Sound is a type of longitudinal, mechanical wave. They need a medium (the elasticity of the molecules) to propagate and will not travel through a vacuum. Sound travels at different speed in different media. The speed of sound is determined by the density and compressibility of the medium. Density is the amount of material in a given volume, and compressibility is a measure of how much a substance could be compacted for a given pressure. The denser and the more compressible, the faster the sound waves would travel. Water is denser than air therefore the molecules through which sound travels are closer allowing sound to go faster.