No description available.

A novel noise control system is developed using adaptable tuned vibration absorbers (ATVA) to interact with a vibrating host structure in such a way as to reduce radiated acoustic energy. ATVA's are single-degree-of-freedom resonant devices that can change their resonant frequency and damping over a range. This ATVA noise control system is targeted at applications with tonal disturbances such as propeller aircraft. The motivation for this work is to better understand and experimentally demonstrate the noise control performance of globally detuned vibration absorbers (i.e. tuned away from the disturbance) compared to that of perfectly tuned devices on complex structures. A two-tier hierarchical control approach is used where a global control algorithm attempts to minimize a global parameter such as radiated acoustic energy by directing the adaptation of subordinate ATVA's. The global control algorithm uses an adaptive simplex search algorithm that requires no initial knowledge of the structure or the ATVA's. The ATVA's also require no model of the structure, each utilizing only the local vibration of its own mass and control gains set by the global controller. Noise control using a single ATVA is first studied on a small simply supported plate. Then, a multiple ATVA system is tested on a large plate structure at several test frequencies where many structural modes participate. Noise reductions up to 22 dB are achieved at locations in the radiated field. Further, it is found in some cases, classic tuning of the ATVA results in increased structural noise radiation. ATVA's are realized by outfitting typical inertial (proof-mass) actuators with a classical feedback loop. The device's resonant frequency and damping can be controlled independently, yet simultaneously via two control gains. The ATVA's are designed, built, and characterized for their adaptable domain and power requirements. A cohesive analytical model of the ATVA is also developed and used to compliment the experimental results. / Ph. D.

Reduction of surface pressure fluctuations is desirable in various aerodynamic and hydrodynamic applications. Over the past few years, studies on canopy surface treatments have been conducted to investigate the fundamental mechanisms of surface pressure attenuation termed as pressure shielding. This work talks about the design, development and experimental testing of unidirectional canopy surface treatments which are evenly spaced arrays of streamwise rods placed parallel to the wall without an entrance condition. The canopy designs are based on surface treatments tested by Clark et al. (2014) inspired by the downy coating on owl wings. The main objective of the work is to establish fundamental physical and mathematical basis for treatments that shield aerodynamic surfaces from turbulent pressure fluctuations, while maintaining the wall-normal transport of momentum and low aerodynamic drag. Experimental testing of these canopy treatments are performed in the Anechoic Wall-Jet facility at Virginia Tech. Different canopy configurations are designed to understand the effect of various geometric parameters on the surface pressure attenuation. The treatment is found to exhibit broadband reduction in the surface pressure spectrum. Attenuation develops in two frequency regions which scale differently depending on two different mechanisms. Canopies seems to reduce the large-scale turbulent fluctuations up to nearly twice the height. Semi-analytical model is developed to predict surface pressure spectra in a wall-jet and canopy flow. The rapid term model shows that the inflection in the streamwise mean velocity profile is the most dominant source of surface pressure fluctuations. Synchronized pressure and velocity measurements elucidate significant features of the sources that could be affecting surface pressure fluctuations. Overall, this study explores the qualitative and quantitative physics behind pressure shielding mechanism which find application particularly in trailing edge noise reduction. / Doctor of Philosophy / Unsteady pressure fluctuations originating from interaction of turbulent flow over surfaces often cause undesirable effects. Trailing edge noise in wind turbines and helicopter blades, cabin noise and interior wind noise are some of noise sources which originate from surface pressure fluctuations. Previous studies have demonstrated that surface treatments help in reducing the unsteady surface pressure fluctuations therefore shielding surfaces and this phenomenon is termed as 'Pressure Shielding'. These are surface treatments inspired from the downy coating on owl's wings. This study is motivated by recent works conducted at Virginia Tech on experimental investigation of unidirectional canopy treatments. These are evenly spaced arrays of streamwise rods held horizontal at the downstream end. Most previous surface treatments contain some entrance condition such as steps, supports or gaps which effect the surface pressure measurements and disturb the incoming flow. In this study, the canopies are developed without any entrance condition therefore assist in capturing the fundamental mechanisms of the flow interaction with the canopy rods.

