Active control of the sound radiation from a finite line source using novel directional secondary sources

Abstract

This thesis is concerned with the active control of the sound radiated from a finite line source, which is a frequently encountered theoretical acoustic source in traffic noise control problems, in free field. To achieve an active noise control in three-dimensional free space is mostly challenging, and the primary noise considered in this study of extended spatial range, that produces a complex sound field, add extra difficulties. Some previous researches on active noise control are focused on the noise reduction capability to create required and acceptable quiet zones in restricted areas. This study is focused on a much wider region of noise reduction from the sound wave radiated from the finite line source, in order to reduce the possibility of sound pressure amplification. An active control system, consists of multiple secondary sources and error microphones with a typical arrangement where the controllers and sensors are equally spaced and positioned in two parallel rows, is proposed to achieve the research objective. The strengths of secondary sources are determined by the least square algorithm. Further optimization of the locations of control sources and error microphones is investigated by a performance descriptor evaluated on a rectangular strip positioned in the far field where noise attenuation is mostly required. The main contribution of this thesis is the introduction of a novel type of secondary sources specifically for a directional primary sound field. Conventional secondary sources of compact size radiate sound wave omnidirectionally, resulting in spillover effect in off-axis areas. The new secondary source proposed here, being of a multi-part construction, possess a reasonably directional radiation pattern even with very small physical size at low frequencies. Through a comprehensive numerical study, the newly devised directive secondary sources prove to be properly effective for global noise reduction. Though the global control performance becomes weaker as frequency increases, the noise reduction within the central region of the receiver plane remains significant in all the cases included in the present study.