PhD Dissertation submitted to the Faculty of the Virginia Tech in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
Dr. Chris R. Fuller, Chair
Dr. Ricardo A. Burdisso
Dr. Daniel Inman
Dr. Harry H. Robertshaw
Dr. Alfred L. Wicks
Active control of sound radiation from vibrating structures has been an area of much research in the past decade. In Active Structural Acoustic Control (ASAC), the minimization of sound radiation is achieved by modifying the response of the structure through structural inputs rather than by exciting the acoustic medium (Active Noise Control, ANC). The ASAC technique often produces global far-field sound attenuation with relatively few actuators as compared to ANC. The structural control inputs of ASAC systems are usually constructed adaptively in the time domain based on a number of error signals to be minimized. One of the primary concerns in active control of sound is then to provide the controller with appropriate ``error'' information. Early investigations have implemented far-field microphones, thereby providing the controller with actual radiated pressure information. Most structure-borne sound control approaches now tend to eliminate the use of microphones by developing sensors that are integrated in the structure. This study presents a new sensing technique implementing such an approach. A structural acoustic sensor is developed for estimating radiation information from vibrating structures. This technique referred to as Discrete Structural Acoustic Sensing (DSAS) provides time domain estimates of the radiated sound pressure at prescribed locations in the far field over a broad frequency range. The structural acoustic sensor consists of a set of accelerometers mounted on the radiating structure and arrays of digital filters that process the measured acceleration signals in real time. The impulse response of each filter is constructed from the appropriate radiation Green's function for the source area associated with each accelerometer.
Validation of the sensing technique is performed on two different systems: a baffled rectangular plate and a baffled finite cylinder. For both systems, the sensor is first analyzed in terms of prediction accuracy by comparing estimated and actual sound pressure radiated in the far field. The analysis is carried out on a numerical model of the plate and cylinder as well as on the real structures through experimental testing. The sensor is then implemented in a broadband radiation control system. The plate and cylinder are excited by broadband disturbance inputs over a frequency range encompassing several of the first flexural resonances of the structure. Single-sided piezo-electric actuators provide the structural control inputs while the sensor estimates are used as error signals. The controller is based on the filtered-x version of the adaptive LMS algorithm. Results from both analytical and experimental investigations are again presented for the two systems. Additional control results based on error microphones allow a comparison of the two sensing approaches in terms of control performance.
The major outcome of this study is the ability of the structural acoustic sensor to effectively replace error microphones in broadband radiation control systems. In particular, both analytical and experimental results show the level of sound attenuation achieved when implementing Discrete Structural Acoustic Sensing rivaled that achieved with far-field error microphones. Finally, the approach presents a significant alternative over other existing structural sensing techniques as it requires very little system modeling.
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