Array Of Resonators Research Articles

Development of Acoustic Metamaterial Sheets with Two-Dimensional Array of Hollow Local Resonators

Novel practical acoustic metamaterial sheets with hollow local resonators that can be processed with simple integral molding have been developed. This acoustic metamaterial is a variant of the acoustic metamaterial sheet reported by our group in 2021, which consists of two-dimensionally connected non-hollow-shaped stubs embedded with metal weights. Each hollow-shaped stub can work as a local resonator using the vibration mode of a circular membrane. This acoustic metamaterial can handle lower frequencies even with smaller and lighter stubs compared to those with the non-hollow-shaped stubs we previously reported. It has a particular advantage when used for objects with limited space and weight. It is also possible to control the resonant frequency by altering the material properties or adjusting the shape and size of the stub. It has been experimentally shown that significant sound insulation performance exceeding the mass law can be obtained around the resonant frequencies of the hollow stubs. We show the results of finite element method simulations, modal vibration testing, and transmission loss measurements.

Sound absorption of resonators or perforated panels with multiple folded long tubes crammed in small module by additive manufacturing

Resonators or perforated panels give a large sound absorption at a resonant frequency tuned by opening area of the pores, neck length and a cavity depth behind the neck. The author has studied the sound absorption of thin tile-like resonators tuned at low frequencies by including a folded long neck in its surface panel. The resonators are made by an additive manufacturing with a fused deposition modeling (FDM). This paper makes the resonators or the perforated panels with multiple folded long tubes and discusses its measured sound absorption characteristics. One of the multiple folded long tubes is crammed in 12 mm × 12 mm × 19.2 mm of rectangular module even if the tube has 2 mm × 2 mm of opening area and 288 mm of length. A perforated panel consisted of planar periodic arrays of this module gives a large sound absorption at 69 Hz when a 40 mm depth of cavity behind the perforated panel. Furthermore, when the planar periodic arrays of small Helmholtz resonators applied single module and single cavity tuned at different resonant frequency, it has potential to form an acoustic metasurface with sound absorption at broadband frequencies.

Combination of a 3D-printed microperforated panel and a Helmholtz resonators array for low-frequency sound absorption

The application of solutions based on conventional porous materials for low-frequency sound absorption is often limited to the available space for their installation, as these require a significant thickness to be efficient. A system combining of a microperforated panel with an array of Helmholtz resonators, properly designed, can provide effective noise absorption at low- and mid-frequencies with a limited thickness. In this work, the sound absorption coefficient of several systems of this type was assessed by analytical calculations, numerical simulations and experiments carried out in an impedance tube according to the standard ISO 10534-1:1996. The prototypes were 3-D printed using recycled materials. The absorption coefficient, analytically obtained, was significantly higher than that measured. However, the results from numerical simulations are in good agreement with measurements, showing that the numerical model carried out can be used for further studies to optimize the system design. An example is given by increasing of the number of Helmholtz resonators in the array, that yields an increase in the sound absorption. Results show the potential of this type of solutions to be used for low-frequency noise absorption in applications where the available space is limited.

Chiral bound states in a staggered array of coupled resonators

We study the chiral bound states in a coupled-resonator array with staggered hopping strengths, which interacts with a two-level small atom through a single coupling point or two adjacent ones. In addition to the two typical bound states found above and below the energy bands, this system presents an extraordinary chiral bound state located within the energy gap. We use the chirality to quantify the breaking of the mirror symmetry. We find that the chirality value undergoes continuous changes by tuning the coupling strengths. The preferred direction of the chirality is controlled not only by the competition between the intracell and the intercell hoppings in the coupled-resonator array, but also by the coherence between the two coupling points. In the case with one coupling point, the chirality values varies monotonously with difference between the intracell hopping and the intercell hoppings. While in the case with two coupling points, due to the coherence between the two coupling points the perfect chiral states can be obtained.

Convergence Rates for Defect Modes in Large Finite Resonator Arrays

Convergence Rates for Defect Modes in Large Finite Resonator Arrays

Ultra-compact scalable spectrometer with low power consumption.

