1,721,164 research outputs found
The effect of enclosure coupling on a dual-layer linear loudspeaker array
No full textLoudspeaker arrays are increasingly being used in a variety of applications where there is a restriction on the electrical power consumption and the array must be robust to changes in the acoustic environment. Previous work has demonstrated that the efficiency and robustness of a two-source endfire loudspeaker array can be improved by acoustically coupling the two loudspeakers via a common enclosure. This paper presents an investigation into the effects of using acoustically coupled enclosures in a dual-layer linear loudspeaker array. Three different dual-layer array designs will be compared. In the first configuration each loudspeaker is mounted in its own enclosure and, therefore, there is no internal coupling between the loudspeakers. In the second configuration, each pair of front-to-back loudspeakers in the dual layer array are coupled via a common enclosure. Finally, in the third configuration all of the loudspeakers are coupled via a single enclosure. The performance of the three array configurations is investigated in terms of the acoustic contrast, the array effort, or electrical power, and the robustness to variations in the electroacoustic responses. Through this investigation it is shown that an increase in efficiency can be achieved by coupling the front-to-back pairs of loudspeakers in the dual-layer linear array
Active control of scattered acoustic fields: cancellation, reproduction and cloaking
No full textThe active control of sound fields has been widely applied in both active noise control and sound field reproduction, however, relatively few studies have focused on active acoustic cloaking. In order to build upon the knowledge and understanding in the areas of active noise control and sound field reproduction, this paper investigates their physical limitations and compares them to the active cloaking problem when the three strategies are employed in the presence of an acoustic scatterer. The three sound field control strategies have been formulated within a consistent framework, and this has enabled insight into the physical control mechanisms. Two different three-dimensional scattering problems have then been simulated and used to investigate the performance limitations of the three strategies. The influence of the number of control sources and their proximity to the scattering object have been investigated, and it has been shown that the requirements for active cloaking differ from those for active noise control and sound field reproduction. Specifically, it has been shown that there is a clear distinction between controlling the internal and external sound fields in the three cases
A parametric study of dynamic stress in acoustic black holes
No full textAcoustic Black Holes (ABHs) make modifications to a structure that act to reduce the speed of a wave travelling through the ABH region. The reduced wavespeed has a corresponding reduced wavelength which increases the effectiveness of damping treatment applied to the ABH. A common way in which the reduction of wavespeed can be achieved is by gradually reducing the thickness of a structure. This is commonly implemented as a beam termination, which results in a thin tip. In this case, the energy focusing effect of the ABH causes high amplitude vibrations to occur in the thin section of the structure, raising concerns about high levels of dynamic stress that could result in fatigue failure. This paper presents an investigation into the effect of changing the ABH taper length, tip height and power law on the dynamic stress in an ABH terminated beam. This is achieved via numerical model of the structure, which enables a full parametric analysis to be carried out. For each ABH parameterisation, the performance is also quantified in terms of reflection coefficient so that trade-offs between performance and stress can be observed
Remote sensing for wave-based nonreciprocal active control
No full textReciprocity is an acoustic property that describes the symmetry of sound transmission between two points. However, this property is undesirable in certain applications, and this has led to significant interest in the development of nonreciprocal acoustic devices that achieve one-way sound transmission. These devices typically achieve nonreciprocal sound transmission by introducing nonlinearities or directional biasing. Previously proposed nonreciprocal acoustic devices generally have limitations; for example, they may not be fully tuneable, they can introduce signal distortions such as additional harmonics, or they can only exhibit nonreciprocal behaviour over a narrow bandwidth. To overcome these challenges, previous work has demonstrated how a wave-based active control system can be used to drive an array of acoustic sources to achieve reversible and broadband non-reciprocal behaviour. However these wave-based active control systems use external far-field pressure sensors to achieve broadband nonreciprocal behaviour and, thus, these active control systems are not self-sufficient. This paper therefore presents an experimental investigation into how remote sensing techniques can be incorporated into the previously proposed wave-based active control systems to create more self-contained nonreciprocal acoustic devices that still achieve broadband nonreciprocal behaviour in a one-dimensional acoustic system
Evaluation of active control concepts for a self-adjusting membrane-type acoustic metamaterial
