Why do soundwaves travel faster through solids?

Answer 1

There are two things that come into play here: elastic properties and inertial properties. Both affect propagation velocity to varying degrees. Elastic propertiesinvolve a substance's ability or tendency to return to its original shape when forces of deformation are removed. Highly elastic substances will snap back to their original shape when the deformation forces are removed. A rubber band is an example of a highly elastic object. A wad of silly putty, on the other hand, is not very elastic. (It is very plastic.) A steel girder is also highly elastic; it will flex when deformation forces are applied, and it will return to its original shape when they are removed. But steel is one thing that a rubber band is not: it's rigid. Whereas a rubber band will flex, bend, or stretch after applying very little force to it, a steel girder will greatly resist any deformation force applied to it. In general, the more rigid and elastic the substance, the faster the propagation of sound through it. Hence, a hunk of steel will have a higher propagation velocity than a hunk of silly putty. Inertial properties involve a substance's mass density. The greater the density, the slower the propagation velocity, which is why sound waves will travel much faster through, say, helium, than through air. Which provokes the question: If substances with lower density have higher propagation velocities than substances with greater density, why do solids have higher propagation velocities than liquids, which have higher propagation velocities than gases? The reason is the elastic properties of solids play a greater role than the inertial ones, and solids are generally more elastic and rigid than liquids and gases. Since liquids and gases are inelastic and non-rigid, the mass density is the key property that affects propagation velocity through them.