Sound Travel Time Calculator
Example: 3 seconds = 1 km away
Sound travels at about 343 m/s in dry air at 20°C, so a one-kilometre trip takes roughly 2.9 seconds. This calculator uses a temperature approximation for air and representative reference values for water and solids. Humidity, wind, altitude, salinity, pressure, material composition and wave direction can change real travel times.
Calculation results updated
Elapsed time as sound travels
Travel Time
Speed of Sound
Echo Time (round trip)
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How to use this calculator
- Enter the distance the sound has to cover. For a lightning strike, this is how far away the storm is; for an echo, it is the distance to the wall or cliff.
- Pick the medium. Air is the default. Water, steel, concrete and wood use representative reference values.
- Set the air temperature if you are using air. Sound moves about 0.6 m/s faster for every 1°C, so a hot afternoon and a freezing night give noticeably different answers.
- Read the one-way time for a sound reaching you directly, or the echo time for a sound that travels out and back.
- To work backwards from a thunder delay instead, count the seconds between the flash and the bang and divide by 3 for kilometres, or by 5 for miles.
How We Calculate This
For dry air near ordinary atmospheric conditions, v ≈ 331.3 + 0.606T m/s over the supported −40°C to 50°C range. One-way time is distance ÷ speed; echo time is twice that distance. Water, steel, concrete and wood use representative values only because temperature, composition, pressure, moisture, grain and wave type matter.
Methodology last reviewed: September 2026. How SparkCalc works
Sources: NASA Glenn Research Center: Speed of Sound · NOAA National Weather Service: Understanding Lightning: Thunder · NOAA National Ocean Service: How Far Does Sound Travel in the Ocean? · Simon Fraser University, Handbook for Acoustic Ecology: Sound Propagation · HyperPhysics (Georgia State University): Inverse Square Law for Sound · HyperPhysics (Georgia State University): Refraction of Sound · Purdue University Chemistry: Noise Sources and Their Effects (decibel levels) · NIDCD (NIH): Noise-Induced Hearing Loss
How to tell how far away lightning struck
Light covers a kilometre in about three millionths of a second, so for any storm you can see, the flash arrives instantly. Thunder does not. It travels at the speed of sound, roughly 343 m/s, which means it needs about 2.9 seconds per kilometre or 4.7 seconds per mile. That gives the rule most people learn as children: count the seconds between the flash and the bang, then divide by 3 for kilometres or by 5 for miles. A 9 second gap puts the strike about 3 km away. A 2 second gap puts it close enough that the next one could reach you. One important caveat. The NOAA National Weather Service advises going indoors as soon as you can hear thunder at all, because lightning regularly strikes well outside the rain area of a storm. The flash-to-bang count tells you where the last strike landed. It does not tell you where the next one will.
Why temperature changes the answer
Sound moves through air by knocking molecules into one another. Warmer air has faster-moving molecules, so each collision passes the disturbance along sooner and the wave travels quicker. The relationship is close enough to linear across normal weather that a simple formula works: v ≈ 331.3 + 0.606T, with T in degrees Celsius. The effect is not trivial. At −20°C sound manages about 319 m/s; at 35°C it reaches about 353 m/s. Over a 5 km distance that is a difference of roughly 1.5 seconds, which matters if you are timing something precisely. Altitude, by contrast, has almost no direct effect. Thinner air is less dense but also less stiff, and the two changes very nearly cancel. Sound is slower high in the atmosphere mainly because it is colder up there, not because the air is thin. Humidity has a small effect, raising the speed slightly, because water molecules are lighter than the nitrogen and oxygen they displace.
Sound in water, steel and other materials
Sound travels about 4.4 times faster in water than in air, and roughly 15 times faster in steel. The instinct is to blame density, but that gets it backwards: water and steel are far denser than air, and density alone would slow a wave down. What actually governs the speed is the ratio of a material's stiffness to its density. Steel resists compression enormously more than air does, and that stiffness advantage far outweighs its greater density. This is why you can hear a train through a rail long before you hear it through the air, and why whales can communicate across ocean basins. The figures used here are representative values, not precise constants. Sound speed in seawater varies with temperature, salinity and pressure, which is why sonar systems carry live measurements rather than a fixed number. In solids it depends on composition, grain direction and whether the wave is compressional or shear. Treat the non-air results as sound order-of-magnitude comparisons rather than engineering figures.
How far can a human voice travel?
Speed and reach are different questions. A voice leaves at the same 343 m/s as thunder, but how far it stays audible depends on how loud it starts, how fast it fades, and what it has to compete with when it arrives. In open air with no walls or ground to reflect it, sound from a small source spreads over an ever larger sphere and obeys the inverse-square law: the level drops by about 6 dB each time the distance doubles. Normal conversation measures roughly 60 dB, with 60 to 70 dB quoted for a typical exchange at close range. A quiet rural area sits near 30 dB and a freeway or busy street near 70 dB. Put those together and the arithmetic is short. A 60 dB voice at 1 m reaches the 30 dB countryside floor after five doublings, about 32 m (105 ft). On a 70 dB street the same voice is below the background before it has travelled a metre, which is why people lean in to talk in traffic. Each 6 dB you add to your voice doubles the reach on paper: 20 dB above conversation gives about 320 m (1,050 ft), and 30 dB above gives about 1 km (0.6 mi). Shouting adds tens of decibels, so a shout crossing a quiet valley for several hundred metres is what this rule predicts. Real air is less tidy. It absorbs high frequencies more than low, by an amount that depends on temperature and humidity, so a distant voice sounds muffled before it disappears. Wind and temperature layering bend sound paths and can move the measured level by as much as 20 dB from the geometric prediction. Over a cool lake at dawn, with warmer air above the water, sound is refracted back down toward the surface and carries much farther than the same voice on land. Under a hot afternoon sun the gradient runs the other way, sound bends upward, and the range shrinks. For a sense of scale, thunder starts at around 120 dB and is normally heard only about 16 km (10 mi) away, according to the NOAA National Weather Service. A voice is at least 50 dB quieter at the source, so a few hundred metres to a kilometre in quiet, still conditions is the realistic ceiling, and in a noisy city it is a small fraction of that.