An ultra-compact on-chip spectrometer was demonstrated based on an array of add-drop micro-donut resonators (MDRs). The filter array was thermally tuned by a single TiN microheater, enabling simultaneous spectral scanning across all physical channels. The MDR was designed to achieve large free spectral ranges with multimode waveguide bends and asymmetric coupling waveguides, covering a spectral range of 40 nm at the telecom waveband with five physical channels (which could be further expanded). Benefiting from the ultra-small device footprint of 150 µm2, the spectrometer achieved a low power consumption of 16 mW. Additionally, it is CMOS-compatible and enables mass fabrication, which may have potential applications in personal terminals and the consumer industry.

Lossy dielectric resonator array absorber made of multiwalled carbon nanotubes/epoxy composite

A cylindrical lossy dielectric resonator array absorber is designed and verified by experiments. The designed cylindrical resonator array is made of a lightweight carbon multiwalled nanotube/epoxy composite material, and the gaps filled with polypropylene are used to fix the cylindrical array. Studies have shown that the lossy dielectric resonator array structure has dual-mode resonance characteristics, and the hybrid array composed of cylinders with different diameters can further generate multi-resonant coupling effects, thereby significantly improving the absorptivity. The total thickness of the designed absorber is 4 mm, and the normal incidence absorptivity is greater than 97% in the frequency range of 8–12 GHz. The measured results are consistent with the simulation, which verifies the reliability of the design concept and preparation method.

Rapid Detection of SARS-CoV-2 in Clinical and Environmental Samples via a Resonant Cavity SERS Platform within 20 min.

The coronavirus disease 2019 (COVID-19) epidemic has given a warning that it is important to explore the rapid detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in clinical specimens or environmental samples for public health strategies and future variants. The surface-enhanced Raman spectroscopy (SERS) technique was demonstrated to achieve this goal. However, the consistency of signals originating from the poor compatibility of virions with SERS hotspots remains a key scientific challenge for the practical applications of SERS. Herein, we develop a SERS platform for the ultrasensitive and rapid detection of SARS-CoV-2 antigen within 20 min by the combination of a highly consistent SERS substrate and a supervised deep learning algorithm. A V-shaped resonant cavity array (VRC) substrate was fabricated to trap SARS-CoV-2 virions in the periodic V cavity array and stimulate the integral SERS signal of the virus via a resonance coupling effect. Benefiting from the unique architecture of the VRC substrate, we were able to directly detect the SARS-CoV-2 virus with high sensitivity and high consistency. These excellent performances enabled us to identify five different kinds of SARS-CoV-2 variants and detect SARS-CoV-2 from clinical and environmental samples with high accuracies.

A compact low-frequency sound absorption metastructure realized by resonators with wavy bending necks

In this work, a compact low-frequency sound absorption metastructure composed of multiple resonators with embedded wavy bending necks is proposed. By arranging this metastructure in parallel and optimizing the parameters, it exhibits excellent broadband sound absorption capability in low-frequency range and has a much more compact volume. Compared with the traditional resonators, an individual resonator of this metastructure can move down the absorption frequency about 120 Hz while maintaining the same thickness. Furthermore, different resonator units are combined into a sound absorption array by employing appropriate design techniques. We first built a small metastructure composed of four units to demonstrate the correctness and accuracy of our design method. Both theoretical models and finite element simulation models are built and experimental results show good agreement between them. To achieve the same absorption value and frequency range, the thickest resonator in the traditional resonator array must be 30% thicker than the one in the wavy bending neck resonator array, which means the overall size of the structure is 30% larger. Following this design method, perfect sound absorption within the frequency range of 248 Hz–420 Hz is achieved with a compact volume of 53 mm in radius and 47 mm in height. The design strategy presents a new approach to achieve perfect broadband low-frequency sound absorption.

Enhancing the brain MRI at ultra-high field systems using a meta-array structure.

