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This paper presents an acoustic source with a small thickness and high bending stiffness. The high bending stiffness is obtained with a sandwich structure in which the face of the sandwich structure internal to the source is perforated to increase the acoustic compliance, thereby leading to increased electroacoustic conversion efficiency. Multiple actuators are used to drive the moving component of the acoustic source. Control of the acoustic resonances and structural resonances is required to obtain an even frequency response. The use of collocated decentralized feedback control based on velocity sensing was found to be ineffective for controlling these resonances due to the destabilizing asymmetric modes caused by the coupling of the internal acoustic cavity and the rigid body vibration of the moving component. Resonances can be controlled by a set of independent combinations of symmetric driving patterns with corresponding velocity feedback controllers such that the fundamental mass-air resonance is effectively controlled, as is the lowest bending mode of the moving component. Finally, a compensation scheme for low frequencies is used which enables a flat frequency response in the range of 30 Hz to 1 kHz with deviations smaller than 3 dB.

This paper compares various decentralised control strategies, including structural and acoustic actuator-sensor configuration designs, to reduce noise transmission through a double panel structure. The comparison is based on identical control stability indexes. The double panel structure consists of two panels with air in between and offers the advantages of low sound transmission at high frequencies, low heat transmission, and low weight. The double panel structure is widely used, such as in the aerospace and automotive industries. Nevertheless, the resonance of the cavity and the poor sound transmission loss at low frequencies limit the double panel's noise control performance. Applying active structural acoustic control to the panels or active noise control to the cavity has been discussed in many papers. In this paper, the resonances of the panels and the cavity are considered simultaneously to further reduce the transmitted noise through an existing double panel structure. A structural-acoustic coupled model is developed to investigate and compare various structural control and cavity control methods. Numerical analysis and real-time control results show that structural control should be applied to both panels. Three types of cavity control sources are presented and compared. The results indicate that the largest noise reduction is obtained with cavity control by loudspeakers modified to operate as incident pressure sources. © 2013 Elsevier Ltd.

This paper presents a novel flat loudspeaker with improved sound frequency response. Flat loudspeakers provide advantages of compact dimensions and high durability. Known flat loudspeaker technology is based on high modal density. However, the resonances in the panel are complex and difficult to control, which often leads to complicated computations and insufficient low frequency response. The flat loudspeaker in this paper comprises a novel panel structure, which offers low density, high stiffness, and efficient space utilization. Furthermore we have developed a simple and stable control mechanism to obtain flat sound frequency response. Experimental results show that our method effectively improves the performance of the flat loudspeaker with extended low-frequency response.

Air cavities between plates are often used to improve noise insulation by passive means, especially at high frequencies. Such configurations may suffer from resonances, such as due to the mass-air-mass resonance. Lightweight structures, which tend to be undamped, may suffer from structural resonances as well. Active methods have been suggested for improved noise insulation of plates, using piezoelectric patch actuators or inertial mass actuators. Other active methods for improved noise insulation can be based on acoustic control of the sound field in the cavity, using acoustic sources and acoustic sensors. Methods based on feedforward control of the sound radiated from such panels with air cavity usually suffer from an irregular frequency response of the actuators on the radiating panel and insufficient acoustic control authority at low frequencies. This paper presents a method to realize the combination of the air cavity and a radiating surface with a well controlled vibration distribution over the radiating surface. A specific distributed controller results in a smooth and well defined frequency response over a broad frequency range, enabling effective feedforward control of the radiated sound. Experimental results agree with numerical predictions.

This paper presents a modified loudspeaker source for decentralized feedback cavity control in a double panel structure to reduce the noise transmission. The double panel structure con-sists of two panels with air in between and offers the advantages of low weight, low sound transmission at high frequencies, and thermal insulation. The main issues of the double panel structure are the resonance of the cavity and the high noise transmission at low frequencies. Many papers have discussed applying active structural acoustic control to the panels or active noise control to the cavity. In our previous study, we considered the resonance of the panels and the cavity simultaneously and numerically compared various decentralized structural and cavity feedback control strategies basing on identical control stability margins. Cavity control by loudspeakers, which are modified to operate as incident pressure sources, was found to-provide the largest noise reduction. The incident pressure source loudspeaker can be realized by using a dynamic loudspeaker, a microphone, and a velocity sensor with a feedforward controller. In this paper, experimental results of a one dimensional realization with a feed-forward controller are presented.