Broadband Sound Absorption Research Articles

SeMSA: a compact super absorber optimised for broadband, low-frequency noise attenuation

The attenuation of low-frequency broadband noise in a light, small form-factor is an intractable challenge. In this paper, a new technology is presented which employs the highly efficient visco-thermal loss mechanism of a micro-perforated plate (MPP) and successfully lowers its frequency response by combining it with decorated membrane resonators (DMR). Absorption comes from the membranes but primarily from the MPP, as the motion of the two membranes causes a pressure differential across the MPP creating airflow through the perforations. This combination of DMR and MPP has led to the Segmented Membrane Sound Absorber (SeMSA) design, which is extremely effective at low-frequency broadband sound absorption and which can achieve this at deep sub-wavelength thicknesses. The technology is compared to other absorbers to be found in the literature and the SeMSA outperforms them all in either the 20–1000 Hz or 20–1200 Hz range for depths of up to 120 mm. This was verified through analytical, finite element and experimental analyses.

Broadband sound absorption by a nested doll metasurface using multi-slit synergetic resonance

A sound-absorbing metasurface with a multi-slit synergetic resonant effect was studied that can achieve broadband sound absorption in a range betwen 500 Hz and 1150 Hz by using a nested doll supercell with a thickness of only 44.1 mm (the thickness being one sixteenth of the wavelength). The basic unit of the improved micro-slit absorber (IMSA), made by designing the slit of the micro-slit absorber as a type of step and locating it at the edge of the surface panel, is proposed for sound absorption at lower frequencies. The improved step slit can not only change the structure’s impedance to match the medium of air at a lower frequency, but can also guide sound waves into the slit and increase the air velocity step by step, which results in a strong friction loss in the lower micro-slit. On this basis, a nested doll supercell with eight integrated parallel IMSAs was designed and tested, and was able to achievesound absorption in a range between 500 Hz and 1150 Hz. In addition, the bandwidth was further broadened (to between 330 Hz and 1150 Hz) by adding four additional IMSAs to the previous supercell, while the surface area of the panel only increased by about 20%. This work highlighted the potentially extensive application of broadband noise reduction due to its depth, i.e. subwavelength thickness, and the relatively small surface area of the panel.

Underwater metastructure with broadband sound absorption capability in low-frequency range above 20 Hz

We present an underwater metastructure with excellent sound absorption effect below 50 Hz. The periodic metastructure unit consists of a conical cavity, rubber matrix, and two metal disks. FEM results show that, in the range of 20–300 Hz, the proposed metastructure demonstrates the excellent sound absorption within 279 Hz bandwidth when the reference absorption coefficient is considered to be 0.5. Displacement vibration diagrams illustrate the addition of two layers of metal disks break the propagation law of acoustic wave in rubber matrix. An anti-phase motion of the rubber matrix emerges due to the presence of the metal disks, then consumes the energy of incident acoustic waves. The geometric parameters of lattice constant [Formula: see text], the thickness [Formula: see text], and the height [Formula: see text] of the upper metal disk are positively correlated with the sound absorption coefficient, while the upper radius [Formula: see text] and the height [Formula: see text] of the conical cavity are negatively correlated with the sound absorption coefficient. The novel design presented in this study could have the potential applications in the realization of an acoustic underwater anechoic layer.

Teaching-learning-based optimization of a composite metastructure in the 0-10 kHz broadband sound absorption range.

This paper proposes a strategy to broaden the sound absorption region of porous materials by embedding ribs. The theoretical solution and the numerical simulations of the optimization model show that the composite metastructure exhibits ultra-wide high absorption characteristics and an average sound absorption coefficient of 0.937 in the 0-10 kHz range upon its teaching-learning-based optimization. High sound pressures are present only among the embedded ribs. A significant slowing down of the sound takes place inside the metastructure. The impedance tube test confirms the design of the broadband sound absorption region in agreement with the teaching-learning-based optimization method.

