More commonly the term refers to the speed of sound in air. In this case the physical properties of the air, its pressure and humidity for instance, affect the speed. An approximate speed (in metres/second) can be calculated from:
A more accurate expression is
In fluids, using the theory of compressible flow, the speed of sound can be calculated using
The speed of sound is typically measured given a "standard atmosphere". Under these conditions the speed of sound is approximately 343 m/s, or 750 miles/hour.
In solids the speed of sound is given by:
The speed of sound in water is of interest to those mapping the ocean 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 Mach number.