# What Is the Speed of Sound?

> The speed of sound is how fast a sound wave travels through a material. This guide explains why it is roughly 343 metres per second in air, why temperature and the medium change it, and why you see lightning before you hear thunder.

*Section: Science — By Priya Anand (Lifestyle & Travel Editor) — Published July 21, 2023 — 6 min read*

Canonical URL: https://dailyjunction.org/science/what-is-the-speed-of-sound
Tags: speed of sound, sound waves, acoustics, physics, supersonic

## Key takeaways

- The speed of sound is how fast a sound wave travels through a given material, about 343 metres per second in dry air at 20C.
- Sound needs a medium such as air, water or steel; it cannot travel through a vacuum.
- Sound travels faster in liquids and solids than in air because their particles are packed closer together.
- In air the speed rises as temperature increases, by roughly 0.6 metres per second for each degree Celsius.
- Light travels far faster than sound, which is why you see lightning before you hear the thunder.

Watch a cricket match from the far side of the boundary and you notice something strange: the batter swings, and a heartbeat later you hear the crack of the ball on the bat. The two have come apart. The same thing happens in a thunderstorm, where the flash arrives long before the rumble. Both are everyday demonstrations of a single fact — sound takes time to travel, and that speed can be measured. This guide explains what the speed of sound is, what changes it, and why it matters.

## What it is

**The speed of sound is how fast a sound wave travels through a given material, and in dry air at about 20C it is roughly 343 metres per second.** That works out to around 1,235 kilometres per hour, or about 767 miles per hour — fast by everyday standards, but a snail's pace compared with light.

To understand the figure, you first need to know what sound actually is. Sound is a *vibration* that travels as a wave. When something vibrates — a guitar string, your vocal cords, a slammed door — it pushes on the air particles next to it. Those particles bump into their neighbours, which bump into theirs, and so a ripple of squashing and stretching passes outward. Your ear detects that ripple as sound. The speed of sound is simply how quickly that disturbance moves through the material, known as the *medium*.

## Sound needs something to travel through

A crucial point follows from this: **sound cannot travel through a vacuum.** Because sound is particles passing a vibration along, it needs particles to be present. Remove them and there is nothing to carry the wave.

This is why space is silent. Despite what films suggest, an exploding spaceship would make no noise at all, because the near-empty vacuum of space has almost no particles to vibrate. Here on Earth we are surrounded by air, so we take it for granted that sound has a medium to travel through. Light is different — it is an electromagnetic wave and needs no medium, which is part of why it behaves so differently from sound. If you want to dig into how charged particles and energy move, our explainer on [what is electricity](/science/what-is-electricity) is a useful companion.

## Why the medium matters so much

The speed of sound is not a single fixed number; it depends heavily on *what* the sound is travelling through. As a rule:

- **Solids** carry sound fastest of all.
- **Liquids** are next.
- **Gases** such as air are the slowest.

The reason comes down to how closely the particles are packed and how strongly they are linked. In a solid such as steel, particles sit very close together and are tightly bonded, so a push is passed on almost instantly — sound travels through steel at around 5,000 metres per second, roughly fifteen times faster than in air. In water the particles are still close, and sound manages about 1,480 metres per second. In air the particles are far apart and loosely connected, so the message travels comparatively slowly.

| Medium | Approximate speed of sound |
| --- | --- |
| Air (20C) | 343 m/s |
| Water | 1,480 m/s |
| Steel | ~5,000 m/s |

This is why you can sometimes hear an approaching train through the rail before you hear it through the air, and why whales can communicate across vast distances underwater. The closer and more tightly bound the particles, the faster the message spreads. The way packing affects a material's behaviour also lies behind [what is density](/science/what-is-density), a closely related idea.

## Temperature changes the speed too

Even within air, the speed of sound is not constant. **It rises as the air gets warmer.** A handy rule of thumb is that the speed increases by roughly 0.6 metres per second for every one degree Celsius rise in temperature.

