ARISES in fields like acoustics, electromagnetics, and fluid dynamics. Historically, the problem of a vibrating STRING such as that of a MUSICAL INSTRUMENT was STUDIED by Jean le Rond d Alembert, Leonhard Euler, Daniel Bernoulli, and Joseph-Louis Lagrange. In 1746, d Alembert discovered the one-dimensional wave equation, and within ten years Euler discovered the three-dimensional wave equation.