Sound, like heat and light, is a form of energy. Sound occurs when a sound source (e.g. a whistle, a drum, a tuning fork or the vocal cords) is set into vibration. The vibrating surface of the sound source moves back and forth and disturbs the particles of air. The vibration is passed across the medium (Figure 14.1) as a series of compressions (areas of high pressure) and rarefactions (areas of low pressure). The sound wave represented as a diagram of the changing sound pressure over distance can be seen in Figure 14.2 (b).