What is the speed of sound?

Short answer

The speed of sound isn't a moment in time but a measurable quantity: in air at 15 °C at sea level it equals 340 m/s, or 1,225 km/h (661 kn), and an aircraft reaches it when flying at Mach 1. Since it depends only on temperature, it drops to 295 m/s (1,062 km/h) at 11,000 m altitude, where the air is at −56.5 °C.

Published: September 21, 2026 · Updated: September 21, 2026 · Edited by Above Ten Thousand · Editorial policy

NASA’s X-59 Continues Supersonic Flight Testing (nasas x-59 continues supersonic flight testing 3)

A figure that changes with altitude

The question is usually asked while thinking of the moment an aircraft "breaks the sound barrier." That moment arrives when its airspeed equals the local speed of sound, and that speed is not fixed. In the standard atmosphere it is 340 m/s at sea level (1,225 km/h), 328 m/s at 3,000 m, 316 m/s at 6,000 m, and 295 m/s from 11,000 m up to 20,000 m, where temperature holds steady at −56.5 °C. A fighter jet accelerating at high altitude reaches Mach 1 at around 1,060 km/h; the same aircraft at sea level would need to reach 1,225 km/h. That's why pilots and engineers speak in terms of Mach number rather than kilometers per hour: it's the figure that describes what happens to the air around the aircraft.

What happens when you get there

Below Mach 1, the pressure disturbances an aircraft generates travel ahead of it, warning the air to move out of the way. Once it reaches the speed of sound, those disturbances can no longer outrun it; they pile up and form shock waves — first over areas where the airflow locally accelerates (the upper surface of a wing can be supersonic at Mach 0.8 even if the aircraft itself isn't), then at the nose and leading edges. Drag increases sharply, the center of pressure shifts aft, and conventional controls lose effectiveness — which is what made World War II fighter dives so dangerous. For an observer on the ground, the passage of the shock waves is perceived as a sonic boom, a double crack that trails the aircraft the entire time it remains supersonic, not just at the instant it crosses Mach 1.

The first time

The first documented level supersonic flight was Chuck Yeager's on October 14, 1947, flying the Bell X-1 after being launched from a B-29 over the Mojave Desert: he reached Mach 1.06, about 1,127 km/h, at 13,100 m (43,000 ft). The X-1 used an all-moving horizontal tail, the solution that restored pitch control in the transonic regime and that every later supersonic aircraft adopted. Concorde, the only passenger aircraft to routinely exceed that speed, stopped flying in 2003; fighter jets cross it routinely, and several supersonic business jet projects are trying to bring the speed of sound back to civil aviation.