All sources of sound have some part of them which is vibrating, for example, a violin string or the surface of a drum. Sound travels in the form of longitudinal waves, that is, molecules vibrate to and fro in the direction of travel of the sound (see Section 13.4). When sound is heard, energy is carried from the source of the waves to the ear of the listener; but the molecules of air in between do not move as a whole towards the listener (see Section 13.1). At any instant there are regions where the air is compressed (compressions), separated by regions where the air is rarefied (rarefactions or decompressions). Sound waves consist of a series of alternate compressions and rarefactions travelling away from a source at a certain speed determined by the nature of the medium in which they flow.
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