Sound waves travel in the air in a manner somewhat similar to the way water waves travel from a point of disturbance. As you may have observed, when a stone is dropped into still water, waves move to outward from the point of disturbance in the form of ever-enlarging circles; sound waves in air, on the other hand, travel in all directions from a vibrating substance in ever-enlarging spherical layers. (The cover of a baseball is an example of a spherical layer.) The fact that a sound may be heard in all directions from its source tends to show that sound waves travel outward from their source in the form of spherical layers.
As a sound wave travels, the air vibrates but it does not travel with the sound wave. As you have already learned, when a sound-producing substance, such as a tuning fork, begins to vibrate, the layer of air next to the tuning fork is compressed. As this layer expands, it compresses a second layer of air, and so on. As a result the volume of air affected is composed of alter nating layers of compressed and rare air. The compression and rarefaction move forward, but at a given point the air merely vibrates back and forth just as the prongs of the tuning fork do. To help you understand how a compression and rarefaction travels without the air itself traveling, place. a piece of cork on water in a tub. Drop a small stone into the water to set up waves. Note that the waves move away from the place where the stone was dropped, but that the piece of cork does not move with the waves; it merely bobs up and down. The water wave travels but the water itself merely moves up and down.
If a tub of water is not available, tie one end of a piece of rope to a desk and hold the other end so that the rope is slack. By moving your hand up and down waves are produced which travel along the rope. Mark any point on the rope and notice that, although the wave travels along the rope, the marked point merely moves up and down. As a sound wave moves farther and farther from its source, a larger and larger volume of air vibrates; ‘consequently the sound becomes fainter. Finally the air vibrations become so weak that they cannot be heard.