Background Essay: Sound and Solids: Listening Stick

Sound waves transport energy from one location to another in a chain reaction. An initiating event, such as the pluck of a guitar string or a knock on the door, disturbs nearby molecules and pushes them into each other, creating a region of higher density, called a compression, and leaving a region of lower density, called a rarefaction, in its wake. In wavelike fashion, the alternating compressions and rarefactions move outward in all directions through the medium (metal, wood, air, water, or whatever is transmitting the sound) as sound waves. Waves continue to form until the source of the disturbance stops making the vibrations that generate the waves.

Sound waves move faster through a denser medium because energy is more easily passed between tightly packed molecules. This helps explain why sound travels faster through water than through air, and faster still through steel than through water. But even more influential than a conducting medium's density is its elasticity. Elasticity refers to how well a medium can return to its initial form after being disturbed by a force. Steel has high elasticity. It bounces right back to its original shape after an applied force is removed. At the particle level, the molecules in elastic materials transfer energy more efficiently, so sound waves travel faster through steel than through water or air. But not all solids are good conductors of sound. Cork, for instance, has low elasticity. Its molecules tend to absorb energy rather than conduct it.

Air, a mixture of gases, is less elastic than most solids and all liquids. Despite being the standard medium through which we hear sounds, it is actually a relatively poor conductor. For this reason, you can hear the clanging of a metal hanger against a table much more loudly when your ear is pressed to the table or when the vibrating hanger is touched to your head, that is, when the sounds are transmitted through solid objects instead of through air.