How long does sound take in air?
Approximate one-way times below use dry air at 20°C (343.42 m/s). Wind and atmospheric conditions are ignored.
| Distance | Travel time |
|---|---|
| 1 metre | 2.9 milliseconds |
| 1 kilometre | 2.91 seconds |
| 1 mile | 4.69 seconds |
Thunder delay as an approximate distance
Light arrival is effectively immediate at these distances, so multiply the delay by the speed of sound.
| Lightning-to-thunder delay | Approximate distance |
|---|---|
| 3 seconds | 1.03 km (0.64 mi) |
| 5 seconds | 1.72 km (1.07 mi) |
| 10 seconds | 3.43 km (2.13 mi) |
NOAA advises going indoors when thunder is heard; delay is not a safety radius.
How far a voice carries before it fades into the background
Distances use the 6 dB per doubling rule for a small source in open air, with no atmospheric absorption, wind or reflections, and treat a sound as audible until it drops to the background level. Real conditions can move these figures by 20 dB either way.
| Level at 1 m | Fades into 30 dB countryside | Fades into 70 dB street |
|---|---|---|
| 50 dB (quiet conversation at home) | 10 m (33 ft) | Under 1 m |
| 60 dB (normal conversation) | 32 m (105 ft) | Under 1 m |
| 70 dB (10 dB above conversation) | 100 m (330 ft) | 1 m (3 ft) |
| 80 dB (20 dB above conversation) | 320 m (1,050 ft) | 3.2 m (10 ft) |
| 90 dB (30 dB above conversation) | 1,020 m (0.63 mi) | 10 m (33 ft) |
A 60 dB voice sits below a 70 dB street background at any distance, which is why conversation in traffic needs a raised voice or a shorter gap.
Key terms
- Speed of sound
- How fast a pressure wave moves through a material. In dry air at 20°C it is about 343 m/s (1,235 km/h or 767 mph). It is a property of the material, not of the sound itself, so a whisper and a thunderclap travel at the same speed.
- Flash-to-bang
- The gap between seeing lightning and hearing thunder. Light arrives effectively instantly, so the whole delay is sound travel time. Divide the seconds by 3 for the distance in kilometres, or by 5 for miles.
- Echo time
- The time for a sound to reach a surface and return. It is double the one-way time, because the wave covers the distance twice. A cliff 170 m away returns an echo after about one second.
- Temperature coefficient
- The rate at which sound speed changes with air temperature, roughly 0.6 m/s per °C. This is why the simple formula v ≈ 331.3 + 0.606T works well across ordinary weather conditions.
- Decibel falloff
- In open air with nothing to reflect it, sound from a small source spreads over an ever larger sphere and follows the inverse-square law, which works out to a drop of about 6 dB for every doubling of distance. A voice at 60 dB one metre away is roughly 54 dB at 2 m, 48 dB at 4 m and 30 dB at about 32 m.
Frequently Asked Questions
How do I calculate lightning distance?
Count the seconds between seeing lightning and hearing thunder. Each 3 seconds equals roughly 1 kilometre (or 5 seconds per mile). Light travels almost instantly, so the delay is entirely due to sound travel time.
Why does sound travel faster in water than air?
Sound speed depends on a medium’s elastic stiffness as well as density. Water and steel are much stiffer than air, so their ratio of stiffness to density produces a higher wave speed. Density alone does not explain it.
How does temperature affect sound speed?
Warmer air has faster-moving molecules that transfer sound vibrations more quickly. Sound speed in air increases by about 0.6 m/s for each 1°C rise in temperature.
Why do I see fireworks before hearing them?
Light travels at 300,000 km/s while sound travels at only 0.34 km/s in air. For fireworks 1 km away, you see them instantly but hear them about 3 seconds later.
How far can a human voice travel?
In open air a voice loses about 6 dB every time the distance doubles. Normal conversation, around 60 dB at 1 m, fades into a 30 dB quiet countryside within a few tens of metres, while a much louder shout can carry for hundreds of metres in still air. Wind, temperature layering and background noise shift the answer by 20 dB or more, which is why a voice can carry across a calm lake at dawn yet vanish on a busy street.
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References
- NASA Glenn Research Center: Speed of Sound
- NOAA National Weather Service: Understanding Lightning: Thunder
- NOAA National Ocean Service: How Far Does Sound Travel in the Ocean?
- Simon Fraser University, Handbook for Acoustic Ecology: Sound Propagation
- HyperPhysics (Georgia State University): Inverse Square Law for Sound
- HyperPhysics (Georgia State University): Refraction of Sound
- Purdue University Chemistry: Noise Sources and Their Effects (decibel levels)
- NIDCD (NIH): Noise-Induced Hearing Loss