Development of ``Global" Feedback Control Strategies for Silencing Noise

Iowa State University, Ames IA

Investigators

Abstract

The proposed research is motivated by the need for inventing new technologies and devices that can drastically reduce noise in noisy environments. In particular, the acoustic systems of interest are acoustic enclosures such as passenger cabins in aerospace vehicles, industrial machinery, and household appliances. The goal of the research is to advance the state-of-the-art in active noise control (ANC) technology to a stage where it can lead to the development of silencing devices; for example, a portable ``intelligent" cover that can be snapped on a vacuum cleaner or a lawn mower for reducing the noise by 80\%; small silencing hardware that can be placed in the family room for reducing the effect of outside noise (e.g., jet aircraft flying overhead) by two-thirds; and smart devices installed in the walls and the interior of an aircraft cabin for making it quieter and more vibration-free than a luxury car. Based on the analytical and experimental results obtained to date, such a technology appears to be within the realm of possibility. Development of such technology, if successful, will also have the added bonus of reducing the outside (environmental) noise - an important problem identified by FAA with regard to noise in the neighborhood of the airports. There are many possible non-aerospace spinoffs such as those mentioned above. The technology may also provide an effective solution to noise problems in spacecraft (such as those recently reported by the Space Station crew). The proposed research, with strong analytical and experimental foundation built over last few years through NSF and NASA support, is aimed at enhancing the state-of-the-art in ANC by addressing following key research issues: (a) for a given reverberant acoustic enclosure what is the theoretical limit of performance for ANC and what are the key design parameters that this limit depends on and how one can optimize the choice of these parameters (such as, number and configuration of actuators and sensors, shape of enclosure, etc.) to extend this limit; (b) what are the inherent limitations of various robust control design methodologies for acoustic systems; (c) what are the conditions (analytical) under which a given feedback strategy can OR can not achieve uniform reduction in acoustic pressure levels in space and frequency and how to design controllers that can satisfy these conditions to achieve effective noise reduction. These key research issues will be addressed through the following list of research tasks: (1) Devise automated synthesis techniques for passivity-based controllers, which have been proven to extract energy from the system in the closed-loop configuration. These controllers have ability to reduce noise not only at the sensor location and at isolated frequencies but uniformly over the space and frequency. (2) Devise integrated (structure + acoustic) control strategies that can effectively minimize the cost which reflects the total acoustic energy of the system and not the acoustic energy at a particular location in space or energy in the particular mode of the system. (3) Develop control strategies that are aimed at modifying boundary impedances of the enclosures to optimize noise containment or noise transmission. The idea is to modify the structural geometry (morphing) to change the impedance characteristics of the boundary so as to minimize transmission of acoustic noise or achieve maximum acoustic dissipation. (4) Develop and validate the concept of ``active silencing hood" that can be used for commercial applications to curb noise pollution. (5) Experimentally validate the new technology first on a specially built laboratory apparatus followed by household appliances such as washer and vacuum cleaner and full-scale aircraft fuselage model available at Acoustic Research Facility at NASA LaRC. Intellectual Merit: The proposed project will develop our understanding of the inherent limitations as well as the benefits of potential ANC methodologies compared to the existing technology. The proposed research has potential for development of number of patents particularly in the design of silencing hoods for machineries and appliances, and design of active control software and hardware for large enclosed spaces. It will also develop scientific base for addressing several unsolved problems in vibration isolation and active control of noise in open field. To the best of P.I.'s knowledge, there doesn't exist an active control technique that can guarantee broadband reduction of noise that is uniform in space. The outcome of the proposed research will also provide the knowledge base for innovative design of casings of machinery and appliances for quiet operation. If successful, this research will also benefit the development of technology that can alter the acoustic signature of submerged vehicles such as submarines. Broader Impact: The impact of the proposed research will involve: (a) advancement of discovery and understanding while promoting teaching, training and learning, (b) broadening of participation of under-represented groups with special recruitment initiative for minority students, (c) enhancement of infrastructure for research and education, (d) economic boost to US industry, (e) broad dissemination of information to enhance scientific and technological understanding, and (f) benefit to society from reduced noise pollution and boost to US industry.