Your turn
Each lab is one step from the video. Tap, slide and race, and see what happens.
Each lab isolates one idea from the video. Speeds are real; some motion is slowed or sped up and labelled.
Tap the buttons and slide the sliders.
Move away from the storm and make a flash. How long until the boom?
Sound covers a mile in about 5 s. Change the distance and time the delay.
Walk away from the storm. Does the boom come later?
The rings are sped up 3× so you don't have to wait.
Hit the drum softly or hard. Does the red molecule travel, or just jiggle?
Compare a soft tap with a hard hit: the amplitude changes, the speed doesn't.
Bang the drum. Watch the red dot wiggle!
Send the same beat 1 km through air, water and steel. Which arrives first?
Real relative speeds from OpenStax: 343, 1,480 and 5,960 m/s.
Race the sound! Which one wins?
The race is shown 3× slower than real time.
Pump the air out of the jar. What happens to the ringing?
Remove the medium and watch the sound fade while the bell keeps striking.
Take the air out of the jar. Where did the ringing go?
Set the pitch and loudness. Can you find a sound nobody can hear?
Sweep the frequency past 20 Hz and 20 kHz, and check the wavelength readout.
The drawn wave isn't to scale; the readout is.
Tap a note. Which part of the cochlea picks it up?
High frequencies peak near the base, low ones toward the apex.
Tap a high note, then a low note. Where do they go?
A simple drawing, not to scale.
Every sound starts with a wiggle. When you bang a drum, its skin wiggles back and forth, super fast.
The wiggle squeezes the tiny bits of air next to the drum. Each bit bumps the next one, like falling dominoes. The air doesn't fly to you. Each bit just wiggles in its spot, but the squeeze keeps going, all the way to your ear.
Sound can travel through air, water, and hard things like metal. It is faster in water than in air, and faster still in steel. Whales call to each other across the sea.
Space has no air, so there is nothing to pass the squeeze along. Astronauts talk by radio. Radio is a kind of light, and light doesn't need air.
The squeeze shakes your eardrum, like a tiny drum. Tiny bones pass the shake on to a little snail shape full of water, and it sends a message to your brain.
Light is much faster than sound. That's why you see lightning first and hear thunder later. Read more about what causes lightning.
Look after your ears: very loud sounds can hurt your ears. Cover them, or move away.
Sound travels as a wave of squeezes through a material such as air, water or steel. A vibrating object pushes the molecules next to it closer together (a compression), then pulls back and lets them spread apart (a rarefaction).112 Each molecule bumps into the next one, so the pattern of squeezes moves outward, like a line of falling dominoes.7
The surprising part is that the air itself doesn't fly from the drum to your ear. Each molecule only jiggles back and forth around the same spot. It's the pattern that travels.313 Think of a Slinky: push one end and a bunch of squeezed coils runs along it, but the coils stay where they are.12
Every sound starts with a vibration: something moving back and forth, fast. A drum skin, a guitar string, a speaker cone and your own vocal cords all do this.13 As the object moves out, it squeezes the air in front of it into a spot of slightly higher pressure. As it moves back, it leaves a spot of slightly lower pressure. These highs and lows repeat as often as the object shakes, and travel outward at the speed of sound in that material.1
Thunder works the same way, just bigger. A lightning bolt heats the air around it to as much as 50,000 °F in a split second. The air expands violently, then cools and contracts, and that sudden shove of air is the sound wave we hear as thunder.11 (There's more in our explainer on what causes lightning.)
In air and water, sound waves are longitudinal: the molecules move back and forth along the direction the wave travels.63 A wave on a guitar string or a wave of light is transverse: it wiggles sideways, at right angles to its direction of travel. Inside solids, sound can travel both ways.6
In air at room temperature (20 °C), sound travels about 343 metres per second, roughly 1,125 feet per second. In colder air it's a little slower: 331 m/s at 0 °C.2 Its speed depends on the material, not on how loud or how high the sound is. High notes and low notes arrive at the same time, which is lucky for music.2
| Material | Speed of sound | Time to cross 1 km |
|---|---|---|
| Rubber | 54 m/s | about 18.5 s |
| Air (20 °C) | 343 m/s | about 2.9 s |
| Fresh water | 1,480 m/s | about 0.68 s |
| Sea water | 1,540 m/s | about 0.65 s |
| Steel | 5,960 m/s | about 0.17 s |
Speeds from OpenStax College Physics, Table 17.1.2 Crossing times are simply 1,000 m divided by the speed.
Yes to both, for water and for stiff solids like steel. Sound moves more than four times faster in water than in air, and about seventeen times faster in steel.2 The reason is stiffness. The harder a material is to squeeze, the faster each molecule shoves its neighbour, so the push races along. For materials of similar stiffness, the lighter (less dense) one carries sound faster.2 Soft, squishy rubber is so easy to squeeze that sound crawls through it at just 54 m/s.
