Optimal configuration of adjustable noise suppressors
Gruber, Elliott Ross
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Noise generated by fluid power applications can be treated using bladder-style suppressors, and an optimal operating condition for these devices is sought in this thesis. Bladder-style suppressors employ a compliant nitrogen-charged bladder to create an impedance change within the system, reflecting the noise back to the source and preventing it from propagating downstream. The noise in a hydraulic system is created by a pump, the flow source in a hydraulic system, and can be separated into three categories: fluid-borne noise, structure-borne noise and airborne noise. Fluid-borne noise places addition stress on sealing surfaces, potentially causing leaks. Airborne noise can be uncomfortable, even hazardous depending on the level. Bladder-style suppressors primarily treat fluid-borne noise; however, it is seen in the literature that fluid-borne noise is the cause of structure-borne and airborne noise. This thesis presents an optimization method for finding the optimal charge pressure for implementation with a given system operating over a broad range of system pressures. The optimization weights suppressor performance by the spectral content of the fluid-borne noise as well as the duty cycle of the system. A single charge pressure works well over a small range of system pressures, though many fluid power applications operate over a larger range of system pressure than the usable range of a suppressor. For systems operating over an extremely broad pressure range, two suppressors charged to different pressures are used to treat the noise in the entire system pressure range. To determine suppressor performance experimental measurements were performed, and models developed, of the transmission loss of this type of device. A multi-microphone method using transfer function relationships between six sensors determines the transmission loss of the suppressor under test. An equivalent fluid model modeling the wave behavior both upstream and downstream, as well as within the suppressor, was created to predict suppressor transmission loss. Optimal configurations are found for a set of system pressures, charge pressures and duty cycles. Analysis of the results shows the time weighting has a more significant impact on the optimum charge pressure than the frequency weighting, as shown by duty cycles considered in this thesis. In addition, all charge pressures selected as optimal for either single suppressor optimizations or double suppressor optimizations, exhibit the highest transmission loss for a single system pressure in the pressure duty cycle for a simulated machine.