The main advantage of ultra-high field (UHF) magnetic resonance neuroimaging is theincreased signal-to-noise ratio (SNR) compared with lower field strength imaging. However, the wavelength effect associated with UHF MRI results in radiofrequency (RF) inhomogeneity, compromising whole brain coverage for many commercial coils. Approaches to resolving this issue of transmit field inhomogeneity include the design of parallel transmit systems (PTx), RF pulse design, and applying passive RF shimming such as high dielectric materials. However, these methods have some drawbacks such as unstable material parameters of dielectric pads, high-cost, and complexity of PTx systems. Metasurfaces are artificial structures with a unique platform that can control the propagation of the electromagnetic (EM) waves, and they are very promising for engineering EM device. Implementation of meta-arrays enhancing MRI has been explored previously in several studies. The aim of this study was to assess the effect of new meta-array technology on enhancing the brain MRI at 7T. A meta-array based on a hybrid structure consisting of an array of broadside-coupled split-ring resonators and high-permittivity materials was designed to work at the Larmor frequency of a 7 Tesla (7T) MRI scanner. When placed behind the head and neck, this construct improves the SNR in the region of the cerebellum,brainstem and the inferior aspect of the temporal lobes. Numerical electromagnetic simulations were performed to optimize the meta-array design parameters and determine the RF circuit configuration. The resultant transmit-efficiency and signal sensitivity improvements were experimentally analyzed in phantoms followed by healthy volunteers using a 7T whole-body MRI scanner equipped with a standard one-channel transmit, 32-channel receive head coil. Efficacy was evaluated through acquisition with and without the meta-array using two basic sequences: gradient-recalled-echo (GRE) and turbo-spin-echo (TSE). Experimental phantom analysis confirmed two-fold improvement in the transmit efficiency and 1.4-fold improvement in the signal sensitivity in the target region. In vivo GRE and TSE images with the meta-array in place showed enhanced visualization in inferior regions of the brain, especially of the cerebellum, brainstem, and cervical spinal cord. Addition of the meta-array to commonly used MRI coils can enhance SNR to extend the anatomical coverage of the coil and improve overall MRI coil performance. This enhancement in SNR can be leveraged to obtain a higher resolution image over the same time slot or faster acquisition can be achieved with same resolution. Using this technique could improve the performance of existing commercial coils at 7T for whole brain and other applications.

Fluid Identification Method of Nuclear Magnetic Resonance and Array Acoustic Logging for Complex Oil and Water Layers in Tight Sandstone Reservoir

In order to improve the logging interpretation accuracy for complex oil and water layers developed in tight sandstone reservoirs, this study takes the Chang 8 member of the Yanchang Formation in the Huanxian area as the research object, and two new fluid identification methods were constructed based on nuclear magnetic resonance (NMR) logging and array acoustic logging. Firstly, the reservoir characteristics of physical properties and conductivity were studied in the research area, and the limitations of conventional logging methods in identifying complex oil and water layers were clarified. Then, the sensitive parameters for identifying different pore fluids were established by analyzing the relationship between NMR logging and array acoustic logging with different pore fluids. On this basis, the fluid identification plate, composed of movable fluid apparent diffusion coefficient and effective porosity difference (Da-Δφe) by NMR logging data of D9TWE3 observation mode, and the other fluid identification plate, composed of apparent bulk modulus of pore fluid and elastic parameter sensitive factor (Kf-Fac), were constructed, respectively. Finally, these two fluid identification methods were used for reservoir interpretation of actual logging data. This study shows that the two new fluid identification methods constructed by NMR logging and array acoustic logging can effectively eliminate the interference of rock skeleton on logging interpretation, which make them more effective in identifying complex oil and water layers than the conventional logging method. Additionally, the two methods have their own advantages and disadvantages when used separately for interpreting complex oil and water layers, and the comprehensive interpretation of the two methods provides a technical development direction for further improving the accuracy of logging the interpretation of complex oil and water layers.

Adiabatic limit for scattering-free waveguiding in space-graded arrays of micro-resonators

We report on the dynamics of graded metamaterials in the context of micro-electromechanical systems (MEMS). Graded metamaterials are known to exhibit space confinement of mechanical energy - or rainbow trapping - in response to a tailored wave speed reduction. Such a behavior, in turn, is controlled by a gradual variation of the resonant characteristics of an array of resonators dressed on a host structure which, as a result, exhibits arbitrarily slow waves. The paper shows quantitatively that there is an adiabatic limit for the rate of change of the grading. Below this limit, the rainbow effect takes place with a negligible scattering of energy, which is otherwise detrimental to practical applications such as harvesting, sensing, and communication. The paper also paves new ways to manipulate wave motion in MEMS through elastic metamaterials.

