From a physics point of view, sound waves are changes in pressure within an elastic medium. Individual molecules in the medium, which may be gaseous, fluid or solid, are forced out of their equilibrium position by external excitement. They then oscillate back and fore around their original position. This motion is transferred to the adjacent molecules as they collide. This causes alternate compression and rarefaction in the medium and this transmits the sound wave. If the oscillations take place in air, we talk of airborne sound. If, on the other hand, the oscillations occur in a solid body, we refer to structure-borne sound. Sound also travels well in fluids. In a vacuum there is no transmission of sound because there is no "medium". What we perceive to be sound is, in fact, nothing more than changes in air pressure superimposed on the natural air pressure.