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(a) Asymmetric acoustic transmission by coupled resonators for the forward (F) and backward (B) incident waves. (b) Transmitted power spectra ( ${T}_{F}$ and ${T}_{B}$ ) and transmission contrast ( ${T}_{F}/{T}_{B}$ ). (c), (d) Simulated pressure field for the forward and backward incident waves at 795 Hz. The gray arrows indicate the locations of the surrounding resonators.
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We investigate acoustic wave coupling between two resonators of very different intrinsic losses, e.g. lossy and lossless resonators as an extreme case. We find that the resonator pair placed on a reflector exhibits angle-dependent absorption, showing an order of magnitude difference for opposite angles of incidence. The results obtained from a harm...
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Citations
... For 2D cases, Γ and K are analytically calculated, as shown in Eq. 3(a). For a single resonator (width: s) on a semi-infinite rigid body, the complex leakage rate ( Γ) is given by [43,44]. ...
... A finite number of coupled resonators exhibit intriguing wave scattering and absorption characteristics, as summarized in Figure 7. Coupled resonators of asymmetric intrinsic losses show different absorption cross section spectra for opposite incidences [44], as shown in Figures 7A,B. In the referred work, the opposite Frontiers in Physics frontiersin.org ...
Coupled resonance enables many intriguing physical phenomena, leading to wave control and sensing. This review discusses fundamental understanding of coupled resonance by providing detailed comparison between lumped parameter-based models including coupled mode theory (CMT) and harmonic oscillator model (HOM). While reviewing recent progress in research concerning coupled resonance, emerging research areas related to coupled resonance are discussed.
... To date, several noticeable theoretical and experimental parity-time symmetric acoustic structures have been proposed [30][31][32][33][34][35][36][37]. Moreover, non-Hermiticity, specifically in terms of losses [38,39], is used to achieve, for instance, an extreme asymmetric acoustic response [40], experimental demonstration of acoustic asymmetric diffraction grating [41], unidirectional wave-vector manipulation in two-dimensional space [42], experimental realization of a higher-order topology in an acoustic crystal [43], a topologically protected exceptional point [44], asymmetric loss-induced perfect sound absorption [45], perfect absorption through a subwavelength medium [46], loss-induced angle-dependent absorption [47], anomalous energy transport with exceptional points [48], stabilization of acoustic modes using Helmholtz and quarter-wave resonators tuned to exceptional points, [49], conditional simultaneous unidirectional zero reflection, and extraordinarily high transmission [50]. ...
We propose, design, and experimentally test a non-Hermitian acoustic superlattice that acts as a tunable precise filter. The superlattice is composed of two concatenated sublattices. The first sublattice is Hermitian, while the other can be adjusted to be Hermitian or non-Hermitian. The existence of non-Hermiticity, in terms of an induced loss in the second sublattice, results in the generation of absorption resonances that appear in the reflected spectrum. This provides us with a powerful knob to absorb or reflect several frequencies at will with high accuracy. The number of filtered frequencies can be controlled by designing the resonances in the first sublattice. Our proposed tunable acoustic filter can be extended to higher-frequency ranges, such as ultrasound, and other areas, such as photonics.
... First, Eq. 5 can be assumed to be independent (Wang et al., 2014) of the azimuthal angle φ if the lateral size of the supercell is deep-subwavelength. Otherwise, an asymmetric absorption (Lee and Iizuka, 2019;Wang et al., 2019) would be observed because of the geometric asymmetry, when the incident wave is from the same polar angle but different azimuthal angles. Second, this frequently used method is an approximation in terms of the accuracy of the mean specific impedance. ...
... These assumptions can also be understood in a mathematical manner: Only the fundamental mode wave dominates the scattering field and thus needs to be considered, when λ ≥ 2D (Kim, 2010), regardless of the angle of incidence. When these assumptions are invalid, the coupled-mode theory and radiation impedance method (Kim et al., 2006;Lee and Iizuka, 2019) can be used, which either implicitly or explicitly take into account the coupling of evanescent waves between unit cells. Third, the acoustic-structure interaction is neglected. ...