Bandgap widening and resonator mass reduction through wave locking

Elastic metamaterials made of locally resonant arrays have been developed as effective ways to create band gaps for elastic or acoustic travelling waves. They work by implementing stationary states in the structure that localise and partially reflect waves. A different, simpler, way of obtaining band gaps is using phononic crystals, where the generated band gaps come from the periodic reflection and phase cancellation of travelling waves. In this work a different metamaterial structure that generates band gaps by means of coupling two contra-propagating modes is reported. This metamaterial, as it will be shown numerically and experimentally, generates larger band gaps with lower added mass, providing benefits for lighter structures.

Thermoplasmonic Controlled Optical Absorber Based on a Liquid Crystal Metasurface.

Metasurfaces can be realized by organizing subwavelength elements (e.g., plasmonic nanoparticles) on a reflective surface covered with a dielectric layer. Such an array of resonators, acting collectively, can completely absorb the resulting resonant wavelength. Unfortunately, despite the excellent optical properties of metasurfaces, they lack the tunability to perform as adaptive optical components. To boost the utilization of metasurfaces and realize a new generation of dynamically controlled optical components, we report our recent finding based on the powerful combination of an innovative metasurface-optical absorber and nematic liquid crystals (NLCs). The metasurface consists of self-assembled silver nanocubes (AgNCs) immobilized on a 50 nm thick gold layer by using a polyelectrolyte multilayer as a dielectric spacer. The resulting optical absorbers show a well-defined reflection band centered in the near-infrared of the electromagnetic spectrum (750-770 nm), a very high absorption efficiency (∼60%) at the resonant wavelength, and an elevated photothermal efficiency estimated from the time constant value (34 s). Such a metasurface-based optical absorber, combined with an NLC layer, planarly aligned via a photoaligned top cover glass substrate, shows homogeneous NLC alignment and an absorption band photothermally tunable over approximately 46 nm. Detailed thermographic studies and spectroscopic investigations highlight the extraordinary capability of the active metasurface to be used as a light-controllable optical absorber.

Low frequency sound isolation by a metasurface of Helmholtz ping-pong ball resonators

We study both numerically and experimentally an acoustic metasurface based on coupled Helmholtz resonators to obtain broadband low-frequency spectral responses for acoustic insulation. A hierarchical approach is proposed, starting from single and coupled Helmholtz resonators, up to a periodic array of resonators. To this end, we performed numerical simulations using the finite element method, in which the resonators are modeled as drilled rigid spheres in airborne environment and experimental demonstrations based on ping-pong balls as Helmholtz resonators in an acoustic reverberation box. We showed the alteration of the low-frequency response of acoustic insulation resulting from inter-unit coupling in acoustic metasurfaces, and the apparition of additional attenuation by inserting a plexiglass board as support for the structure.

Near-field imaging and spectroscopy of terahertz resonators and metasurfaces [Invited

Terahertz (THz) metasurfaces have become a key platform for engineering light-matter interaction at THz frequencies. They have evolved from simple metallic resonator arrays into tunable and programmable devices, displaying ultrafast modulation rates and incorporating emerging quantum materials. The electrodynamics which govern metasurface operation can only be directly revealed at the scale of subwavelength individual metasurface elements, through sampling their evanescent fields. It requires near-field spectroscopy and imaging techniques to overcome the diffraction limit and provide spatial resolution down to the nanoscale. Through a series of case studies, this review provides an in-depth overview of recently developed THz near-field microscopy capabilities for research on metamaterials.

