Active control is applied to reduce noise emission from a vibrating elastic cylinder by exerting forces on the cylinder that cancel the noise-generating vibration. This technique is called Active Structural Acoustic Control (ASAC) (Fuller, 1987). Sensors are implemented using piezoelectric film, and actuators are implemented using piezoceramic material. Both analog and digital noise cancellation control algorithms are used to reduce the noise emission from the cylinder. Two-cylinder boundary conditions are taken as case studies. The first boundary condition is the open cylinder case. The second boundary condition is where the cylinder has an end plate bolted to each end. Actuator placement and the sensor design are done by first obtaining the natural frequencies and mode shapes of the cylinder using both analytical and experimental methods. Modal sensors developed and tested in previous work (Lee, 1989) are applied. After preliminary control experiments with analog feedback loop show that control can be done with the sensors and the actuators, digital signal processing hardware programmed with the filtered-x Least-Mean-Square adaptive control algorithm is used to control the vibration of the cylinder. The excitation is single-tone on-resonance. Acoustic - testing demonstrates that ASAC reduces the sound pressure level generated by the vibrating cylinder by up to 29 dB in the reverberant field. Vibration measurement reveals that the reduction in sound emission from the cylinder is a result of reduction in vibration. The adaptive controller reduces the vibration level by up to 68 dB. / Master of Science

An analytical expression for the sound pressure radiated into the far-field by a coupled plate-cylinder system is derived. The system is composed of a rigid plate mounted inside a finite length simply-supported cylindrical shell via a fixed number of active-passive mounts. A harmonic point-force disturbance is applied to the plate. Various active control approaches are applied to minimize the acoustic pressure radiated by the coupled system. The Active Structural Acoustic Control (ASAC) approaches include the control of the acoustic pressure in one or several directions of radiation, the control of the total radiated power, the control of the power radiated in a sector and the control of selected components associated with circumferential cylinder modes. The Active Vibration Control (AVC) approach is the control of the radial vibration at the points of attachment of the mounts on the cylinder. Numerical calculations show that the radiated pressure can be controlled using active-passive mounts for all these approaches. However, comparisons in terms of control efficiency and control effort show that ASAC yield better results than AVC. Moreover, ASAC enables directional control of sound and AVC does not. Opposed and parallel active-passive mount configurations are compared. The results show that the first arrangement requires much larger control forces on-resonance, but the two methods show similar performance off-resonance. / Master of Science

A double loop control scheme is developed to control broadband acoustic radiation from a cylinder. An analog feedback loop is investigated and developed to add damping to the cylinder at particular frequencies of interest. Circuitry is developed and refined to condition Polyvinylidene Fluoride Filnl (PVDF) sensor outputs as strain rate signals. The strain rate signals are used in the feedback loop to provide damping to the structure. In conjunction with the feedback loop a feedforward loop is also implemented. The feedforward loop utilizes the filtered-x LMS algorithm. The result of combining the two control laws was unknown prior to implementation.The resulting control scheme shows that the feedback control law is effective in attenuating undesirable frequency components in the feedforward error sensor. This results in an error sensor signal which is highly correlated with the disturbance. With a more correlated error signal a more effective feedforward control is achieved. The resulting control system provides acoustic control over a wide range of frequencies. The filtered-x LMS algorithm is applied to an effective acoustic radiator. The feedback loop provides for broadband control of the structure. Typical double loop controller results show power spectrum reductions of 35 dB for an effective acoustic radiator and reductions of 10 dB for other frequencies in the excitation range. In addition, the measured controlled plant transfer functions show significant reductions in the transfer of energy through the structure. Overall Sound Pressure Level (SPL) reduction in the acoustic field generated by the cylinder in response to a random excitation with a harmonic component was 4.9 dB for feedback, 18.4 dB for feedforward, and 25.2 dB for the double loop controller. / Master of Science