A broadband sound absorption attained by a deep-subwavelength structure is of great interest to the noise control community especially for extremely low frequencies (20–100 Hz) in room acoustics. Coupling multiple different resonant unit cells has been an effective strategy to achieve a broadband sound absorption. In this paper, we report on an analytical, numerical and experimental study of a low-frequency broadband (50–63 Hz, one third octave band), high absorption (average absorption coefficient ≈ 93%), near-omnidirectional (0–75°) acoustic metasurface absorber composed of four coupled unit cells at a thickness of 15.4 cm (1/45 of the wavelength at 50 Hz). To further broaden the bandwidth (50–100 Hz, one octave band), a design with 19 unit cells coupled in a supercell is analytically studied to achieve an average absorption coefficient of 85% for a wide angle range (0–75°) at a thickness of 20 cm (1/34 of wavelength at 50 Hz). Two additional degrees of freedom, the lateral size of supercell and the number of unit cells in the supercell, are demonstrated to facilitate such a causally optimal design which is close to the ideally causal optimality. The proposed design methodology may solve the long-standing issue for low frequency absorption in room acoustics.
... To model such a system and study intrinsic loss effects, we start by considering coupled harmonic oscillators (HOS), as illustrated in Fig. 1. With the same mass (m) and stiffness (k), the equation of motion is given by [37][38][39][40] ...
We investigate a duct silencer consisting of coupled resonators for perfect sound absorption. The device is composed of pairs of compact, ring-shaped Helmholtz resonators stacked along a duct. To study the effect of asymmetric intrinsic losses on the absorption performance, the resonators within each pair have the same dimensions (i.e., the identical resonance) but with different intrinsic losses. We find that the perfect absorption is realized by carefully choosing asymmetric intrinsic losses to the coupled resonators within each pair. The device with optimized losses exhibits exceptional point-like characteristics. This study provides not only a solution for practical duct systems but also allows thorough understanding of the role of asymmetric loss in degenerate resonators.
... The response of the two-resonator sensor can be analyzed by modeling each HR as a harmonic oscillator consisting of a mass (m), damper (δ), and spring (k). For these coupled harmonic oscillators, the equation of motion is given by [28][29][30] ...
Directional sound sensing plays a critical role in many applications involving the localization of a sound source. However, the sensing range limit and fabrication difficulties of current acoustic sensing technologies pose challenges in realizing compact subwavelength direction sensors. Here, a subwavelength directional sensor is demonstrated, which can detect the angle of an incident wave in a full angle range (0°∼360°). The directional sensing is realized with acoustic coupling of Helmholtz resonators each supporting a monopolar resonance, which are monitored by conventional microphones. When these resonators scatter sound into free‐space acoustic modes, the scattered waves from each resonator interfere, resulting in a Fano‐like resonance where the spectral responses of the constituent resonators are drastically different from each other. This work provides a critical understanding of resonant coupling as well as a viable solution for directional sensing. Fano‐like resonances supported by deep‐subwavelength coupled resonators enable directional sensing. Owing to the coupled resonance, drastically different characteristics depending on incident angles are exhibited by the constituent resonators, which are used to identify the angle of incident acoustic waves in a full‐angle range.
... Another interesting question is whether scatterers composed of multiple resonators can excite a sufficient number of angular momentum channels for superscattering. In such a case, the constituent resonators strongly couple with each other [9,22], and thus, the orthogonality of resonances is not guaranteed. Moreover, new features, such as angle-dependent superscattering, can be useful for directional sensing [23,24] and scattering pattern shaping [25,26], while maintaining superior scattering performance. ...
... Here, the diagonal elements of C describe radiation leakage (c ii ) and intrinsic loss (δ i ), and the off-diagonal elements (c ij for i = j ) describe coupling between the resonators [22]. By using Eq. ...
We demonstrate acoustic superscattering from a subwavelength scatterer consisting of multiple deep-subwavelength resonators. We show that superscattering is enabled by strong coupling between these resonators and the discrete rotational symmetry of the scatterer results in angle-dependent superscatter-ing. From the cylindrical-wave expansion of an incident plane wave, we observe that radially arranged resonators can effectively couple to these expanded cylindrical waves. In addition, we provide an analytical model characterizing the coupled resonators, providing a critical understanding of superscattering. As an experimental demonstration, we present a superscatterer with six acoustic resonators that shows angle-dependent superscattering.
... The achieved reflection coefficient (87% in simulation and 83% in experiment) is lower than the desired high theoretical value (over 97%), and the reflection coefficient is not easily controlled. Moreover, a pair of lossy and lossless resonators was used to obtain angle-dependent absorption based on resonance coupling [40], which makes this structure suffer from limited bandwidth. Therefore, it is necessary to explore alternative methods to realize angle-dependent asymmetric absorption with improved performance. ...