The work described in this thesis was directed towards the development of a compact hybrid Passive/Active noise absorption system that would be effective over a wide frequency range. The Passive/Active system efficiently integrates both passive and active noise control methods. Both theoretical and experimental investigations were carried out in this work. A simple numerical model was developed to simulate the Passive/Active system. The passive sound absorbing material used in the system was the partially-reticulated polyurethane foam. This material was characterized using its sound propagation constant and its characteristic impedance which were empirically determined. Three different control strategies were investigated for the Passive! Active system. These control strategies were: (i) directly minimizing the reflected wave, (ii) inducing a pressure-release boundary condition on the back surface of the absorbing layer and (iii) a new approach consisting of minimizing the reflected wave in the airspace. The latter results in a match between the impedance in the air cavity and the impedance of a plane wave in air. This impedance-matching control approach was selected for the Passive/Active system because it meets the goal of optimum absorption over a wide frequency range and offers practical advantages. In this system, the error sensing process takes place inside of the airspace which results in a compact design with all the necessary sensors and actuators built into the system. Parametric analysis were performed on the impedance-matching control approach to investigate the sensitivity of the performance of the system to variation in absorbing layer thickness and airspace depth. The performance of the system was determined to be independent of the airspace depth and marginally sensitive to absorbing layer thickness. In addition, in order to study the feasibility of using the Passive/Active system for other sound absorbing materials, an experimental investigation was performed with the polyurethane foam replaced by a porous metal sheet, commercial name FELTMETAL®. The result of this investigation showed an improvement in the absorption property of the FELTMETAL, opening a number of potential applications for the Passive/Active system by using different absorbing materials. / Master of Science

The work presented in this thesis can be divided into two main subjects. First, lightweight designs for acoustic devices such as Helmholtz resonators and loudspeakers used for noise control in rocket payload compartments are developed. Second, active control using a hybrid control system (with structural and acoustic actuators) was tested experimentally. Due to the weight limitations for this application, Helmholtz resonators and loudspeakers are re-designed in order to reduce the device weight as much as possible while maintaining performance. For Helmholtz resonators, this is done by modeling the resonator for different structural shapes, wall materials and wall thicknesses using a finite element analysis software. The final design is then compared to the rigid resonators and is shown to perform effectively. These designs are then successfully applied to the full-scale fairing at Boeing facilities. In order to design a lightweight loudspeaker, a comparative approach was used. A standard 12' loudspeaker is taken as the reference loudspeaker and weight reduction solutions are applied to it while maintaining performance. The loudspeaker is characterized using mechanical, electrical and acoustical theories, and an optimization process is applied in order to minimize a defined cost function, which was taken as the total sound pressure output over a targeted frequency range per mass of the actuator. The results are used to build a lightweight loudspeaker together with a lightweight box, and the new designs are tested for comparison with the reference loudspeaker and shown to increase performance by 1.7 dB over 60-200 Hz band while reducing the mass by 78%. The second part of this thesis investigates the performance of a hybrid active control treatment featuring distributed vibration absorbers (DAVAs) and loudspeakers applied on a scale payload fairing. Several aspects such as causality, reference signals, and maximum controllable levels of this feedforward control scheme are the subjects of analyses. The results show that this active control approach can achieve significant amount of interior noise attenuation, and the total actuator weight required to control an external level of 138 dB can be reduced to 9.2kg using lightweight loudspeakers. However, it is shown that the attenuation levels can still be improved further by actuator positioning that gives more effective coupling of the actuators with the structural and acoustic modes and by using multiple references for the control system. / Master of Science

A common method of noise control is the implementation of passive transmission loss enclosures. An acoustical enclosure surrounds a noise source for the purpose of interrupting the noise transmission path. The effectiveness of these enclosures is, however, limited by various drawbacks. These drawbacks include leaks in the enclosure walls, transmitted vibrations to the enclosure due to physical coupling, and the limited reduction of noise at low frequencies. Recent advances in active control have shown the potential for applying some of this technology to passive acoustical enclosures to improve their performance. / Master of Science

