How to use it
- Choose the medium. For air, helium, hydrogen and oxygen, enter the temperature.
- Choose whether to find the travel time over a distance or the distance from a delay.
- Enter the distance or the delay in seconds.
- Read the speed in four units and the time or distance.
Formula and values
For gases the speed is taken at 0 °C and scaled with temperature like an ideal gas:
v = v₀ × √(1 + T ÷ 273.15), with v₀ = 331.3 m/s for dry air, 972 m/s for helium, 1286 m/s for hydrogen and 317 m/s for oxygen.
Liquids are taken at about 25 °C and solids as longitudinal waves in bulk material, with typical textbook values: water 1493 m/s, sea water 1533 m/s, mercury 1450 m/s, glycerol 1904 m/s, rubber 1600 m/s, lead 1960 m/s, gold 3240 m/s, copper 5010 m/s, Pyrex glass 5640 m/s, iron 5950 m/s and aluminium 6420 m/s. Real samples vary with composition and temperature.
Worked example
Air at 20 °C: v = 331.3 × √(1 + 20 ÷ 273.15) = 331.3 × 1.03596 = 343.21 m/s, or 1,235.6 km/h. Sound takes 1000 ÷ 343.21 = 2.9136 s to travel 1 km.
Thunder heard 3 s after the flash: 343.21 × 3 = 1,029.6 m, so the lightning struck about 1.03 km away.
Tips
- Cold air is slower. At −20 °C sound travels at about 319 m/s.
- Mach 1 is not a fixed speed. It is the local speed of sound, which is lower at the cold altitudes where jets fly.
- In helium your voice sounds higher because sound travels about three times faster, which shifts the resonances of your vocal tract.