Sound is one of those rare physical phenomena that has been correctly explained by the ancient Greeks. They grasped the essential by focusing on the vibrations of musical instrument cords: sound is an undulating phenomenon. Caused by the initial vibratory movement of a body (cord, diaphragm), it moves around the immediate area via the movement of successive units of air in the same way that a wave spreads along the surface of water by means of vertical oscillatory movement. Source: Flammarion science dictionary.