Broadband thin sound absorber based on hybrid labyrinthine metastructures with optimally designed parameters

Broadband acoustic absorbers with thin thickness are highly desired in practical situations such as architectural acoustics, yet it is still challenging to achieve high absorption by using structure with limited thickness. Here we report the theoretical optimal design, numerical simulation and experimental demonstration of a planar acoustic absorber capable of producing broadband sound absorption with deep-subwavelength thickness. The mechanism is that, we use a hybrid design of individual unit cell comprising multiple resonators with a coiled configuration for expanding the working bandwidth and downscaling the resulting device, and, on the other hand, the geometries of the constituent resonance elements are optimally designed by using genetic algorithm. Based on an analytical formula we derive for an efficient prediction of the absorption efficiency, the optimization process is accelerated and gives rise to an optimally maximized amount of absorbed energy with limited device thickness. As a result, the proposed absorber features planar profile, broad bandwidth, wide absorbing angle (the absorber works well when the incident angle of sound wave reaches 60°) and thin thickness (< 1/25 wavelength). In addition, the proposed scheme does not rely on extra sound-absorptive materials or the type of constituent solid material, which significantly simplifies the sample fabrication and improves the application potential of resulting device. The measured data agree well with the theoretical predictions, showing high sound absorption in the prescribed frequency range. We envision our design to further improve the performance of acoustic absorbers and find applications in practical situations in need of elimination of broadband acoustic waves within limited spaces.

Enhancement of sound absorption via vegetation with a metasurface substrate

The sound absorption via an absorber constructed by a vegetation layer and a metasurface substrate (MS) is numerically and experimentally investigated. By combining the vegetation and MS, high-efficiency sound absorption (>90%) can be achieved in a broader frequency range (from 0.93 kHz to 1.15 kHz). The MS subunit is composed of two 180°-twisted ellipse-shaped split tubes, which can achieve high absorption at the resonance frequency. According to the applications, the resonance absorption peak can be controlled by adjusting the length of the semi-major axis of the elliptical subunit. The acoustical properties of the vegetation layer can be described as a porous material, and its parameters in the simulation are obtained by fitting the measured sound absorption coefficients. Integrating the vegetation layer into the proposed absorber can further improve the absorptions in a wide frequency range of interest while it does not affect the resonance frequency of the MS subunit. Five different subunits are selected to form a deep-subwavelength MS array which can extend the absorption bandwidth. The broadband sound absorption of the absorber is robust under oblique incidence even at large incident angles. The proposed absorber has potential applications in acoustic engineering such as traffic and building noise control as well as green façades and green walls.

Broadband low frequency sound absorption using a monostable acoustic metamaterial.

This Letter presents a monostable acoustic metamaterial that has the potential to implement broadband sound absorption in low frequencies. The proposed metamaterial is realized by placing a flexible panel with a magnetic proof mass in a symmetric magnetic field. A theoretical model of the metamaterial is established and experimentally validated. Predictions and measurements demonstrate that the sound absorption peak frequency significantly shifts downwards with the increasing magnetic field. The relative bandwidth of the metamaterial is also broadened with the increasing magnetic field due to its inverse proportionality to the absorption peak frequency.

Design of subwavelength broadband hybrid sound absorption structure based on micro-perforated plate and coiled channels

In this paper, we propose a hybrid subwavelength broadband sound absorber based on micro perforated plate and multiple coiled channels. And the mechanism of low frequency broadband sound absorption of the hybrid sound absorber is analyzed in detail. Based on this, the theoretical analysis model and the finite element numerical analysis model are established, and the mutual verification of theoretical and numerical solutions is completed. The structure can theoretically achieve the low-frequency and high-efficiency sound absorption with an average absorption coefficient of 0.8 in a frequency band of 200–500 Hz when the overall thickness of the sound absorbing structure is 60 mm. At the same time when the overall thickness is 90 mm, quasi-perfect sound absorption with peaks up to 0.95 in a frequency range of 180–350 Hz is realized theoretically. The composite sound absorption structure has a certain application prospect in engineering low frequency noise in future.

