How far is the speed of sound?

Sound has no fixed range; it is a pressure wave that travels outward until it fades below hearing. What is fixed, for a given air temperature, is how fast that wave moves. At 20 °C and sea-level pressure the speed of sound is about 343 m/s, or 1,235 km/h. The figure is set by how quickly air molecules can pass a disturbance to their neighbours, which depends on how fast they are moving, and that in turn depends on temperature. Pressure and altitude affect it only through their effect on temperature: at the same temperature, thin air and dense air carry sound at the same speed.

If the question is about distance, it helps to turn the speed into travel times. At 343 m/s, sound covers 100 m in about 0.3 s, one kilometre in roughly 2.9 s and 10 km in about half a minute. This is why a lightning flash is followed by thunder after a delay of about three seconds per kilometre, and why the noise of a high-flying jet seems to come from a point well behind where the aircraft actually is: by the time the sound reaches the ground the aircraft has moved on. An airliner at 11,000 m is roughly 37 s of sound away from someone directly below, and in that time it has flown about 9 km.

Aviation expresses the speed of sound as Mach 1 and measures aircraft speed as a fraction of it, because the physics of the airflow changes as an aircraft approaches that value. In the cold air of the upper troposphere, standard at −56.5 °C from 11,000 m upward, sound moves at only about 295 m/s (1,062 km/h, 573 kn). A jet cruising at 900 km/h up there is at about Mach 0.85, close to the point where shock waves and drag rise sharply, which is why airliners cruise where they do and no faster. Concorde flew at about Mach 2, twice the local speed of sound, covering 2,150 km/h at 17,000 m. The same Mach number nearer the ground, in warmer air, would correspond to a higher speed in km/h, so the Machmeter, not the airspeed indicator, is what tells a pilot how close the aircraft is to the barrier.