Active control of sound transmission into enclosure through a panel

Abstract

The thesis is concerned with the effectiveness of active sound transmission control inside an enclosure under the potential energy, squared pressure and energy density control algorithms using a purely vibration actuator, a purely acoustic source and combined source system. The principal application is to effectively use various control algorithms and sources on the active control of sound transmission into an enclosure and control of its resultant sound fields. The main objective of this thesis is to clarify the role of acoustic and vibration actuators, as well as error sensors and sensing schemes, in the global and local control of harmonic sound transmission into a structural-acoustic coupled system with different strength of structural-acoustic coupling. The effects of the edge rotational and translational flexibility of the flexible structure on the active sound transmission control are investigated in detail. The objective is tackled by way of investigations into i) the physics of structural-acoustic coupled systems and ii) the physical limitation and control mechanisms of active sound transmission control. The investigations were performed by employing three concepts: sound-field visualization for the analysis of active noise control, normalized impedance and mobility for the analysis of structural-acoustic coupled systems, and the effects of the edge rotational and translational flexibility of the flexible structure on the active sound transmission control. Effectiveness of global and local sound field control inside an enclosure under the potential energy, squared pressure and energy density control algorithms were investigated numerically in this thesis. Modal analysis with three-dimensional visualization of the sound field, as well as resultant total acoustic potential energy attenuation, was performed. Significant localized sound attenuation can be achieved in specific areas even with an overall amplification of total acoustic potential energy in the enclosure, showing the limitation of traditional potential energy analysis. The present results also showed the occurrence of the detrimental effects and spillovers under the squared pressure control, while they can be removed by using the energy density control. The spillover effect results from the control source inadvertently exciting higher order modes. The energy density control results in more uniform sound fields and a performance similar to the theoretical solution of potential energy control. However, it is not effective if the error sensor is located near to the secondary sound source. The present finding on producing large quiet zones using a simple system has significant implication for building noise control. The concept of impedance-mobility approach was extended to the energy density based active control in structural-acoustic coupled systems. Formulae based on the normalized impedance-mobility approach were developed for the active sound transmission control with consideration of flexible structures with edges elastically supported against translation and rotation. Full coupling between the flexible structure and the interior acoustic cavity is considered. The criteria of weak coupling were defined in detail. Formulae for the coupled eigen-frequencies of the structural-acoustic coupled systems were also derived. For active sound transmission control, previous work has considered the classical boundary conditions of the flexible structures (i.e. simply supported and clamped edges). The analysis of active sound transmission control in this thesis was extended to the cases where the edges of the panel structure are elastically restrained in rotation and translation. Results showed that effective active control is obtained for pure vibration control at low frequency, but poor performance occurs at low rotational or translational flexibility at high frequency. Pure acoustic control gives a more uniform performance for various combinations of edge rotational and translational flexibilities. Also, effective pure acoustic control can be achieved at some eigen-frequencies of the enclosure, at high frequencies and at some pairs of rotational and translational flexibilities with ineffective transmission of acoustic energy.