The speed of shear waves is determined only by the solid material's shear modulus and density. The speed of compression waves in solids is determined by the medium's compressibility, shear modulus and density. Shear waves in solids usually travel at different speeds than compression waves, as exhibited in seismology. Sound waves in solids are composed of compression waves (just as in gases and liquids), and a different type of sound wave called a shear wave, which occurs only in solids. 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. 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). However, the speed of sound varies from substance to substance: typically, sound travels most slowly in gases, faster in liquids, and fastest in solids.
In colloquial speech, speed of sound refers to the speed of sound waves in air. The speed has a weak dependence on frequency and pressure in ordinary air, deviating slightly from ideal behavior. The speed of sound in an ideal gas depends only on its temperature and composition. It depends strongly on temperature as well as the medium through which a sound wave is propagating. At 20 ☌ (68 ☏), the speed of sound in air is about 343 metres per second (1,125 ft/s 1,235 km/h 767 mph 667 kn), or one kilometre in 2.91 s or one mile in 4.69 s.
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium.
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