Longitudinal Waves and Tuning Forks
Sound waves are produced by vibrating objects. Whether it be the sound of a person's voice, the sound of a piano, the sound of a trombone or the sound of a physics book slamming to the floor, the source of the sound is always a vibrating object.
A tuning fork serves as a useful illustration of how a vibrating object can produce sound. The fork consists of a handle and two tines. When the tuning fork is hit with a rubber hammer, the tines begin to vibrate. The back and forth vibration of the tines produce disturbances of surrounding air molecules. As a tine stretches outward from its usual position, it compresses surrounding air molecules into a small region of space; this creates a high pressure region next to the tine. As the tine then moves inward from its usual position, air surrounding the tine expands; this produces a low pressure region next to the tine. The high pressure regions are known as compressions and the low pressure regions are known as rarefactions. As the tines continue to vibrate, an alternating pattern of high and low pressure regions are created. These regions are transported through the surrounding air, carrying the sound signal from one location to another.
In solids, sound can exist as either a longitudinal or a transverse wave. But in mediums which are fluid (e.g., gases and liquids), sound waves can only be longitudinal. The animation above depicts a sound wave as a longitudinal wave. In a longitudinal wave, particles of the medium vibrate back and forth in a direction which is parallel (and anti-parallel) to the direction of energy transport. In the animation above, the energy is shown traveling outward from the tuning fork - from left to right. The air molecules are vibrating about a fixed position from left to right and from right to left. This is what makes a sound wave a longitudinal wave.
There is another important characteristic of waves depicted in the animation above. A careful inspection of the particles of the air (represented by dots) reveal that the air molecules are nudged rightward and then move back leftward to their original position. Air molecules are continuously vibrating back and forth about their original position. There is no net displacement of the air molecules. The molecules of air are only temporarily disturbed from their rest position; they always return to their original position. In this sense, a sound wave (like any wave) is a phenomenon which transports energy from one location to another without transporting matter.
For more information on physical descriptions of waves, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:
What is a Wave?
Sound as a Mechanical Wave
Sound as a Longitudinal Wave
Sound as a Pressure Wave
Pitch and Frequency
Forced Vibration and Resonance