How to Work Out the Speed of Sound

“Sound travels at 340 metres per second” is one of those figures everyone half-remembers. It’s approximately right on a mild day and wrong by several metres per second on any day that isn’t.

For most purposes that doesn’t matter. For acoustic measurement, ultrasonic distance sensing, or anything where a few milliseconds counts, it does — and the correction is a single term.

The temperature relationship

In air, sound speed rises with temperature because the molecules are moving faster and transmit the pressure wave more quickly. Over ordinary conditions the relationship is nearly linear:

v ≈ 331.3 + 0.606 × T(°C) metres per second

At 0°C that gives 331 m/s. At 20°C, 343 m/s. At 35°C, 352 m/s.

So the range across a normal year is around 20 m/s — about 6%. An ultrasonic sensor calibrated in a warm workshop and deployed outdoors in winter will read systematically long unless it compensates.

What doesn’t matter much: humidity changes the speed by well under 1% across the full range, and pressure has essentially no effect on its own, because density and stiffness change together.

Other media

Sound travels faster in stiffer materials and slower in denser ones. The relationship depends on the ratio, and both water and solids are far stiffer than air relative to their density.

Fresh water carries sound at roughly 1,480 m/s, sea water slightly faster. Steel is around 5,000 m/s — about fifteen times air. This is why you can hear a train through a rail long before you hear it through the air, and why sonar works so effectively underwater.

The practical use: travel time

Most of the time what you actually want is how long sound takes to cover a distance.

time = distance ÷ speed

At 343 m/s, sound covers 340 m in about a second. That’s the basis of the lightning rule: count seconds between the flash and the thunder, divide by three for kilometres or five for miles. Light’s travel time over those distances is negligible.

It’s also why a delay of 20 milliseconds in a sound system corresponds to about 7 metres of path difference — useful when aligning speakers, where the audible effect of misalignment is comb filtering rather than an obvious echo.

Common mistakes to avoid

• Using 340 m/s for measurement work without checking the temperature.
• Worrying about humidity, which matters far less than temperature.
• Assuming sound speed changes with frequency in air — it doesn’t, to any meaningful degree, which is why music arrives from a distant stage in the right order.
• Applying air values to underwater or structural measurements.
• Forgetting that a temperature gradient bends sound paths, which is why sound carries further over water on a cold morning.

How to do it with Sound Speed Calculator

The Sound Speed Calculator applies the temperature correction and converts to travel time.

1. Enter the air temperature — the dominant variable by a wide margin.
2. Pick a medium if you’re not working in air.
3. Enter a distance to get the travel time, which is what most practical uses need.

Other acoustics and physics tools are in the tools directory.

Frequently asked questions

Does humidity change the speed of sound?

Slightly — humid air is marginally less dense, so sound travels a little faster, but the effect is under one percent across the full humidity range. Temperature dominates.

Why is sound faster in water and steel?

Because speed depends on stiffness relative to density, and both are far stiffer than air. Steel carries sound roughly fifteen times faster.

Can I estimate lightning distance this way?

Yes — about three seconds per kilometre, or five per mile, between the flash and the thunder. Light’s travel time is negligible at those distances.

Final thought

Put the temperature in. It’s one term, it takes a second, and it’s the difference between an estimate and a measurement.

Try the free Sound Speed Calculator