Soun 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). 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.