and elasticity of the medium. In general, the speed of sound in a medium is given by: where d = density of the medium and E = the bulk modulus of elasticity of the medium (the higher E, the more difficult the medium is to compress). Clearly, the equation above suggests that sound travels fastest in media which are least compressible and which have low density. At first glance this statement seems to disagree with our assertion that sound travels faster in solids than in liquids and faster in liquids than in gases, since in general gases have lower densities than liquids and liquids have lower densities than solids.  This apparent contradiction is resolved when we examine the relative elasticity values of the different states of matter.  In general, solids have much higher values of elasticity than liquids, which in turn have much higher values of elasticity than gases.  In fact, it is the elasticity values that have the main effect on the speed of sound in materials.  Thus, the above equation supports our assertion that sound travels faster in solids than in liquids and faster in liquids than in gases.  The equation shows, however, that where two materials have similar elasticity values, the relative densities of the two materials will determine which material provides the higher speed of sound.  For example, sound travels almost three times as fast in helium than it does in air at the same temperature mainly due to the much lower density of helium compared