Topology optimization of array of split-ring resonators in two-dimensional acoustic waveguide for low-frequency transmission problems

This work focuses on the topology optimization of an array of split-ring resonators to achieve nearly zero transmission in the low-frequency regime ranging from 500 Hz to 1000 Hz in 2D waveguide, under the condition of normal-incident plane wave and based on a homogenization scheme known as the Effective Medium Approach (EMA). The EMA transforms each individual resonator defined by its outer radius (rout), neck thickness (h), and the slit width (d), into a homogenous medium characterized by frequency-dependent physical parameters. The transmission coefficient (Tcoeff) of the array can thus be easily evaluated without relying on computationally expensive Finite Element (FE) software. Two optimization algorithms, namely “Sequential Quadratic Programming (SQP)” and “Genetic Algorithm (GA),” have been implemented to the abovementioned topological parameters with sum of Tcoeff being the objective function to minimize. Accuracy of the results has also been validated using software COMSOL Multiphysics® and the satisfactory level of accuracy has proven this approach to be a more efficient and time-saving method to conduct topology optimization of split-ring resonators in low-frequency regime. Moreover, it is noteworthy that the optimized dimensions of resonators are over 20 times smaller than the wavelength in the low frequency range where almost-zero transmission is achieved.

Ventilated silencer based on layered Helmholtz resonators

This study investigates the performance of a multilayer sound-attenuating metamaterial with ventilation capabilities. The design incorporates arrays of Helmholtz resonators embedded within the wall structure. Each cell of the barrier's front wall is a Euclidean polygon, triangular, square, or hexagonal, housing a parallel array of resonators interconnected via an axial ventilation duct. The sound attenuation performance of these barriers is meticulously examined using the lumped parameter theory and the transfer matrix method (TMM), complemented by finite element (FE) simulations. The results indicate that sound attenuation in the audible frequency range (300–2000 Hz) can be significantly enhanced by employing multiple layers of resonators, each layer assigned a unique peak resonance frequency. Despite the design's inherent trade-offs between barrier thickness, ventilation capacity, operational frequency range, and overall sound attenuation, its structural simplicity and straightforward theoretical performance estimation methods make it a promising candidate for applications requiring a specific attenuation spectrum. The sound-blocking performance of the design is further validated through impedance tube experiments, which show a reasonable agreement with the numerical predictions, thereby affirming the efficacy of the proposed design. The performance of multilayer design is also compared with the similar ventilated barriers proposed in the recent studies.

Broadband noise absorption by time-varying devices

A resonator array can broaden the effective sound absorption bandwidth by using the structure with multiple cavities in parallel or in series in space. However, this kind of fixed mechanical structure cannot flexibly adapt to different noise sources, which severely limits the development prospect of the resonator array in practical applications. In this paper, a time-varying resonator is proposed based on a shunted electrical circuit, realizing the equivalent effect of multi-cavities in space. Based on the working principle of spatial and temporal resonators, the theoretical models are established, and the three designs for achieving broadband effects are analyzed theoretically. Through time-domain numerical calculation, the sound absorption performance of the two spatial designs and the temporal design with the same cavity volume is obtained. This study focuses on the scheme design and performance optimization of the temporal resonator. Compared with the traditional spatial resonator, the proposed temporal resonator has the advantages of tunable frequency and high adaptability to varying sources while achieving similar or even better sound absorption performance.

An acoustic design project for final year engineering students using resonant arrays

This paper reviews the creation of a design course for mechanical engineering students at the University of Auckland. The primary objective of the course is to introduce students to the field of acoustics, teaching basic theoretical and experimental acoustics principles, together with product design and fabrication methods, in an engaging and educational manner. During the course, students were assigned the task of developing a noise reduction duct jacket that reduced sound propagation whilst allowing free airflow, all within specified dimensions. Students were required to create designs that attenuated a predefined sound spectrum, with both narrow-band and wide-band components. Over six weeks students developed their designs using mathematical modelling and fabricated their prototypes. The performance of their suppressors was assessed through sound pressure level and impedance tube transmission loss measurements. Students then modified their designs to maximise performance before final submission. Four-person teams produced a diverse range of implementations. The most successful designs achieved impressive performance with peak transmission loss up to 70dB, 2kHz bandwidths of 10 + dB transmission loss and a delta dBA reduction of 21.5 dB. Student feedback indicates a high level of satisfaction with the course, highlighting its effectiveness in imparting key knowledge and skills in acoustics engineering.