The use of hydraulic units to power tools, lifts, and other industrial equipment in both indoor and outdoor settings is widespread. The noise radiated by these units and the hydraulic lines used to convey the fluid continues to be a problem. The dominant noise components arise from pressure fluctuations in the hydraulic fluid ocurring at harmonics of the pump vane or piston passage frequency. In this study, the effectiveness of in-line tuning cables (after a patent by Klees 1967) for quieting the harmonic noise from these hydraulic lines is determined. This approach, which combines ease of installation with relative economy, amounts to the insertion of a quarter-wavelength side branch into the acoustical circuit. Critical to effective design is the determination of the wave speeds in the flexible hosing. Theoretical predictions and measurements are applied to determine the wave speeds in the fluid-loaded hydraulic line. Multiple PVDF sensors measure wave speeds and the wave amplitude insertion loss achieved by the device. Experiments performed on a full-scale 60 h.p., 1500 p.s.i. unit will be compared to the theory. Sound pressure levels determine design effectiveness.