low-pressure parts than quiet sounds.
For sounds through the air on Earth’s surface, the average of the high-pressure and low-pressure parts of the wave is our normal atmospheric pressure—what we call 1 “atmosphere” of pressure. So a sound wave might have a high pressure component of 1.0001 atmospheres and a low pressure component of .9999 atmospheres, and a louder sound might be 1.01/.99 instead—but in both cases, the average of the two is 1 atmosphere.
We often measure loudness using a unit called the decibel (named after Alexander Graham Bell). If you want to be confused, read the Wikipedia page on decibels. It’s a super icky unit. And rather than bore us both by explaining it, let’s just talk about how we use decibels to measure sound.
The scale of loudness has a very tiny minimum. The faintest sounds are far softer than any human could hear—even softer than any of our finest scientific instruments could detect. But depending on where you are, sound has a hard maximum. The reason is that sound isn’t a thing in itself—it’s a pressure wave moving through a medium. And since, as we talked about, the average of the high and low pressure points of a sound wave has to be the normal pressure of the medium, loudness is limited by the fact that eventually, the low pressure point hits zero-pressure—a vacuum. Since the low pressure can’t go any lower, that point determines the max amplitude of a sound wave, and the loudest a sound can be, in any given place.
The convenient thing about decibels (dB) is that the absolute faintest sound detectable to the human ear is, by definition, 0 dB—we call that “the threshold of hearing.” Scientists do their best to study sounds far down into the negative decibel scale and there are man-made rooms on Earth that register as low as -9.4 dB—where it’s so quiet you can hear the blood pumping through your own brain—but we can only hear sounds in the dB positives. The loudest a sustained sound can possibly be on Earth’s surface is 194 dB—which is when the amplitude of the sound wave is so intense that the low pressure part is a perfect vacuum (the wave alternates between double the normal atmospheric pressure and no air at all—not something you want to be present for). Let’s take a look at the full scale, starting with the really quiet.
One thing to keep in mind is that with decibels, each increase of 10 dB doubles the loudness. So 20 dB is twice as loud as 10 dB, 30 dB is four times as loud as 10 dB, and 80 dB is 128 times louder than 10 dB.69
The scale stops at 194 because there’s no such thing as a louder sound on Earth’s surface. But we can go beyond here in two ways:
1) Shock Waves
When enough energy is released to pass the 194 dB mark, it’s too much to create a sustained pressure wave because we’ve bottomed out on low pressure—but things still happen. Very, very intense things.
At 194 dB, there’s a maxed out wave alternating between double the normal pressure and a total vacuum—but once we get to 195 dB, the energy stops moving through the air and starts pushing the air outward with an expanding vacuum. The more dBs above 194 there are, the farther reaching and higher-impact that vacuum bubble will be. It expands outward as a rapidly-growing half-sphere:10
On the edge of the bubble is a barrier of super-compressed gas, and when this barrier sweeps over the land, it usually flattens whatever’s in its path:11
As the hemisphere expands, it loses energy and eventually dissipates. But if you found yourself in the path of a shock wave before that happened, you’d have a bad time. First, the impact of the super-compressed barrier would be like hitting a brick wall (in the same way and for the same reason falling on water from a bridge is like falling on concrete). Second, compressed air is hot. Third, it wouldn’t just hit all parts of your body, it would go through your body, and if it were powerful enough it could turn your bones to powder and your organs to soup.
Here are some famous 194dB+ events:
Saturn V launch: The Saturn V was a beast, and the sound waves from its launches were so intense that they could light grass on fire a mile away. Even at three miles away, an observer would experience ear-splitting 135 dB sound.12 Rocket launches create such a powerful sound, that space agencies flood the launchpad with water as the rocket launches to absorb the sound so the force of the pressure wave doesn’t damage the rocket.
The Hiroshima and Nagasaki bombs: According to sources I read, these clocked in at well over 200 dB. The shock wave was so charged that it traveled 7 mi / 11 km in 30 seconds.
The 1883 Krakatoa volcano eruption:13 I’m overwhelmed by the amount of things I need to tell you about the Krakatoa. Let’s do bullets.
It was a big eruption.
2) Other Mediums
There can be louder sound than 194 dB—just not on the Earth’s surface. There can be louder sounds in the ocean, in the land, or on other planets. The gas giants in our Solar System, for example, have denser atmospheres than Earth’s, which allow for higher pressure wave amplitudes, and with incredibly fast winds and powerful storms, there’s plenty of opportunity there to make loud things.
