The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 meters per second (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn), or one kilometer in 2.9 seconds or one mile in 4.7 seconds. It depends strongly on temperature as well as the medium through which a sound wave is propagating. At 0 °C (32 °F), the speed of sound is about 331 m/s (1,086 ft/s; 1,192 km/h; 740 mph; 643 kn).

In colloquial speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance: typically, sound travels most slowly in gases, faster in liquids, and fastest in solids. For example, while sound travels at 343 m/s in air, it travels at 1,481 m/s in water (almost 4.3 times as fast) and at 5,120 m/s in iron (almost 15 times as fast). In an exceptionally stiff material such as diamond, sound travels at 12,000 metres per second (39,000 ft/s), about 35 times its speed in air and about the fastest it can travel under normal conditions.

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