How fast does sound travel?

Sound is a pressure wave transmitted by molecules colliding with their neighbors, so its speed depends on the medium rather than on the source. In dry air at 20 degrees Celsius it travels about 343 meters per second, roughly 767 miles per hour, which is where the mile-every-five-seconds rule for thunder comes from. Temperature matters because warmer molecules move faster and transmit the disturbance more quickly, so sound is measurably slower on a cold day. Air pressure and humidity have much smaller effects, and altitude changes the speed principally through temperature.

Denser and stiffer media generally transmit sound faster because the particles are closer together and more strongly coupled. Sound travels roughly four times faster in water than in air, about 1,480 meters per second, and around 5,000 meters per second in steel. This is why you can hear a distant train through a rail before hearing it through the air, and why whale calls and sonar work over enormous ocean distances. In a vacuum there is nothing to transmit the wave, which is why space is silent.

The speed defines the sound barrier. An aircraft approaching it compresses the air ahead of it into a shock wave, and passing through produces a sonic boom, which is a continuous cone trailing the aircraft rather than a single event at the moment of transition. Chuck Yeager first exceeded it in level flight in 1947. Mach number expresses speed as a multiple of the local speed of sound, which is why Mach 1 corresponds to different absolute speeds at different altitudes and temperatures. Overland supersonic flight is restricted in the United States and elsewhere because of the boom, which is the constraint current quiet supersonic research aims to overcome.