Side branch resonator

Liquid-filled, quarter-wavelength, side branch accumulators reduce pulsations in a narrow frequency band and can be effective on constant-speed positive displacement pumps. However, in variable-speed systems, accumulators with or without a bladder can be made more effective by partially charging them with a gas (nitrogen or air) since the gas charge cushions hydraulic shocks and pulsations [1].

[1] J. C. Wachtel, F. R. Szenasi, 
Section 3.6, Displacement Pump Performance, Instrumentation, and Diagnostics, pp. 3.113