A Sandwich Coating Containing Micro-perforated Panel for Underwater Sound Absorption

This paper focuses on the underwater application of Micro-perforated panel (MPP) for broadband sound absorption. In this paper, MPP-rubber coating (MPPRC), a novel sandwich anechoic coating embedded with MPP in high-viscosity condition, is proposed. The acoustic impedance for MPP is derived from Maa’s equation and vibration modal modification, while the acoustic propagation property of the viscoelastic rubber layer is obtained by the wave propagation equation. The theoretical calculation for the absorbency of the composite structure is conduced based on electro-acoustic analogy method and transfer matrix approach. The sound absorption validation is carried on in a hydroacoustic impedance tube filled with water, and the experimental results match with the theoretical model. Compared with the equal-thickness rubber, the MPPRC performances better at a lower frequency of 1.6 kHz ∼15 kHz due to MPP, which shows the potential to be utilized as an underwater anechoic coating.

Broadening sound absorption coefficient with hybrid resonances

In the last years, a great research effort has been focused on the noise mitigation at low frequencies. Membrane-type acoustic metamaterials (AMM) are one of the most promising solutions to meet the growing demand for low frequency sound absorbers. Typically, acoustic membrane absorbers require large back cavities to achieve low frequency sound absorption, which is usually categorised by a single narrow absorption peak. This paper presents an acoustic resonator unit cell, comprising of a thin elastic silicone plate with an air gap cavity with broadband absorption in a frequency range between 250 and 400 Hz. The broadband and multiple peak sound absorption showed by the proposed resonator is due to hybrid resonances which occur in the frequency range due to coupling of the structural dynamic response of the plate with the acoustic response of the air cavity. A numerical model based on acoustic-structural interaction, validated for experimental data, has been used to explain how the broadening gain in the sound absorption level is strictly related to the hybrid resonances of the unit cell resonator. We demonstrated that hybrid resonances are a function of the geometrical parameters and the ratio between the Young’s modulus and the density of the material plate, thus the proposed resonators absorption frequency range is tuneable at low frequencies allowing a wider broadband not achievable with acoustic membrane absorbers.

Between Science and Art: Thin Sound Absorbers Inspired by Slavic Ornaments

Acoustic metamaterials have opened promising opportunities for manipulation of low-frequency sound and development of compact structures with a broadband acoustic performance for noise mitigation applications, room, and architectural acoustics. So far, several mechanisms have been studied to achieve broadband sound absorption at subwavelength frequencies, when the system thickness is much smaller than the wavelength of sound waves. In this paper, we analyze two other approaches based on the use of interacting and coupled resonators with the aim to further reduce a structural thickness. We numerically demonstrate that properly designed panels with interacting resonators allow extending a single-peak absorption to broadband frequencies, while coupled resonators enable achieving broadband absorption at several frequency ranges. The developed designs are inspired by ancient Slavic folk patterns, attracting attention by their delicate ligature, multi-layered structure, and concealed meaning. Our results open new possibilities for creating acoustic metamaterials with art effects, which provide meaning to occupied space and make these metamaterials more appealing for applications.

2D underwater acoustic metamaterials incorporating a combination of particle-filled polyurethane and spiral-based local resonance mechanisms

This paper develops an underwater acoustic metamaterial plate with potential advantages of low-frequency broadband sound absorption and high hydrostatic pressure resistance. In this research, a new acoustic metamaterial embedding particle-filled polyurethane (PU) and spiral-based local resonance mechanism for underwater sound absorption is investigated numerically and experimentally. Specifically, the proposed metamaterial is composed of particle-filled PU damping materials and a square lattice of spiral resonators. The thickness of the proposed plate structure occupying most of the area is thin and equals to 4 mm. Wave propagation properties of the proposed metamaterial are obtained by using the finite element (FE) method. Compared to the underwater sound absorption coefficient, the formation mechanisms of the locally resonant band gaps are investigated based on the modal analysis of plate modes and local resonances. Results show that the location and bandwidth of newly generated locally low-frequency resonant band gaps are greatly affected by the interaction between particle-filled PU and the spiral resonator within every periodic cell. In addition, the theoretical results of the underwater sound absorption coefficient of the proposed metamaterial are compared with the experimental results. In the frequency domain [0.8 kHz, 6 kHz], the average sound absorption coefficient of the proposed metamaterial plate under normal atmospheric pressure is 0.54. Furthermore, the sound absorption coefficient of the proposed structure is experimentally studied under different hydrostatic pressure conditions. Specifically, the proposed metamaterial structure achieves average sound absorption coefficient of 0.51 in the frequency range [1.5 kHz,6kHz] under 0.5 MPa.

