Active control of sound radiation due to subsonic wave scattering from an infinite or a finite fluid-loaded plate excited below the critical frequency is analytically studied. The disturbance is caused by a flexural wave in an infinite plate, or by a point force on a finite plate at subsonic frequencies. The wave scattering is caused by discontinuities on the plate or by the boundary conditions. A feed-forward control approach is applied by implementing either point/line forces or piezoelectric actuators on the plate. The amplitude and phase of control forces are determined by the optimal solution of a cost function which minimizes the far-field radiated acoustic power over a prescribed surface in the half space of the fluid field. The results show that for subsonic excitations, high global reduction in radiated pressure is possible with properly located active control forces. The number and location of control forces employed in order to obtain high control performance are related to the excitation frequency. The far-field sound radiation directivity pattern, the modal amplitudes of the plate vibration, the plate vibration autospectrum in the wave number domain, and the near-field intensity distribution are extensively studied in order to uncover the mechanisms of control. / Ph. D.

This study presents a basis for the analytical and experimental procedures as well as design techniques required in achieving adaptive structures for active structural acoustic control (ASAC). Test structures studied in this work included a baffled simply supported beam and a baffled simply supported plate which were subjected to a harmonic input disturbance created physically with a shaker and modelled by a point force input. Structural acoustic control was achieved with piezoelectric actuators bonded to the surface of the test structure. The primary focus of this work was devoted to studying alternative sensing techniques in feed forward control applications. Specifically, shaped distributed structural sensors constructed from polyvinylidene fluoride (PVDF), distributed acoustic near-field sensors constructed from PVDF, and accelerometers were explored as alternatives to microphones which are typically implemented as error sensors in the cost function of the control approach. The chosen control algorithm in this study was the feed forward filtered-x version of the adaptive LMS algorithm. A much lower level of system modelling is required with this method of control in comparison to state feedback control methods. As a result, much of the structural acoustic coupling (i.e. system modelling) must be incorporated into the sensor design. / Ph. D.

Despite many advances in aviation noise control over the past 50 years, the industry is continually striving to reduce noise emissions. Turbofan engine acoustic liners are efficient attenuators of engine noise. Plasma actuators have been used as flow control devices in other settings and will now be studied as an enhancement for acoustic liners. A plasma actuator can excite oscillatory flow or a single direction (bias flow). Both flow types are studied as possible means to excite turbofan liners in order to improve the acoustic performance. Experiments revealed the oscillatory flow as the dominant factor in controlling resonator performance. The phase control of the actuator signal is an important parameter when dealing with the oscillatory flow. The actuator is first applied to a single resonator and then a set of six resonators. The experiments show that with the correct phase, the actuators improved the performance of a single resonator by 3 dB to 5 dB. The results for the array of actuators/resonators mirror the results of a single device. Beyond the improvements in performance, a number of other factors affect the usefulness of the plasma actuator technology in a turbofan environment. The ability of the actuator to produce plasma is susceptible to small imperfections in the device, and this property will likely be amplified in a perforated sheet with embedded actuators. Additional weight and energy consumed by the actuators is another factor to consider. Finally, plasma actuator operation produces ozone, so environmental effects deserve consideration as well. / Master of Science

Noise from fluid flow over rough surfaces is an important consideration in the design and performance of certain vehicles with high surface-area-to-perimeter ratios. A new method of noise control based on the anatomy of owls is developed and consists of fabric or fibrous canopies suspended above the surface. The method is tested experimentally and is found to reduce the total far-field noise emitted by the surface. The treatment also is found to reduce the magnitude of pressure fluctuations felt by the underlying surface by up to three orders of magnitude. Experimental investigations into the effects of geometric parameters of the canopies lead to an optimized design which maximizes noise reduction. The results obtained during the canopy experiment inspired a separate new device for the reduction of trailing edge noise. This type of noise is generated by flow past the wing of an aircraft or the blades of a wind turbine, and is a source of annoyance for those in surrounding communities. The newly developed treatment consists of small fins, or "finlets," placed near the trailing edge of an airfoil. The treatment is tested experimentally at near-full-scale conditions and is found to reduce the magnitude of far-field noise by up to 10 dB. Geometric parameters of the finlets are tested to determine the optimal size and spacing of the finlets to maximize noise reduction. Follow-up computational and experimental studies reveal the fluid mechanics behind the noise reduction by showing that the finlets produce a velocity deficit in the flow near the trailing edge and limit the magnitude and spanwise correlation lengthscale of turbulence near the trailing edge, factors which determine the magnitude of far-field noise. In a final experiment, the finlets are applied to a marine propeller and are found to reduce not only trailing edge noise, but also noise caused by the bluntness of the trailing edge. The results of this experiment show the potential usefulness of finlets to reduce noise from rotating systems, such as fans or propellers, as well as from structures which feature blunt trailing edges. / Ph. D.