Asymmetric wave manipulation has attracted growing interest due to its great importance in practical applications. We design and demonstrate a planar acoustic metamirror for realizing asymmetric sound absorption with a controllable retroreflection coefficient, which is absent in conventional lossless meta-surfaces. We design the metamirror by realizing the required theoretical surface impedance profile, and then numerically and experimentally demonstrate its asymmetric response. The measured and simulated acoustic fields agree well with each other, which shows that the proposed metamirror can yield strongly asymmetric sound absorption: retroreflection and nearly full absorption for two opposite incident angles. In addition, the measured retroreflection coefficient and retroreflection angle are very close to the design values. We find that the asymmetrical excitation of evanescent waves plays a key role in the realization of asymmetric absorption. The proposed metamirror enriches the functionalities for acoustic wave manipulation and has prospective applications in many fields, such as acoustic antennas, acoustic sensing, and angle-encoded steganography.
... Under the assumption ( λ d ), the leakage rate can be expressed as a simple form. For two-dimensional (2D) resonators on a reflector with a period of λ d , the radiation leakage (kg/s per unit depth) is given by 17 36 , or the period is comparable to the wavelength, the acoustic reactance (i.e. imaginary part of C leak ) is non-negligible 37 . ...
We demonstrate broadband perfect acoustic absorption by damped resonances through inclusion of lossy porous media. By minimally placing the lossy materials around the necks of single-resonance Helmholtz resonators, where acoustic energy is concentrated, we show an increase in absorption bandwidths (>100% of the resonance frequency). Using the damped resonance, we demonstrate three types of broadband acoustic absorbers in one-port and two-port systems: broadband absorbers (one-port), broadband sparse absorbers (two-port), and broadband duct absorbers (two-port). Our approach for broadband absorption allows to minimize the number of resonances for compact absorbers, while it is beneficial for practical applications owing to the minimum use of porous materials.
Recently, with advances in acoustic metamaterial, the Fano resonance has been widely introduced into the field of noise reduction for simultaneous airborne soundproofing and airflow permeability. The conventional Fano-based mechanism, manifesting as an ultrasharp spectrum, just serves a narrow working-frequency range, which deviates from the demands of broadband sound insulation. Here, we theoretically present and experimentally demonstrate the concept of acoustic consecutive multiple Fano resonances (ACMFRs), unlocking an unprecedented soundproof bandwidth (80%) while maintaining highly efficient ventilation. We first develop an analytical model to analyze the transmission spectra of ACMFRs in the bilayer metamaterial. Owing to the superposition of multiorder Fano resonance modes, the response strength from the discrete and continuum states of ACMFRs remains quasibalanced over a broad frequency range, which can be further interpreted by a consecutive single-negative nature. Subsequently, we implement this concept with an ultraopen helical metamaterial, which yields broadband sound attenuation at 770–1768 Hz and high air permeability (90%) simultaneously. The experimental results coincide with both theoretical and numerical ones, validating the effectiveness of the proposed ACMFRs. Our work lays the groundwork for the next generation of Fano-based metamaterial applications, with far-reaching implications for noise control and related fields.
Acoustic metasurfaces are at the frontier of acoustic functional material research owing to their advanced capabilities of wave manipulation at an acoustically vanishing size. Despite significant progress in the last decade, conventional acoustic metasurfaces are still fundamentally limited by their underlying physics and design principles. First, conventional metasurfaces assume that unit cells are decoupled and therefore treat them individually during the design process. Owing to diffraction, however, the non-locality of the wave field could strongly affect the efficiency and even alter the behavior of acoustic metasurfaces. Additionally, conventional acoustic metasurfaces operate by modulating the phase and are typically treated as lossless systems. Due to the narrow regions in acoustic metasurfaces' subwavelength unit cells, however, losses are naturally present and could compromise the performance of acoustic metasurfaces. While the conventional wisdom is to minimize these effects, a counter-intuitive way of thinking has emerged, which is to harness the non-locality as well as loss for enhanced acoustic metasurface functionality. This has led to a new generation of acoustic metasurface design paradigm that is empowered by non-locality and non-Hermicity, providing new routes for controlling sound using the acoustic version of 2D materials.
This review details the progress of non-local and non-Hermitian acoustic metasurfaces, providing an overview of the recent acoustic metasurface designs and discussing the critical role of non-locality and loss in acoustic metasurfaces. We further outline the synergy between non-locality and non-Hermiticity, and delineate the potential of using non-local and non-Hermitian acoustic metasurfaces as a new platform for investigating exceptional points (EPs), the hallmark of non-Hermitian physics. Finally, the current challenges and future outlook for this burgeoning field are discussed.
