The influence of structural-acoustic coupling on the dynamic behaviour of a one-dimensional vibro-acoustic system

Abstract

The aim of this thesis is to investigate the structural-acoustic coupling effects on the dynamic behaviour of a vibro-acoustic system under passive/active control. The simplest model of a vibro-acoustic system one can consider is a one-dimensional acoustic cavity driven by a single-degree-of-freedom (SDOF) structure. This simple model is used to demonstrate the physical characteristics of the coupling phenomenon. This simple analytical model can provide various degrees of structural-acoustic coupling, which are dependent upon (i) the structural-acoustic stiffness ratio, (ii) structural-acoustic natural frequency ratio, (iii) structural damping, and (iv) acoustic damping. In this case, although the geometric coupling factor is not included because the SDOF structure has a single mode, 80 percent of the factors that determine the degree of coupling can be accounted for by the simple analytical model. The coupling mechanism, in the simple vibro-acoustic system, is investigated using the mobility-impedance approach. In order to provide the threshold of the degree of coupling, a coupling factor is calculated in terms of non-dimensional structural-acoustic parameters. Vibro-acoustic responses are represented by the acoustic potential energy in the cavity and the kinetic energy of the structure coupled to the acoustic cavity.
The vibro-acoustic responses are investigated for various coupled cases. The principles are demonstrated by controlling the acoustic potential energy in the one-dimensional finite acoustic tube driven by the SDOF structure. Three control strategies are applied; passive control, active feedforward control and decentralised velocity feedback control. Passive control is investigated to achieve physical insight into the relative benefits of passive control treatments. In the more strongly coupled case, acoustical modifications were preferable for the reduction of the acoustic potential energy. On the other hand, in the more weakly coupled case, structural modifications were more effective. For harmonic disturbance, an active feedforward control strategy is considered for the control of the acoustic potential energy in the cavity driven by the structure under external harmonic excitation. For the active feedforward control systems, this study uses the concept of optimal impedance, which is defined as the ratio of the control force to the velocity of a secondary source when the acoustic potential energy is minimised. In the more strongly coupled case, all the acoustic modes were effectively suppressed at the resonance frequencies. On the other hand, in the more weakly coupled case, all the acoustic modes were controllable as in the more strongly coupled case. However, the structural mode was generally uncontrollable. For broadband disturbance, decentralised velocity feedback control is formulated to investigate the relative control effectiveness of structural and acoustic actuators for the control of the acoustic potential energy. In the more strongly coupled case, the control configuration of using the acoustic actuator was preferable. On the other hand, in the more weakly coupled case, the decentralised velocity feedback control strategy using both the actuators was desirable.

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