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Sound Waves: Characteristics and Applications 189 You observe regions in the slinky where its turns are closer together than usual, and other regions where they are more spread out. These closely spaced and spread out regions appear to travel along the slinky. However, the mark does not travel along the slinky, it only oscillates about its position of rest parallel to the direction of the disturbance. Each turn oscillates about its own position while the disturbance travels from one end of the slinky to the other. Sound moves in a similar way through a medium. In our analogy, the closeness or spreading out of the turns of the slinky represents the higher or lower density of air through which sound is travelling. Let us consider a long tube filled with air that has a piston at one end and is open at the other. The piston can be made to oscillate back and forth inside the tube. Here, the oscillating piston is used as a simple and idealistic model of the source of sound to help understand how sound travels through air. (a) Piston not oscillating (average air density in the When the piston is not oscillating, the air entire tube) inside the tube has a certain uniform density, C which we call the average density (Fig. 10.9a), where the dots represent the air particles. As the piston moves forward, it pushes and displaces the nearby air particles in a forward direction, (b) Forward motion of piston produces a compression (region of air with a higher density) thereby compressing the air and increasing R C its density in a small region close to the piston (Fig. 10.9b). This region of air with higher density (compared to the average density) is (c) Backward motion of piston produces a rarefaction called a compression (C). (region of air with a lower density) The compressed air particles collide with the C R C R particles further ahead passing the compression forward. The compressed particles further collide with their neighbouring particles, and so on. As a result, the higher density compression (d) Oscillating piston produces alternate compressions and rarefactions moves forward through the air even though the Fig. 10.9: Density of air in a tube with a piston air particles themselves do not travel with it. As the piston moves backwards, the air particles move backwards in the direction of the piston and the air in a small region close to the piston becomes less dense (Fig. 10.9c). This region of air with lower density (compared to the average density) is called a rarefaction (R). Again, due Note to collisions with nearby particles, the rarefaction moves ahead while the The particles particles themselves oscillate only about their mean positions. of the medium As the piston oscillates in forward and backward directions, do not travel with the wave. compressions and rarefactions are produced alternately (Fig. 10.9d). These They just travel away from the source, and a series of compression and rarefaction are produced. vibrate about their mean The disturbance consisting of a series of alternating compressions and positions. rarefactions propagating through a medium, without the actual flow of the particles of medium, is called a sound wave. The direction in which the wave travels is known as the direction of propagation of the wave.