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Withtheadventofmoresophisticatedelectronicsandelec- tronic control mechanisms, the approach of active-mode cancellation or control has become increasingly attractive and thus increasingly studied. The basic concept behind ac- tive control involves first measuring or predicting the acous- tic or vibration response and then utilizing active elements such as piezoelectric transducers to generate wave fields to either enhance or, more commonly, reduce the response. A primary complication to this approach stems from the vi- brational complexity of most structures, with most struc- tures having complex, superpositioned, or superimposed vibratory modes. The responses of these structures are noto- riously difficult both to predict and then to selectively con- trol. Additionally, active control techniques require exten- sive electronics and complex control systems, which can be expensive and require precise tuning.
One of the newest fields of study in structural acoustics and vibration is the field of engineered acoustic or structural ma- terials termed metamaterials (see the article by Haberman and Norris in this issue of Acoustics Today). Metamateri- als, which came to the field of acoustics in 2000, is a field of advanced materials that are designed to provide a desired response by introducing subwavelength features into the studied domain. Extending the idea of composite structures, which provide effective properties that are a mixture of the constituent components, metamaterials enable acoustic and vibrational response beyond the properties of any of mate- rials from which the structures are made (Haberman and Guild, 2016). By controlling the effective bulk modulus and sound speed of a region, propagation of acoustic and elastic waves can be tailored to behave in ways previously unachiev- able. Indeed, by exploiting resonance of local, subwave- length inclusions, it is possible to achieve negative effective bulk modulus and density either alone or in combination. As both modulus and density become negative as a func- tion of frequency, a double-negative material is achieved, leading to subdiffraction limit hyperlensing. These effective properties are available only under excitation and should not be confused with the static properties of the constitu- ent materials. Because of the range of achievable properties, the applications of acoustic and mechanical metamaterials are limited only by imagination. Although the most high- profile application of metamaterials is acoustic cloaking or the reduction in scattering profile by application of a coat- ing over a structure (Norris, 2015), significant advances have been made using this general approach in the fields of sound insulation, lensing, and antennas.
With comprehensive understanding of the optimal ex- pected acoustic and vibrational response of a structure, it is then possible to detect changes or damage to the region via changes in the acoustic response. This type of structural healthy monitoring can involve the identification of changes to vibrational resonance modes via measured modal analy- sis or detection of acoustic emission events during damage processes (Barre and Benzeggagh, 1994). Structural health monitoring via structural acoustics and vibration analysis provides a significant advantage over other types of damage detection in that is generally noninvasive so that the struc- ture is not damaged during evaluation. Changes to the struc- ture can thus be detected early, before catastrophic failure occurs. Although simple in concept, practical implementa- tion is challenging due to the inherent complexity of most inspected structures. Resonance and modal responses from fasteners, structural reinforcement, and holes all cause mul- tiple wave scattering and mode conversion responses that must be accounted for in detection of structurally destruc- tive geometries such as cracks (Duroux et al., 2010).
To address control and design challenges, a range of phys- ics-based numerical modeling and simulation techniques has been employed to the great benefit over time within the structural acoustics and vibration community. These tech- niques have ranged wildly in both complexity and accuracy. These techniques, available as both commercially distributed and custom software packages, include finite-element analy- sis (FEA), boundary-element analysis, energy-based FEA, and statistical-energy analysis. As a general approach, these techniques involve the following overall steps: defining the geometry, imposing physical conditions such as boundary conditions and deformation or frequency-domain loading excitations, and then utilizing the solver algorithms in com- puting the results such as eigenvalues, eigenvectors (mode shapes), transient structural response, and/or acoustic radi- ation response. Each of the techniques presents its own ad- vantages and pitfalls, which must be accounted for via thor- ough understanding of the underlying physics of the system and also the computational cost penalty associated with the varying degrees of model fidelity and accuracy available and required. Without that background knowledge of the funda- mental physics, it is possible, and indeed fairly common, to produce nonphysical results that do not represent the reality of the system. Regardless, these techniques present a power- ful tool, with applications in diverse industries such as auto- motive, aerospace, and general commercial goods.
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