Similarly, sound travels about 1.41 times faster in light hydrogen ( protium) gas than in heavy hydrogen ( deuterium) gas, since deuterium has similar properties but twice the density. For instance, sound will travel 1.59 times faster in nickel than in bronze, due to the greater stiffness of nickel at about the same density. All other things being equal, sound will travel more slowly in spongy materials, and faster in stiffer ones. In a real material, the stiffness of the springs is called the elastic modulus, and the mass corresponds to the density. Effects like dispersion and reflection can also be understood using this model. The speed of sound through the model depends on the stiffness of the springs (stiffer springs transmit energy more quickly). Sound passes through the model by compressing and expanding the springs, transmitting energy to neighboring balls, which transmit energy to their springs, and so on. For real material the balls represent molecules and the springs represent the bonds between them. The transmission of sound can be illustrated by using a toy model consisting of an array of balls interconnected by springs. 7 Effect of frequency and gas composition.4 Implications for atmospheric acoustics.

3 Dependence on the properties of the medium.

The speed of shear waves, which can occur only in solids, is determined by the solid material's stiffness, compressibility and density. The speed of an elastic wave in any medium is determined by the medium's compressibility and density. Therefore, they arrive at an observer at different times, an extreme example being an earthquake, where sharp compression waves arrive first, and rocking transverse waves seconds later. These different waves (compression waves and the different polarizations of shear waves) may have different speeds at the same frequency. In general, transverse waves occur as a pair of orthogonal polarizations. A transverse wave, often called shear wave, is due to elastic deformation of the medium perpendicular to the direction of wave travel the direction of shear-deformation is called the 'polarization' of this type of wave. A longitudinal wave is associated with compression and decompression in the direction of travel, which is the same process as all sound waves in gases and liquids. In solids, sound waves propagate as two different types. It travels about 4.3 times faster in water (1,484 m/s), and nearly 15 times as fast in iron (5,120 m/s), than in air at 20 degrees Celsius. Sound travels faster in liquids and non-porous solids than it does in air. However, the speed of sound varies from substance to substance. In common everyday speech, speed of sound refers to the speed of sound waves in air. Although, in the case of gases only, the speed of sound may be expressed in terms of a ratio of both density and pressure, these quantities are not fully independent of each other, and canceling their common contributions from physical conditions leads to a velocity expression using the independent variables of temperature, composition, and heat capacity noted above. For different gases, the speed of sound is inversely dependent on square root of the mean molecular weight of the gas, and affected to a lesser extent by the number of ways in which the molecules of the gas can store heat from compression, since sound in gases is a type of compression. It is a function of the square root of the absolute temperature, but is nearly independent of pressure or density for a given gas. The speed of sound in an ideal gas is independent of frequency, but it weakly depends on frequency for all real physical situations.

Objects moving at speeds greater than Mach 1 are traveling at supersonic speeds. The speed of an object (in distance per time) divided by the speed of sound in the fluid is called the Mach number. In fluid dynamics, the speed of sound in a fluid medium (gas or liquid) is used as a relative measure of speed itself. This is 1,236 kilometres per hour (768 mph), or about one kilometer in three seconds or approximately one mile in five seconds. In dry air at 20 ☌ (68 ☏), the speed of sound is 343.2 metres per second (1,126 ft/s). The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium.

Speed of sound at altitude