Noise and vibration are two of the main drawbacks with fluid power  systems. The increasing requirements concerning working environment as well as machines' impact on surroundings put components and systems to harder tests. The surrounding machines, e.g. combustion engines, have made considerable progress regarding the radiated noise. This allows the fluid power system's noise to become more prominent. Noise from fluid power systems has been a research topic for several decades and much improvement has been achieved. However, considerable potential for improvement still remains.

In addition to the legislation governing working environment, the machines tend to be used as more multi-quadrant machines, which require more flexible noise reduction features. One of the main benefits with fluid power is the high power density. To increase this value even more, the system's working pressure increases, which correlates with increased noise level.

The main source of noise is considered to be the pump and motor unit in the fluid power system. The noise can be divided into two parts: fluid-borne noise and structure-borne noise. The fluid borne noise derives from flow pulsation which is subsequently spread through pipeline systems to other parts of the fluid power systems. The flow pulsation is created due to the finite stiffness of oil and the limited number of pumping elements. The structure-borne noise generates directly from pulsating forces in the machine. The pulsating forces are mainly created by the pressure differences between high and low pressure ports.

Effective and accurate tools are needed when designing a quiet pump/motor unit. In this thesis simulation based optimisation is used with different objective functions including flow pulsation and pulsating forces as well as audible noise. The audible noise is predicted from transfer functions derived from measurements. Two kinds of noise reduction approaches are investigated; cross-angle in multi-quadrant machines and non-uniform placement of pistons. The simulation model used is experimentaly validated by source flow measurements. Also, source flow measurements with the source admittance method are investigated.

In addition, non-linear flow through a valve plate restrictor is investigated and the steady state restrictor equation is proposed to be extended by internal mass term.