Core answer: Sound travels 343 m/s in 20 °C air — about 1 km in 3 seconds, or 1 mile in 5. That is why you see lightning long before you hear it: count the seconds, divide by 3 for kilometers. Sound needs a medium and moves faster in denser-phase materials: ~1,480 m/s in water, ~5,900 m/s in steel. In vacuum, silence.
Speed by medium
| Medium | Speed (m/s) | Roughly × air |
|---|---|---|
| Air (0 °C) | 331 | 0.96× |
| Air (20 °C) | 343 | 1× |
| Air (30 °C) | 349 | 1.02× |
| Hydrogen | 1,284 | 3.7× |
| Water (25 °C) | 1,497 | 4.4× |
| Concrete | ~3,200 | 9× |
| Steel | ~5,900 | 17× |
| Diamond | ~12,000 | 35× |
In air the handy formula is v ≈ 331 + 0.6 × T(°C). Humidity raises it slightly (~0.3% at 100% RH).
Worked examples
Example 1 — Lightning distance. Flash-to-bang 7 s → 7 × 343 ≈ 2,400 m ≈ 2.4 km. The storm is moving if the gap shrinks; take shelter under ~10 s.
Example 2 — The rail trick. Put an ear to a rail and a distant train's clank arrives through steel (5,900 m/s) seconds before the airborne sound — the classic Western-movie trick is real physics.
Example 3 — Stadium echo. A 170 m wall returns an echo after 2 × 170 / 343 ≈ 1 s — easily heard as a distinct repeat, which is why stadium sound design uses distributed speakers with delays.
Example 4 — Sonar depth. A fish-finder pings and hears the bottom 0.08 s later: depth = 1,497 × 0.08 / 2 ≈ 60 m. (Divide by 2 — the pulse made a round trip.)
Mach number and the sound barrier
Mach 1 = local sound speed. Because sound slows with altitude's cold (at 11,000 m it is ~295 m/s), "Mach 2" cruising is slower in m/s than at sea level. Crossing Mach 1 piles pressure waves into a shock cone — the sonic boom is not a one-time "break" but a continuous carpet dragged behind the jet.
Common mistakes and myths
- "Sound is instant over short distances" — a drummer 30 m away is ~0.09 s late, enough to confuse musicians; big stages use in-ear monitors for this reason.
- Using 343 m/s at all altitudes — at −50 °C cruise altitude it is ~295 m/s, 14% slower.
- "Sound can't travel in space — but the movie explosion was audible" — movies lie; vacuum carries no sound. The ISS crew hears only structure-borne vibration.
- Forgetting the round trip — echo and sonar distances are v × t / 2, the most common homework error.
- Thinking louder = faster — amplitude changes loudness, not speed; a whisper and a shout arrive together.