The reason is that warmer particles move faster and collide more often, so they pass the vibration along more quickly. On a hot summer afternoon sound travels slightly faster than on a freezing winter morning. This is also why the figure of 343 metres per second always comes with a temperature attached — quote the speed of sound and you should really say at what temperature.

Humidity and air pressure have smaller effects, but temperature is by far the biggest factor in everyday conditions. This is a good example of how scientific measurements are stated carefully, with their conditions spelled out so the result can be checked and repeated.

## Lightning, thunder and counting the gap

The gap between light and sound gives us one of the most useful party tricks in physics. Light travels at about 300 million metres per second, so a flash of lightning reaches your eyes effectively the instant it happens. The thunder, travelling at the speed of sound, lags far behind.

You can turn this into a distance estimate. Count the seconds between seeing the flash and hearing the thunder, then divide by three to get the approximate distance to the strike in kilometres (or divide by five for miles). A gap of nine seconds means the storm is roughly three kilometres away. If the flash and bang arrive together, the storm is right on top of you.

## Breaking the sound barrier

When an aircraft flies, it constantly produces pressure waves that spread out at the speed of sound. As the aircraft approaches that speed, the waves ahead of it cannot get out of the way fast enough and begin to pile up. Push past the speed of sound — to *supersonic* speed — and these compressed waves form a sharp **shock wave**.

On the ground, that shock wave is heard as a sudden, startling crack known as a **sonic boom**. This is the famous moment of "breaking the sound barrier". There is no physical wall to smash through; the barrier is simply the point at which the aircraft outruns its own sound. The pitch changes you hear from a fast-moving siren or jet, by contrast, are explained by [the Doppler effect](/science/what-is-the-doppler-effect), which is about how wave frequency shifts when a source moves.

## Why any of this is useful

The speed of sound is not just trivia. Knowing it precisely matters for designing concert halls and recording studios, where engineers manage how sound waves bounce and arrive. Ships and submarines use **sonar**, timing how long a sound pulse takes to return in order to map the seabed or detect objects. Doctors use the same idea in **ultrasound** scans, and geologists study how sound waves travel through the Earth to understand its hidden structure. In each case, the speed of sound is the constant that turns a measured time into a useful distance.

## The bottom line

The speed of sound is how fast a sound wave travels through a material — about 343 metres per second in air at 20C, but far quicker in water and quicker still in solids, where particles are packed close and tightly linked. It needs a medium, so it cannot cross the vacuum of space, and it speeds up as the air warms. Because light vastly outruns sound, we see events before we hear them, a gap we can even use to measure how far away a storm is. From sonic booms to ultrasound scans, this single, measurable speed quietly shapes a surprising amount of the world around us.

## Frequently asked questions

### How fast does sound travel?

In dry air at about 20C, sound travels at roughly 343 metres per second, which is around 1,235 kilometres per hour or 767 miles per hour. The exact figure depends mainly on the temperature of the air and the material the sound is passing through. Sound moves much faster in water and faster still in solids such as steel.

### Why do you see lightning before you hear thunder?

Light travels at about 300 million metres per second, while sound manages only around 343 metres per second in air. The flash therefore reaches your eyes almost instantly, but the thunder takes time to arrive. Counting the seconds between the flash and the bang and dividing by three gives a rough distance to the storm in kilometres.

### Can sound travel through space?

No. Sound is a vibration that needs particles to pass it along, so it requires a medium such as air, water or a solid. Space is very close to a vacuum, with almost no particles, so there is nothing to carry the vibration. This is why an explosion in space would, in reality, be silent.

### What does breaking the sound barrier mean?

It means an object such as an aircraft is travelling faster than the speed of sound in the surrounding air. The pressure waves it creates pile up into a shock wave, which reaches the ground as a loud sonic boom. Speeds above the speed of sound are described as supersonic.

## Sources

- [Encyclopaedia Britannica: sound](https://www.britannica.com/science/sound-physics)
- [NASA Glenn Research Center: speed of sound](https://www.grc.nasa.gov/www/k-12/airplane/sound.html)
- [BBC Bitesize: sound waves](https://www.bbc.co.uk/bitesize/topics/zwz8nbk)

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