What isn’t loud is almost everything else in space. You’ve probably heard the term, “Sound doesn’t travel in a vacuum,” but it makes sense now, right? Sound is pressure waves through matter. If there’s no matter, there’s no sound. There can be immense heat, and radiation, and force, but to a nearby observing human, it’s all dead silent.
If, hypothetically, there were air filling the universe, then suddenly things would get very loud. Forget the terrifying concept of the sound of a supernova—just the dumb sun sitting there hanging out would ring in at an astounding 290 dB. According to one solar physicist, we’d hear that on Earth as a 100 dB sound—the volume of a motorcycle—all the time, every day, everywhere. Be happy that sound doesn’t travel in a vacuum.
One last thought—
Researching for this post and learning about what sound is gave me a new perspective on the tree falling in the forest with nobody there to hear it question. I now think that no, it doesn’t make a sound. It makes an air pressure wave and that’s it. The concept of sound is by definition a biological being’s perception of the pressure wave—and if there are no ears around to perceive the pressure wave, there’s no sound. It’s a little like asking, “If humans go extinct, and somewhere in the post-apocalyptic rubble, there’s a photo of a beautiful woman lying there—is she still beautiful?” I kind of don’t think she is. Because the only thing that’s beautiful about her is that humans found her beautiful, and without humans, she’s no more beautiful than the female beetle a few feet away, rummaging around the rubble. Right?
Three things I want you to read:
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If you liked this, here are a few more Wait But Why explainers:
How Tesla Will Change the World
The AI Revolution: The Road to Superintelligence
Putting Time in Perspective
The awesome GIFS: Dan Russell and ISVR
CDC: Noise and Hearing Loss Prevention
US Department of Labor: Occupational Noise Exposure
Nautil.us: The Sound So Loud That It Circled the Earth Four Times
UNSW: What is a Decibel?
Decibelcar.com: Decibel Equivalent Table
Make it Louder: Ultimate Sound Pressure Level Decibel Table
NASA: Sound Suppression Test Unleashes a Flood
Idiom Zero: How Loud is the Sun?
Gibson.com: It Might Get Loud: The 10 Loudest Rock Bands of All Time
GC Audio: Decibel (Loudness) Comparison Chart
Mathpages.com: The Speed of Sound
Turn it to the Left: Noise Levels
Extreme Tech: Can a Loud Enough Sound Kill You?
Abelard.com: Loud Music and Hearing Damage
Soundproof Cow: Loudest Sound Ever Heard
Chalmers: Quantum microphone captures extremely weak sound
born.gov.au: The eruption of Krakatoa, August 27, 1883
- Here’s a real slinky doing a longitudinal wave.↩
- Fun fact: The loudest animal on Earth is a blue whale, whose calls can reach an outrageous 188 decibels, far louder than a jet engine.↩
- If you want to see this in action, watch these two guys sit in a car while playing unthinkably loud bass music—at 2:11 you can see the guy’s shirt flapping about 20 times/second as the low and high pressure air passes through it (only watch the 10 seconds where I cued it up—I watched the whole video and it’s time I can’t get back now).↩
- Also—dog whistles sound silent to us but work on dogs because dogs can hear pitches as high as 44,000 Hz. Cats can go even higher—up to 79,000 Hz. But neither animal can hear pitches as low as we can. In fact, neither animal can hear the lowest seven keys on the piano—who knew.↩
- At first, I was using “volume” here but it seemed weird to use volume when talking about the natural world—like, you wouldn’t say, “What’s the volume of that waterfall?” So is volume just for man made things like stereos? In any case, we’ll be going with loudness.↩
- Going up by 10 dB multiplies the sound intensity, or the power, of the wave by ten—not two. But a 10x increase in power only sounds about 2x as loud to our ears, so for our purposes, the 2x increase is more relevant.↩
Citations & Sources
- GIF: ISVR↩
- GIF: GIF Soup↩
- GIF: GIF Soup↩
- GIF: Dan Russell↩
- GIF: ISVR↩
- Thanks to Christopher Reiss for that analogy.↩
- GIF: ISVR↩
- Thanks to commenter rebel28 for suggesting the pitch link.↩
- Sources for the info on this chart is down below. For each of these, I tried to find several sources that all agreed on the same number. It’s surprising, in general, how much disagreement there is online regarding dB figures.↩
- I couldn’t find the original source.↩
- Most of my info on this came from this excellent article on Nautilus↩