How much is the sound barrier?

The phrase sound barrier suggests something that could be measured in metres or paid for, but it describes a speed. Sound travels through air as a wave of pressure passed from molecule to molecule, and the speed of that wave sets the speed at which any aircraft begins to encounter shock waves and a steep rise in drag. In air at 20 °C at sea level the figure is about 343 m/s, which is 1,235 km/h or 667 knots. Because the speed of sound depends on temperature and nothing else in practice, it is lower in cold air: at the typical airliner cruise level of 11,000 m, where the standard atmosphere is −56.5 °C, sound moves at about 295 m/s, or 1,062 km/h. Pilots therefore do not talk about the barrier in km/h at all but in Mach number, the ratio between the aircraft's true airspeed and the local speed of sound.

If the question is what it costs to fly through Mach 1, the answer is energy. Drag can multiply several times over in the transonic range as shock waves form and then remains well above subsonic levels once through. An afterburning fighter burns fuel many times faster in supersonic flight than in cruise, which is why most can hold it for only minutes. Concorde, the only supersonic airliner to see long service, cruised at about Mach 2 and used roughly four times as much fuel per passenger as a subsonic jet of its era, one of the reasons its fares were among the highest in the industry. Designing an aircraft for supersonic flight also means thin wings, powerful engines and heat-resistant structures, all of which add cost before the first flight.

Because the local speed of sound changes with temperature, aircraft carry a Machmeter that combines airspeed and outside air temperature into a single ratio. Airliners cruise at around Mach 0.78 to 0.85, close enough to the barrier to be efficient but below the point where drag rises steeply. A fighter accelerating to Mach 1.0 at 11,000 m is doing about 1,062 km/h, while the same Mach number near the ground would require 1,225 km/h or more. The sound barrier, then, moves with the weather and the altitude, and its cost is counted in thrust and fuel rather than in currency.