This paper discusses the application of plane wave theory to the modelling of pressure ripples generated by positive displacement pumps in high-pressure hydraulic systems. These ripples can lead to structural vibration of pipework and associated components and hence result in airborne noise. A clear understanding of the mechanism of pressure ripple generation is essential in the development of quiet systems. The characteristics of the pump and the circuit components have a considerable influence on the amplitude of the pressure fluctuations. Methods of evaluating these characteristics are described and typical results presented. It is shown that, from a knowledge of these characteristics, the pressure ripple levels at any position in a circuit may be accurately determined using plane wave theory.