Speed of sound

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The speed of sound is the rate of travel of a sound wave through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343 meters per second (1,125 ft/s). This equates to 1,236 kilometers per hour (768 mph), or about one kilometer in three seconds and about one mile in five seconds. This figure increases with temperature (equations are given below), but is nearly independent of pressure or density for a given gas. For different gases, the speed of sound is dependent on the mean molecular weight of the gas, and to a lesser extent upon the ways in which the molecules of the gas can store heat energy from compression (since sound in gases is a type of compression). The speed of sound in air is referred to as Mach 1 by aerospace physics.

Although "the speed of sound" is commonly used to refer specifically to the speed of sound waves in air, the speed of sound can be measured in virtually any substance. Sound travels faster in liquids and non-porous solids (5,120 m/s in iron) than it does in air, traveling about 4.3 times faster in water (1,484 m/s) than in air at 20 degrees Celsius.

Additionally, in solids, there occurs the possibility of two different types of sound waves: one type (called "longitudinal waves" when in solids) is associated with compression (the same as all sound waves in fluids) and the other is associated with shear stresses, which cannot occur in fluids. These two types of waves have different speeds, and (for example in an earthquake) may thus be initiated at the same time but arrive at distant points at appreciably different times. The speed of compression-type waves in all media is set by the medium's compressibility and density, and the speed of shear waves in solids is set by the material's stiffness, compressibility and density.

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