Sound waves transport energy from one location to another in a kind of chain reaction. An initiating event disturbs nearby molecules and pushes them into each other. In wavelike fashion, alternating regions of higher density (compressions) and lower density (rarefactions) move outward in all directions through a solid, liquid, or gas medium. As with other types of waves, such as surface waves in water, it's not the medium or material carrying the waves that moves outward; rather, what moves is the disturbance in the material itself. The material then returns to the position it was in before the wave came by.

Sound waves move faster through a denser medium partly because energy is more easily passed, or conducted, between tightly packed molecules. This helps explain why the speed of sound in water is about five times faster than the speed of sound in air. Interestingly, sound can't travel at all in space because space is a vacuum containing no molecules, or almost none. Electromagnetic waves, such as light and radio waves, can travel through empty space because, unlike sound, they are self-sustaining, and no material or medium must vibrate for them to exist.

Perhaps even more influential than a conducting medium's density is its elasticity. Elasticity refers to how well a medium can return to its original form after being disturbed. Generally speaking, the stronger the bond between a medium's molecules, the higher its elasticity. At the particle level, molecules bounce right back to their original positions after an applied force is removed. Because the molecules in such highly elastic materials as steel transfer energy more efficiently, sound waves travel at higher speed through them.

The fact that water is a relatively good sound conductor explains why many marine animals have adapted to life underwater in ways that emphasize sound. Dolphins and whales, probably more than any other marine animals, have developed a sophisticated use of sound. Not only do they hear extremely well, but many have also evolved vocalizations specialized for communication, navigation, and locating food. Fish also make sounds underwater.