Above are some typical measured flow ripple waveforms for an axial piston pump, at 40 bar (red) and 120 bar (blue). The most striking feature is the reverse flow spike which occurs repeatedly when the delivery port starts to open and the fluid in the cylinder is suddenly pressurised. This increases with pressure. Also apparent is a smaller ripple due to the sinusoidal motion of the pistons.

This pump contained no relief (silencing) grooves in the portplate and was consequently very noisy. The reverse flow spikes could be reduced by the use of relief grooves. Flow ripple measurements such as the above could be extremely useful in the design of relief grooves; the effect on the reverse flow spikes could be examined in detail.

The corresponding amplitude spectra are shown below. These measurements were taken up to 3.5 kHz and a large number of significant harmonics are apparent.