Bimodal hybrid lightweight sound-absorbing material with high stiffness

An iron foam/silica aerogel composite (IFSA) with bimodal structure was designed and fabricated to solve the contradictions between acoustic damping and mechanical strength of a material. IFSA exhibits excellent broadband sound absorption performance with average absorption of ∼57% (500–6400 Hz) and maximum absorption coefficient of 0.995 at 4336 Hz, and high specific stiffness (1.6 × 105 m2 s−2). The dissipation mechanism of sound energy in IFSA is well predicted by a designed bimodal model. Such fascinating artificial composite not only provides new insights for developing structural materials with high noise reduction and strength, but also has great potential applications for various fields.

Design of Broadband Sound Absorption Device based on Acoustic Metasurface with Multiple Membranes Resonator

Design of Broadband Sound Absorption Device based on Acoustic Metasurface with Multiple Membranes Resonator

Adaptive metamaterials for broadband sound absorption at low frequencies

We propose and design a new adaptive sound absorption metamaterial targeting broadband airborne noise at extremely low frequencies. The metamaterial consists of two piezoelectric smart elements: a circular aluminum membrane with surface-bonded piezoelectric films controlled by shunting circuits enclosed with an air cavity for nearly total acoustic absorption at narrow-band frequencies; a hybrid-circuit shunted piezoelectric stack which is mechanically grounded attached to the center of the membrane for purely stiffness control to broaden this high-absorption bandwidth. A piezoelectric-structural-acoustic coupled model is firstly developed to evaluate the sound absorption of the metamaterial. We then perform analytical and numerical tests on metamaterials with and without the piezoelectric stack to design a metamaterial with broadband absorption at desired low frequencies. The underlying adaptive mechanism is to automatically regulate the effective acoustic resistance and reactance of the metamaterial to achieve impedance match conditions, according to different frequencies of inputs. Our numerical results demonstrate that the absorption coefficient of the adaptive metamaterial can be greater than 0.9 in the frequency region, 112–236 Hz with the relative bandwidth being around 0.7. The metamaterial thickness is 30 mm, which is nearly 1/65.6 wavelength of the central frequency of the absorption band. The proposed adaptive metamaterial may open a new avenue towards broadband sound absorption at extremely low frequencies.

Acoustical siphon effect for reducing the thickness in membrane-type metamaterials with low-frequency broadband absorption

Acoustical siphon effect in membrane-type metamaterials for low-frequency broadband absorption is proposed, whose physical mechanism is further investigated by the theoretical analysis and finite element (FE) simulation. This kind of membrane-type metamaterials consists of multiple detuned units, each of which is composed of two aluminum platelets fasten on a piece of silicone membrane above an air cavity. For the multi-unit metamaterial illuminated by an incident plane wave with certain frequency, there exists a certain unit at resonance with maximum acoustic absorption, and meanwhile other units nearly keep static due to the narrow absorption peak of the resonant unit. Therefore, almost the whole incident energy can be forced to flow to this resonant unit resulting in a much enhanced vibration and a reduced acoustic impedance that could be more matchable to air medium, and thus much more acoustic absorption appears without increasing the unit thickness, which is called acoustical siphon effect of the unit. On this basis, by precisely designing the acoustical siphon effect of each unit, the broadband absorption can be obtained by a subwavelength six-unit sample in the low-frequency range of 400–650 Hz with the maximum absorption coefficient of almost 100% and the average absorption coefficient of about 80%, which is then verified by the corresponding experiment. The results presented here would offer a new approach for the metamaterials design for low-frequency broadband sound absorption and could have potential applications in controlling vibration and noise.