Low sounds also carry a long way. Whales and elephants use very low calls to keep in touch across huge distances.12
You see a lightning flash almost instantly, because light is incredibly fast. The thunder takes about 5 seconds to travel a mile (about 3 seconds per kilometre). So count the seconds between flash and boom and divide by 5 to get the distance in miles.11 Fifteen seconds means the storm is about three miles away. If you can hear thunder at all, you're close enough to be struck, so head indoors.11
No. Sound is a mechanical wave: it can only travel by molecules bumping into molecules. Space is almost a perfect vacuum, with nothing to bump, so sound can't cross it.712 The famous classroom test shows this. Put a ringing bell inside a glass jar and pump the air out: the bell keeps shaking, but the ringing fades to silence.14
That's why astronauts on a spacewalk talk by radio. Radio waves are a form of light, and light doesn't need any material at all.7 Explosions in films may go "boom", but a real one in space would be silent.
A mechanical wave, such as sound or a water wave, is a disturbance moving through matter: a solid, liquid, gas or plasma. It needs that material (the medium) to travel. An electromagnetic wave, such as light, radio, microwaves or X-rays, is made of changing electric and magnetic fields that keep each other going, so it needs no medium and crosses empty space easily.7
| Sound (mechanical) | Light (electromagnetic) | |
|---|---|---|
| Needs a material? | Yes | No |
| Travels through space? | No | Yes |
| Speed | 343 m/s in air | about 300,000 km/s |
| Wave type | Longitudinal in air and water | Transverse |
Sources: NASA78, OpenStax26. Light is almost a million times faster than sound in air, which is why you see the flash first. Follow light's journey in how does light travel?
Every sound wave has two "dials" you can hear:
Frequency and wavelength are linked by one simple formula:
speed = frequency × wavelength (v = f λ)
So in air a deep 20 Hz rumble has waves about 17 metres long, while a 20,000 Hz squeak has waves under 2 centimetres long (343 ÷ 20 and 343 ÷ 20,000).2
Loudness is measured in decibels (dB). Every extra 10 dB means ten times the sound intensity (the sound power hitting each square metre).4
| Sound | Level |
|---|---|
| Quietest sound people can hear | 0 dB |
| Whisper at 1 metre | 20 dB |
| Normal conversation | 60 dB |
| Loud rock concert (threshold of pain) | 120 dB |
| Jet plane at 30 metres | 140 dB |
Levels from OpenStax.4 Long or repeated exposure to sounds at or above 85 dBA can damage your hearing, so turn headphones down.10 Clever headphones can even cancel sound with more sound: see how noise cancelling works.
When the wave reaches you, it takes a quick trip through your ear:9
Different pitches land in different places. Hair cells near the wide end of the cochlea pick up high sounds, like a baby crying, and cells near the centre pick up low sounds, like a big dog barking.9
When hair cells are damaged, hearing gets worse. Find out how a tiny computer can help in how do hearing aids work?
Sound also gets quieter with distance, because its energy spreads over a bigger and bigger area and some is absorbed along the way.1
Sound waves bounce off hard surfaces such as cliffs, walls and big buildings. When the bounced sound reaches your ear more than about 0.1 seconds after the original sound, your brain hears it as a separate copy: an echo. If it comes back sooner, the two blend together and the sound just seems to last longer (that lingering sound from many overlapping bounces is called reverberation).15 Bats send out ultrasound calls and listen to the echoes to find their prey, and doctors use echoes to make pictures of the inside of the body (medical imaging).215
A vibrating object, like a drum skin or a guitar string, pushes and pulls on the air next to it. That makes squeezed-together areas (compressions) and spread-out areas (rarefactions). Each molecule bumps the next one, so the pattern of squeezes moves outward while each molecule only jiggles back and forth. When the pattern reaches your eardrum, you hear it.
Not through empty space. Sound needs molecules to carry it, and space is almost a vacuum, so astronauts on a spacewalk talk by radio. Radio waves are a kind of light, which needs no material to travel. Inside the space station there is air, so astronauts can talk normally there.
In air at room temperature (20 °C), sound moves about 343 metres per second, so in 10 seconds it covers about 3.4 km, a little over 2 miles. That's why the National Weather Service rule is about 5 seconds of thunder delay per mile.
Water. Sound moves about 1,480 metres per second in fresh water, more than four times faster than the 343 m/s in air. In steel it's about 5,960 m/s. The stiffer the material, the faster it passes the push along.
In air and water, sound waves are longitudinal: the molecules move back and forth in the same direction the wave travels. Waves on a guitar string and light waves are transverse (they wiggle sideways). In solids, sound can travel both ways.
Every sound starts with a vibration: something moving back and forth, like vocal cords, a speaker cone, a drum skin, or air heated suddenly by lightning. The vibration squeezes and stretches the material around it, and that travelling pattern of pressure is sound.
As a sound wave spreads out from its source, its energy is shared over a larger and larger area, so each bit of air is squeezed less. Some energy is also absorbed by the air and by objects it hits. Smaller squeezes mean a smaller amplitude, and that sounds quieter.
An echo is sound bouncing back off a hard surface, like a cliff or a big wall. If the bounced sound reaches you more than about a tenth of a second after the original, your brain hears it as a separate echo.