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| The '''speed of sound''' c varies depending on the medium through which the ] waves pass. It is usually quoted in describing properties of substances (e.g. see the article on ]). | |||
| More commonly the term refers to the speed of sound in ]. The ] affects very little the speed of sound nor does it the sound pressure, but more important is the temperature. An approximate speed (in metres/second) can be calculated from: | |||
| :<math>c_{\mathrm{air}} = (331{.}5 + 0{.}6 \ \cdot \vartheta) \ \mathrm{m/s}</math> | |||
| where <math>\vartheta</math> (theta) is the temperature in degrees ] or centigrade. | |||
| A more accurate expression is | |||
| :<math>c = \sqrt{\gamma RT}</math> | |||
| where ''R'' is the ] (287 J/kgK for air), γ is the ] (1.4 for air), and ''T'' is the absolute temperature in ]. In the standard atmosphere, ''T''<sub>0</sub> is 298.15 K, giving a value of 346 m/s (25°C = 77°F). | |||
| In fluids, using the theory of ], the speed of sound can be calculated using | |||
| :<math>c = \sqrt{{\gamma p}\over\rho}</math> | |||
| This is correct for adiabatic flow; Newton famously used isothermal calculations and omitted the γ from the numerator. | |||
| The speed of sound is typically measured given a "standard atmosphere". Under these conditions the speed of sound is approximately 346 ] at 25°C, or 750 miles/hour. | |||
| In solids the speed of sound is given by: | |||
| :<math>c = \sqrt{\frac{E}{\rho}}</math> | |||
| where ''E'' is ] and ρ is ]. Thus in ] the speed of sound is approximately 5100 m/s.<br> | |||
| For air, see ]. | |||
| The speed of sound in water is of interest to those mapping the ] floor. In saltwater, sound travels at about 1500 m/s and in freshwater 1435 m/s. These speeds vary due to pressure, depth, temperature, salinity and other factors. | |||
| See also ]. | |||