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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

Active control of noise and vibration has been previously demonstrated in finite and infinite systems undergoing single and multiple-frequency excitations. Control of broadband noise and vibration has also been reported, but it tends to be limited to infinite and semi-infinite systems. Here, four new adaptive feedforward control algorithms were developed for attenuating the response generated by finite structural systems. The algorithms are based on the filtered-X Least Mean Square (LMS) adaptive algorithm. A system identification of the plant control loop is required to implement this algorithm. An autoregressive moving-average (ARMA) model was used for the system identification since it provides the most computationally-efficient means of representing the frequency response function (FRF) of a lightly-damped structure. In the first control system, an adaptive finite impulse response (FIR) or nonrecursive filter was used as the compensator. A second control approach was realized by employing a recursive compensator. These two algorithms were modified using an equation error minimization technique to form two additional control systems, which eliminate certain stability requirements of the ARMA system identification. Each algorithm was simulated and then demonstrated experimentally. Lastly, an analysis of control system causality was developed to determine the importance of this topic with regard to controlling finite structural systems. An exemplary parametric study of one of the four control systems presented, will demonstrate the analytical tool by examining the effects of system damping, compensator order, and a time delay in the control path, which is responsible for acausal control solutions. It was determined that control is always achievable, despite a delay in the control path, and also that control system performance can be improved by increasing the order of the control compensator. Both of these results were verified experimentally. / Master of Science

This research project was aimed at investigation of speech communication issues in industrial noise environments where workers utilize hearing protection devices (HPDs). A controlled empirical study was conducted to determine the effects of several independent variables on speech reception and discrimination including: l). subject's hearing configuration (unoccluded or earplug, earcap, earmuff-occluded), 2). ambient noise intensity level (60, 83 dBA), 3). ambient noise spectral type (low, white approximation, high frequency), 4). speaker's voice level (63 or 65 dBA in 60 dBA noise, 82 or 88 dBA in 83 dBA noise), and 5). subject's hearing level (normal hearing, slight loss, or moderate loss) used as a blocking variable. Isophonemic word discrimination, with male-voiced word lists presented through loudspeakers in an anechoic field, served as the experimental task. Twenty-three males and twenty-two females participated in the experiment and a mixed-factors, partial hierarchical design was used for data collection. Analysis of variance and Newman-Keuls multiple-range tests were applied to the data. All main effects, with the exception of hearing level blocks, were significant, in addition to several interactions. These are discussed in detail and depicted graphically. One fundamental finding was that none of the hearing protection devices degraded speech discrimination (in comparison to an unoccluded condition) in the 83 dBA ambient noise level. In fact, the most protective HPD significantly enhanced speech discrimination in the high noise level. In the low ambient noise level, there was some reduction in discrimination due to the wearing of an HPD, but this effect is not of concern because HPDs are not needed at low ambient levels for protection purposes. From the results, it appears that properly selected HPDs can be expected to at least maintain speech discrimination levels (equivalent to unoccluded levels) in moderately-high intensity industrial noises of varied spectral characteristics. / Master of Science

Adaptive FeedforWard algorithms have been successfully applied in the active control of sound and vibrations~ Current approaches, such as the Filtered-X Least Mean Square algorithm, are limited in adaptability and applicability by the computationally intensive system identification process. The Time Average approach is a technique that promises to be a major advance in the active control of sound and vibration. This approach is based on statistical estimates of the error signals, and does not require having previous knowledge of the system. A new minimization algorithm has been developed, which enhances the performance of the Time Average controllers. A new technique to estimate the cost function is also presented. This technique is called the Real Time Cost Function Evaluation method. Based upon these new controller design concepts, single channel and multi-channel controllers were developed and experimentally tested. The experiments included control of a harmonically excited, simply supported beam, and the control of inlet tonal noise radiated from a JT1SD-l turbofan engine. / Master of Science