How much the speed of sound?

The speed of sound in dry air at 20 °C is 343 m/s. Converted, that is 1,235 km/h, 767 mph, 667 kn or about 1,125 ft/s. Aviation uses the International Standard Atmosphere instead, which fixes sea level at 15 °C, giving 340.3 m/s (1,225 km/h, 661 kn). The two numbers are often quoted interchangeably; the difference is the 5 °C between them.

The formula behind these values is the square root of γRT/M, where γ is the ratio of specific heats (1.4 for air), R the gas constant, T the absolute temperature and M the molar mass. A useful rule of thumb follows from it: sound speed in air changes by about 0.6 m/s for each degree Celsius.

Because only temperature matters, the speed of sound tracks the atmosphere's temperature profile. In the standard atmosphere it falls steadily through the troposphere, reaching 295.1 m/s (1,062 km/h, 573 kn) at the tropopause at 11 km, then holds constant up to 20 km where temperature is uniform, before rising again higher in the stratosphere as ozone warms the air. This is why an airliner cruising at 900 km/h true airspeed at 11 km is at Mach 0.85, whereas the same speed at sea level would be only Mach 0.73. Pilots and flight computers use local Mach number, not a sea-level constant, because the aerodynamic limits of the wing depend on it.

Humidity has a small effect, adding a few tenths of a metre per second in very moist air, because water vapour is lighter than the nitrogen and oxygen it displaces. Pressure alone has none: on a mountain top the air is thinner but sound is slower only because it is colder.

Liquids and solids carry sound much faster because their molecules are closer and more tightly bound. In fresh water at 20 °C the figure is about 1,480 m/s; in sea water slightly more, around 1,530 m/s, which sonar systems must correct for with temperature and salinity. In steel it is close to 5,900 m/s, in aluminium about 6,300 m/s and in diamond over 12,000 m/s. Among gases, hydrogen at about 1,270 m/s and helium at around 1,000 m/s are the fastest; carbon dioxide, denser and with a lower γ, is slower than air at about 270 m/s.

Mach number is the ratio of an aircraft's true airspeed to the local speed of sound, so the same Mach 1 corresponds to different speeds in different conditions. Concorde's Mach 2.02 cruise at 17 km was about 2,150 km/h, whereas Mach 2 at sea level on a hot day would be closer to 2,500 km/h. When a figure for the sound barrier is quoted without an altitude, it is almost always the sea-level standard value of 1,225 km/h.