No full textMembrane-type acoustic metamaterials (MAM) consist of a thin pre-stretched membrane with masses periodically attached to the membrane. MAM can be designed to be very lightweight and exhibit anti-resonances at low frequencies with sound transmission loss values that can be much higher than the mass-law. This makes MAM very appealing for different noise control applications. However, the typically narrow bandwidth of the anti-resonances is a big challenge for applying MAM to noise sources that consist of changing tonal frequencies and/or broadband noise. In this contribution the preliminary results of a concept study for a self-adjusting MAM using active control are presented. In this concept, the added mass is replaced by an inertial electrodynamic shaker which enables the actuation of the MAM. The performance of different control strategies is evaluated using analytical and numerical models of the MAM. In particular, this evaluation will focus on methods to enable the MAM to adjust its anti-resonances to changing tonal frequencies and to improve the broadband noise reduction of the MAM. Additionally, it is investigated how the performance of the active MAM is affected when not every unit cell of the MAM contains an actuator in order to reduce the complexity of the system
Control of vibration using surface mounted acoustic black holes
No full textAcoustic Black Holes (ABHs) are tapered features that can either be surface mounted or embedded into structures to provide effective broadband structural damping. Generally, structures with embedded ABHs require material to be removed from the structure to create the required geometric features. However, for thin plates, it is not straightforward to embed ABHs, since the removal of the material in this case will significantly impact the strength of the structure. Therefore, this paper presents an numerical model based investigation into how surface mounted ABHs can be used to control the vibration of a thin beam over a broad frequency range. The numerical investigation compares the performance of the proposed surface mounted ABHs with a beam treated using conventional passive measures via evaluation of the total kinetic energy of the primary structure. The results presented in this paper show that the surface mounted ABHs achieve greater total kinetic energy attenuation compared to the conventionally treated beam over the presented bandwidth due to the ABH effect. The proposed surface mounted ABHs could be used as an effective broadband vibration control solution for thin structures, where it is not straightforward to design effective embedded ABHs
Limitations of FxLMS in feedforward active vibration control of a nonlinear two degree of freedom system
No full textActive control systems are often used to surmount the challenges associated with passive noise and vibration control measures to control low frequency disturbances, since they achieve control without the application of large or heavy control treatments. Historically, linear active control strategies have been used in feedforward control systems to drive the control source to minimise the signal measured at the error sensor. Amongst the various control algorithms available, the Filtered-reference Least Means Squares (FxLMS) algorithm has become extremely popular in the last few decades due to its relatively good performance and high level of robustness, as well as simplicity in both design and application. However, when the system under control contains nonlinearities, either in the primary or secondary paths, the performance of the FxLMS algorithm can degrade dramatically. This paper explores the performance limitations of the FxLMS algorithm when applied to the control of a two degree of freedom mass-spring-damper system with linear and cubic nonlinear stiffness terms. The aim of this study is to improve understanding of and inspire better design of nonlinear control systems. The effect of the nonlinearity on the statistical uncertainty in the plant model is discussed, as well as the effect on reliable control performance
Experimental validation of a modified acoustic black hole profile
No full textAcoustic Black Holes (ABHs) use modifications to a structure to reduce the structural wavespeed, which increases the effect of damping treatment applied in the ABH region, resulting in greater vibration attenuation. The most common type of ABH uses a gradually tapering thickness to reduce the wavespeed, resulting in part of the structure becoming very thin. Due to the focusing effects of the ABH, high amplitude vibrations occur in the thin part of the structure, and this raises concerns about vibration fatigue. This paper uses a numerical model to investigate the fatigue life of an ABH taper used to terminate a beam. A comparison is made between the results from a standard ABH taper and a modified ABH profile that aims to reduce fatigue while preserving vibration performance. Following this, the dynamic performance of both ABHs is assessed experimentally to validate the results from the numerical model
Topology optimisation of microphone arrays for remote microphone virtual sensing in diffuse sound fields
No full textPast research has shown that virtual sensing techniques can enhance the performance of active noise control systems by projecting the control points towards remote locations of interest. However, accurate sound field estimation using virtual sensing is critical to the performance of such active control systems and depends on both signal processing and the physical microphone array. Typically, microphone placement is determined by practical considerations such as convenience, spatial constraints and cost, resulting in limited exploration of optimal microphone positioning. The current study employs a genetic algorithm to identify optimal microphone array configurations for accurate estimation of the pressure in a diffuse sound field, utilising the Remote Microphone Technique. The optimality criterion is defined as the estimation performance or the robustness of the derived topologies to practical uncertainties. The resulting optimal configurations are evaluated against a conventional uniform linear microphone array, which consist of sub-array elements capable of utilising both pressure and pressure gradient information for enhanced estimation accuracy
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