Sound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time. In dry air at 20 °C (68 °F), the speed of sound is 343 m/s (1235 km/h, or 770 mph, or 1129 ft/s, or approximately 5 seconds per mile).

Although it is commonly used to refer specifically to air, the speed of sound can be measured in virtually any substance. Sound travels faster in liquids and non-porous solids than it does in air.