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Acoustics Of Ducts And Mufflers With Application To Exhaust And Ventilation System Design

Acoustic metamaterial science is an emerging field at the frontier of modern acoustics. It provides a prominent platform for acoustic wave control in subwavelength-sized metadevices or metasystems. However, most of the metamaterials can only work in a narrow frequency band once fabricated, which limits the practical application of acoustic metamaterials. This paper highlights some recent progress in tunable acoustic metamaterials based on various modulation techniques. Acoustic metamaterials have been designed to control the attenuation of acoustic waves, invisibility cloaking, and acoustic wavefront engineering, such as focusing via manipulating the acoustic impedance of metamaterials. The reviewed techniques are promising in extending the novel acoustics response into wider frequency bands, in that tunable acoustic metamaterials may be exploited for unusual applications compared to conventional acoustic devices.

A Review of Tunable Acoustic Metamaterials

The introduction of engineered resonance phenomena on surfaces has opened a new frontier in surface science and technology. Pillared phononic crystals, metamaterials, and metasurfaces are an emerging class of artificial structured media, featuring surfaces, that consist of pillars–or branching substructures–standing on a plate or a substrate. A pillared phononic crystal exhibits Bragg band gaps while a pillared metamaterial may feature both Bragg band gaps and local-resonance hybridization band gaps. These two band-gap phenomena, along with other unique wave dispersion characteristics, have been exploited for a variety of applications spanning a range of length scales and covering multiple disciplines in applied physics and engineering, particularly in elastodynamics and acoustics. The intrinsic placement of pillars on a semi-infinite surface–yielding a metasurface–has similarly provided new avenues for the control and manipulation of wave propagation. Classical waves are admitted in pillared media, including Lamb waves in plates and Rayleigh and Love waves along the surface of substrates, ranging in frequencies from Hz to several GHz. With the presence of the pillars, these waves couple with surface resonances richly creating new phenomena and properties in the subwavelength regime and in some applications at higher frequencies as well. At the nanoscale, it was shown that atomic-scale resonances–stemming from nanopillars–alter the fundamental nature of conductive thermal transport by reducing the group velocities and generating mode localizations across the entire spectrum of the constituent material well into the THz regime. In this article, we first overview the history and development of pillared materials, then provide a detailed synopsis of a selection of key research topics that involve the utilization of pillars or similar branching substructures in different contexts. Finally, we conclude by providing a short summary and some perspectives on the state of the field and its promise for further future development.

/pdf/physics-of-surface-vibrational-resonances-pillared-phononic-50nrk5dsy2.pdf

Physics of surface vibrational resonances: pillared phononic crystals, metamaterials, and metasurfaces.

High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system’s initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system’s excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.

http://absimage.aps.org/image/MAR12/MWS_MAR12-2011-009305.pdf

Probing Thermomechanics at the Nanoscale: Impulsively Excited Pseudosurface Acoustic Waves in Hypersonic Phononic Crystals

Many efforts have been devoted to studying the robustness of topologically protected edge states in acoustics; however, the robustness of conventional edge states is rarely reported. In this work we theoretically study interface acoustic states appearing in finite arrays of resonators on a thin plate with topologically protected and conventional designs. Topologically protected interface states are first analyzed by employing the concept of breaking inversion symmetry within the unit cell of a honeycomb lattice for cylindrical and spherical resonators; we further demonstrate the robustness of the wave propagation along a zigzag path containing sharp corners and defects. In parallel, a conventional interface state is also designed and compared to the same situations. We found that the conventional interface state suffers backscattering in the zigzag path while it can show a more confined wave transport in some cases. The presence of a defect along the propagation path scatters the conventional interface wave and in particular can prohibit full propagation in the presence of a localized state at the defect. Then, we show that the immunity of the topologically protected design needs the interface to be surrounded by at least two hexagons of the phononic crystals on both sides, especially at the sharp corners in the zigzag path, while the conventional design only needs one hexagon of bulk media with the advantage of compact wave transport. Position and height disorders are further introduced to the interface pillars for both designs. It is revealed that in both designs, the transmission decreases quasilinearly with position disorder while it exhibits an abrupt drop with height disorder showing a transition threshold. With high disorder perturbation, waves can hardly enter the interface for the topologically protected design, while waves are trapped at the interface for the conventional design. A certain robustness against disorder is exhibited for conventional edge states. This work provides insight into the interface states in micro- and nanoscale characterization and figures out the behaviors for both topologically protected and conventional interface states.

https://link.aps.org/accepted/10.1103/PhysRevB.98.054307

Robustness of conventional and topologically protected edge states in phononic crystal plates

In this work, Fano resonance is studied in a pillared acoustic metasurface, which originates from the avoided crossing of strongly coupled individual monopolar modes of two dissimilar pillars in one unit cell. The Fano resonance manifests itself as an out-of-phase compressional (monopolar) vibrations of the two pillars coupled through the plate. The Fano asymmetric transmission profile can be analyzed through the interference between the incident wave and the scattered wave emitted by resonant pillars. The peak in the asymmetric transmission is always maintained either the coupling of the two pillars is strong or weak. It is further found that the Fano resonance can be tuned by varying different geometrical parameters, such as the height/diameter of the pillars, the distance between the two pillars, or introducing an inner hole into the pillars which can possibly be filled with a liquid.

https://iopscience.iop.org/article/10.1088/1361-6463/aa8a19/pdf

Tunable Fano resonances of Lamb modes in a pillared metasurface

Wave demultiplexers transporting desired wavelengths towards proper directions or ports are attracting numerous interests and applications in both physical and engineering areas. In acoustics, there is still a lack of compact and simple designs to achieve demultiplexers in three-port systems. In this work, we propose such a design using Helmholtz resonators where the frequency selection is based on the phenomenon of acoustically induced transparency (AIT). First, a modified transfer matrix method is derived to analytically describe and analyze the AIT effect with Helmholtz resonators. Then, the good performances of wave routing in these designs are further demonstrated by both simulation and experiment. These AIT based demultiplexers are subwavelength and simple in their designs. Therefore, they are promising for various potential applications such as signal processing, information communication and sensing.

https://iopscience.iop.org/article/10.1088/1361-6463/abe07c/pdf

EI Houssaine Ei Boudouti

Papers

Tunable Fano resonances of Lamb modes in a pillared metasurface

Induced transparency based subwavelength acoustic demultiplexers