THROUGH AIR
About a kilometre in three seconds
At 20 °C sound covers 343 metres every second. That is quick, but slow enough to time with a count.
Time a storm- Air: 343 m/s
- Water: about 1,500 m/s
- Steel: 5,960 m/s
- Space: silence
00HOW SOUND TRAVELS
Count the gap.
Hear the distance.
Sound is fast, but nowhere near as fast as light.
That small delay is why thunder rumbles after the flash and why a distant hammer seems out of step. Here is how sound moves through air, water and steel, in plain English.
Plain English, with the sources linked.
Try the thunder counter01Principles
02THE SHORT ANSWER
Stiffer stuff, faster sound.
Sound is a vibration handed from particle to particle,
so what it travels through sets the pace.
THROUGH WATER
Over four times faster
In seawater sound travels at about 1,500 metres a second, more than four times its speed in air.
Compare the media
THROUGH STEEL
Fastest of the three
Sound runs through steel at 5,960 metres a second, about 17 times its speed in air.
See the numbers03A THUNDER COUNTER
How far away
was that flash?
Count the seconds from the flash of lightning to the first sound of thunder, then pick the gap that matches. The rule comes from the Met Office: divide by three for kilometres, or by five for miles.
It is an estimate, not a measurement. The speed of sound shifts a little with temperature, and long rumbles make the start hard to pin down.
04ONE KILOMETRE, THREE WAYS
Same distance,
different wait.
How long a sound takes to cross 1 km
depends on what it is crossing.
2.9 s vs 0.7 s
Time to cross 1 km
At 343 metres a second, sound needs almost three seconds to cross a kilometre of air. At about 1,500 metres a second in seawater, it needs well under one.
AIR VERSUS SEAWATER
343 vs 5,960
Speed in metres per second
In steel the same kilometre takes under a fifth of a second. Stiff solids pass the vibration on far more quickly than a gas can.
AIR VERSUS STEEL
The times are simple division from the published speeds: 1,000 m ÷ 343 m/s, 1,000 m ÷ 1,500 m/s and 1,000 m ÷ 5,960 m/s.
05TRY IT OUTSIDE
Four ways to
hear the delay.
Count the next storm
From indoors, count the seconds from flash to thunder and divide by three. A shrinking count means the storm is getting closer.
Watch a distant hammer
Across a wide field or a building site, the blow lands before you hear it. At 100 metres the lag is under a third of a second, but you can notice it.
Clap at a far wall
Stand about 50 metres from a large wall and clap. The echo travels there and back, 100 metres in all, and returns in roughly 0.3 seconds.
Listen through a table
Rest one ear on a long table and tap the far end gently. Solids carry sound well, so the tap often sounds clearer through the wood than through the air.
06SOUND FACTS
Five things worth
hearing.
343 metres every second
A standard university physics text gives the speed of sound in air at 20 °C as 343 metres per second, a little over a kilometre every three seconds.
Cold air is slower
At 0 °C the figure drops to about 331.5 metres per second, against about 344 at 20 °C. Warmer air passes the vibration along a little faster.
The sea is a fast lane
Sound travels about 1,500 metres per second in seawater, and the exact speed varies by a few percent with location, season, time of day and depth.
Steel is faster still
The same physics text lists 5,960 metres per second for steel and 1,480 for fresh water. Solids and liquids resist squeezing far more than air does.
Space is silent
Sound needs a material to vibrate. Space is a near vacuum, with no air or water to carry it, so there is effectively nothing to hear.
07QUICK QUESTIONS
Sound, answered.
Short answers to the questions people ask most.
How fast does sound travel?
In air at 20 °C sound travels at about 343 metres per second. That is roughly one kilometre every three seconds, or a mile in about five.
How can I tell how far away a thunderstorm is?
Count the seconds between the flash and the thunder. The Met Office says to divide by three for kilometres or by five for miles, so a nine-second gap means the storm is about 3 km away.
Why do I see lightning before I hear the thunder?
Light travels at around 300,000,000 metres per second and sound at around 340, so the flash reaches you almost instantly while the sound lags behind. Thunder itself is the sound of air heated suddenly, to up to 30,000 °C, by the lightning.
Does sound travel faster in water?
Yes. Sound travels at about 1,500 metres per second in seawater, more than four times its speed in air, and the exact figure varies by a few percent with location, season, time of day and depth.
Is there any sound in space?
Not in the ordinary sense. Sound is a vibration passed through a material such as air or water, and space is a near vacuum, so there is almost nothing there to carry it.
08Sources
Where the facts come from
- Thunder and lightning: what causes thunder and how far away a storm isMet Office
- Why does lightning always come before thunder?Hong Kong Observatory
- Speed of sound, with a table for gases, liquids and solidsOpenStax University Physics
- Speed of sound in air and its change with temperatureHyperPhysics, Georgia State University
- How fast does sound travel in the sea?DOSITS, University of Rhode Island
- Can you hear sound in space?Adler Planetarium