A multilayer microperforated panel prototype for broadband sound absorption at low frequencies

Microperforated panel (MPP) absorbers are one of the most promising alternatives to porous sound absorbing materials. However, these structures cannot achieve high and broadband absorption at low frequencies. To be effective, once defined the material properties the geometrical parameters of the absorber need to be optimized to match the prescribed absorption level. This paper presents a multiple layer MPP absorber with a high sound absorption coefficient and broadband absorption at low frequencies. An electro-acoustical equivalent circuit model was used for a parametric analysis to study the relationships between the absorption mechanism and the absorbers geometrical parameters in the proposed multilayer MPP. A prototype of this absorber was machined and tested in an impedance tube test ring and the experimental acoustical properties in terms of absorption coefficient were extracted using the transfer function method. It was demonstrated that the five-layer MPP absorber was capable of guaranteeing a high absorption (constantly over 90%) in a frequency range from 400 to 2000 Hz. The results indicate that the proposed multilayer MPP absorber provides a good alternative for sound absorption applications.

Experimental Study of Advanced Helmholtz Resonator Liners with Increased Acoustic Performance by Utilising Material Damping Effects

In aero engines, noise absorption is realised by acoustic liners, e.g., Helmholtz resonator (HR) liners, which often absorb sound only in a narrow frequency range. Due to developments of new engine generations, an improvement of overall acoustic damping performance and in particular more broadband noise absorption is required. In this paper, a new approach to increase the bandwidth of noise absorption for HR liners is presented. By replacing rigid cell walls in the liner’s honeycomb core structure by flexible polymer films, additional acoustic energy is dissipated. A manufacturing technology for square honeycomb cores with partially flexible walls is described. Samples with different flexible wall materials were fabricated and tested. The acoustic measurements show more broadband sound absorption compared to a reference liner with rigid walls due to acoustic-structural interaction. Manufacturing-related parameters are found to have a strong influence on the resulting vibration behaviour of the polymer films, and therefore on the acoustic performance. For future use, detailed investigations to ensure the liner segments compliance with technical, environmental, and life-cycle requirements are needed. However, the results of this study show the potential of this novel liner concept for noise reduction in future aero-engines.

Composite honeycomb metasurface panel for broadband sound absorption.

Composite honeycomb sandwich panels have been adopted in a wide range of applications owing to their excellent mechanical properties. This paper demonstrates a design of a composite honeycomb metasurface panel that can achieve 90% sound absorption from 600 to 1000 Hz with a thickness less than 30 mm. The panel is comprised of periodically and horizontally arranged honeycomb "supercells" which consist of unit cells of different geometric parameters (pore size). Two different analytical models (Helmholtz resonator model and micro-perforated panel model) are used to calculate the sound absorption of the panel, and they are further validated by a numerical model. The relatively broadband sound absorption is found to be attributed to the coupling between unit cells, which is illustrated by both the complex frequency plane theory and the calculated sound intensity field.

Anomalous wavefront manipulation and broadband sound absorption by metasurfaces with periodic subwavelength modulation

Both periodic subwavelength structures and phase gradient metasurfaces have opened up new degrees of freedom to manipulate the acoustic waves respectively. In this study, a periodic subwavelength structure is introduced into the metasurface to modulate the acoustic surface waves deduced by the phase gradient. Anomalous sound reflection beam steering is observed, as well as significant sound absorption is achieved with deep subwavelength thickness in a broadband frequency range as damping is considered. We comprehensively analyze the joint effect of the phase gradient and subwavelength periodic constraints. A local multiple reflections mechanism is proposed to reveal the underlying physics and the acoustic performances, showing an excellent agreement. Our realization opens a new perspective for the study of acoustic metasurfaces and paves the way for the design of novel devices such as acoustic holograms and broadband acoustic absorbers with deep subwavelength thickness.