Why is the speed of sound so slow?

Sound in air is a pressure wave: a region of slightly compressed air pushes on the air next to it, which pushes on the next, and so on. Nothing travels except the disturbance. The speed at which it propagates is set by how quickly molecules can pass on a push, and that is limited by how fast the molecules themselves are moving between collisions. In air at room temperature the average molecular speed is around 500 m/s, and the resulting sound speed is 343 m/s, a little lower because energy is shared among several forms of molecular motion. For a gas the formula is the square root of γRT/M: it depends on temperature, on the molecular mass and on the ratio of specific heats, and on nothing else. Pressure and altitude on their own do not enter.

Warmer air means faster molecules and faster sound. This is why the speed of sound falls with altitude in the lower atmosphere, not because the air is thinner but because it is colder. In the international standard atmosphere it drops from 340 m/s at sea level (15 °C) to 295 m/s at 11 km (−56.5 °C), then stays constant through the lower stratosphere where temperature is uniform. For aviation this has a practical consequence: an aircraft at a fixed true airspeed has a higher Mach number the higher it climbs, which is why airliners reach their cruise Mach limit at altitude. Lighter gases carry sound faster; in helium it is close to 1,000 m/s, which is what makes a voice sound high-pitched after breathing it.

Light is an electromagnetic wave. It needs no medium and propagates at 299,792 km/s, almost 900,000 times faster than sound in air. That is the difference between seeing the lightning and hearing the thunder three seconds later per kilometre of distance. Denser, stiffer media pass a push along more efficiently: sound travels at about 1,480 m/s in water and around 5,900 m/s in steel, because the molecules are bound together and respond almost as a unit rather than through random collisions.

On a human scale, 343 m/s is not slow: it is 1,235 km/h, roughly the speed of a rifle bullet. It only seems slow next to light, or next to modern aircraft. The fastest air-breathing manned aeroplane, the SR-71, cruised at over three times the speed of sound, and re-entering spacecraft exceed it more than twentyfold. The speed of sound is slow only in the sense that engineering has learned how to outrun it.