<div>Fluid power is widely used in a variety of applications such as construction machines, aerospace, automotive, agricultural machinery, manufacturing, etc. Although this technology has many obvious advantages such as compactness, robustness, high power density, and so forth, there is much room for improvement, of which one of the most important and challenging problems is the noise.</div><div><br></div><div>Different institutes have been researching fluid power noise for decades. However, much of the experimental investigation was based on simple measurement and analysis techniques, which left the designers/researchers no method of understanding the complicated phenomena. A microphone array is a powerful tool that unfortunately has not been introduced to the fluid power noise research. By capturing the magnitude and phase information in space, a microphone array enables the noise source identification, separation, localization and so forth.</div><div><br></div><div>This thesis focuses on implementing the microphone array processing techniques on a synthetic microphone array for fluid power noise diagnostics. Differing from traditional scan-based approaches, the synthetic array is created by synchronizing the non-synchronous measurements to achieve the equivalent effect of a multi-microphone snapshot. The final results will show the power of microphone arrays and provide an economical solution to achieve approximate results when a real microphone array is not available.</div>

Thesis (M. Sc.)--University of Hong Kong, 1998. / Includes bibliographical references (leaf 86-87).

Smart foam is a composite noise control treatment that consists of a distributed piezoelectric actuator, known as polyvinylidene fluoride (PVDF), embedded within a layer of partially-reticulated polyurethane foam. The principal function of smart foam is to yield broadband sound attenuation. Passive acoustic foams are a very reliable high-frequency sound reduction method. With regard to smart foam, the embedded piezoelectric actuator is introduced to overcome the limitations of the passive foam in the low-frequency region. The piezoelectric actuator excites the structural and acoustic phases of the foam when driven by an externally supplied control voltage. This generates a secondary acoustic field which destructively interacts with the acoustic field created by a primary noise source. Initial experiments employ the composite "active/passive" treatment to yield attenuation of piston sound radiation. For this simple source, the global farfield pressure is minimized according to the feedforward, Filtered-x LMS control algorithm using one error sensor. Significant broadband sound attenuation is obtained. A more advanced noise control problem is investigated which minimizes plate radiation. The vibrating plate has a distributed modal response requiring a collective array of independently-phased smart foam actuators to yield reduction of the radiated sound power. This is accomplished by minimizing the sound pressure at an array of nearfield microphones. Good broadband sound power reduction is obtained using a MIMO (multiple-input/multiple-output) Filtered-x LMS control scheme. Various techniques for improving smart foam's acoustic control authority are identified during manufacturing and finite element modeling. of the actuator. These improved smart foam actuators are employed as an active/passive liner to suppress the transverse propagating acoustic modes within an anechoically-terminated rectangular duct. A section of a duct wall is lined with an array of smart foam and the sound downstream of the control actuators is minimized at several error microphones. Successful harmonic and broadband noise control is achieved. A full-scale numerical model of the duct acoustic control application is presented based on the finite element method. The purpose of the model is to study the sensitivity of this active/passive control approach relative to the spatial distribution of control channels and error sensors. A comparison of the numerical and experimental results yields similar trends. / Ph. D.

In this work, the potential for a smart foam actuator in controlling interior cabin noise due to a turbulent boundary layer excitation has been experimentally demonstrated. A smart foam actuator is a device comprised of sound absorbing foam with an embedded distributed piezoelectric layer (PVDF) designed to operate over a broad range of frequencies. The acoustic foam acts as a passive absorber and targets the high frequency content, while the PVDF serves as the active component and is used to overcome the limitations of the acoustic foam at low frequencies. The fuselage skin of an aircraft was represented by an experimental test panel in an anechoic box mounted to the side of a wind tunnel. The rig was used to simulate turbulent boundary layer noise transmission into and aircraft cabin. An active noise control (ANC) methodology was employed by covering the test panel with the smart foam actuators and driving them to generate a secondary sound field. This secondary sound field, when superimposed with the panel radiation, resulted in a reduction in overall sound in the anechoic box. An adaptive feedforward filtered-x Least-Mean-Squared (LMS) control algorithm was used to drive the smart foam actuators to reduce the sound pressure levels at an array of microphones. Accelerometers measured the response of the test panel and were used as the reference signal for the feedforward algorithm. A detailed summary of the smart foam actuator control performance is presented for two separate low speed wind tunnel facilities with speeds of Mach 0.1 and Mach 0.2 and a single high speed tunnel facility operating at Mach 0.8 and Mach 2.5. / Master of Science