Question

Why does sound travel faster in solids than in liquids, and faster in liquids than in gases (air)?

Asked by: Will K

 Answer

Sound is nothing more than a local disturbance whose propagation is facilitated by the collisions between particles; this disturbance propagates in a logitudnal wave; imagine one molecule hitting the next molecule, and then that molecule hitting the next, and so forth.

The distances between molecules in solids are very small, i.e., solids are more dense - as compared to liquids and gases. Because they are so close, than can collide very quickly, i.e. it takes less time for a molecule of the solid to 'bump' into its neighborough. Solids are packed together tighter than liquids and gases, hence sound travels fastest in solids. The distances in liquids are shorter than in gases, but longer than in solids. Liquids are more dense than gases, but less dense than solids, so sound travels 2nd fast in liquids. Gases are the slowest because they are the least dense: the molecules in gases are very far apart, compared with solids and liquids.

Answered by: Jonathan Apple


If one solves the wave equation for the propagation of sound, one finds that the square of the sound velocity is proportional to the ratio of an elastic modulus to the mass density of the material. Therefore, by a simple density argument, one should conclude that the sound velocity is higher in gases than in solids and liquids which, of course, is not true. The reason why the sound velocity is usually larger in solids than in liquids and usually larger in liquids than in gases is because of the elastics constants of the material.

What determines the elastic constants of a material is the interatomic bond strength. The stronger the bond, the higher the elastic constants. In gases, the atoms are very weakly bonded together and the elastic constants are very low. In solids, the atoms are more tightly bonded together, and the elastic constants are higher, ... that is most of the time. In some cases, some elastic constants of solids can drop to nearly zero and the (shear) sound velocity can reach nearly zero. This can happen, for example, near a structural phase transformation.

Answered by: André Moreau, Canada

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