Positive displacement pumps do not produce an absolutely steady flow, but produce a flow ripple related to the pumping mechanism and fluid properties. This flow ripple interacts with the hydraulic circuit in a complex way to generate pressure ripple or fluid-borne noise (FBN). This pressure ripple is highly dependent upon the circuit impedance and the pump 'source impedance'. However the pump flow ripple is independent of the circuit and is a unique pump signature.

The source impedance depends on the properties of the fluid in the pump's delivery passageway, and is a combination of compressibility, inertia, leakage, friction and wave effects.

The pump flow ripple and impedance can be measured accurately using the FBN2 package.

Pump flow ripple can be used for:

• noise rating. Relative noise levels of different pumps can be compared.
• analyzing the pumping dynamics. Flow ripple measurements have been used to provide very detailed information about the dynamic behaviour of a wide range of pumps, leading to design improvements.
• predicting system noise. Flow ripple and impedance results from FBN2 can be imported directly into the PRASP program for simulating pressure ripple in circuits. This is a powerful system design tool.