Perfect Sound Absorption Research Articles

Low-frequency perfect sound absorption achieved by a modulus-near-zero metamaterial

We have analytically proposed a mechanism for achieving a perfect absorber by a modulus-near-zero (MNZ) metamaterial with a properly decorated imaginary part, in which the perfect absorption (PA) is derived from the proved destructive interference. Based on the analysis, an ultrathin acoustic metamaterial supporting monopolar resonance at 157 Hz (with a wavelength about 28 times of the metamaterial thickness) has been devised to construct an absorber for low-frequency sound. The imaginary part of its effective modulus can be easily tuned by attentively controlling the dissipative loss to achieve PA. Moreover, we have also conducted the experimental measurement in impedance tube, and the result is of great consistency with that of analytical and simulated ones. Our work provides a feasible approach to realize PA (>99%) at low frequency with a deep-wavelength dimension which may promote acoustic metamaterials to practical engineering applications in noise control.

Aerogel-based metasurfaces for perfect acoustic energy absorption

The unusual viscoelastic properties of silica aerogel plates are efficiently used to design subwavelength perfect sound absorbers. We theoretically, numerically, and experimentally report a perfect absorbing metamaterial panel made of periodically arranged resonant building blocks consisting of a slit loaded by a clamped aerogel plate backed by a closed cavity. The impedance matching condition is analyzed using the Argand diagram of the reflection coefficient, i.e., the trajectory of the reflection coefficient as a function of frequency in the complex plane. The lack or excess of losses in the system can be identified via this Argand diagram in order to achieve the impedance matching condition. The universality of this tool can be further exploited to design more complex metasurfaces for perfect sound absorption, thus allowing the rapid design of efficient absorbing metamaterials.

Acoustic metaporous layer with composite structures for perfect and quasi-omnidirectional sound absorption

To enhance the sound absorption capability of porous materials, an acoustic metaporous layer is proposed. The sound absorption is enhanced based on the capability of reflected wave manipulation using the metaporous layer. The designed structure is periodic and one period of the metaporous layer contains four slits filled with the porous materials and embedded partitions. The partitions in the four slits are designed to form different phase responses of reflected waves, so that a linear phase gradient is generated within one period length. The phase gradient has a deciding influence on the reflected wave manipulation. The capability of manipulating the reflected waves is demonstrated numerically and experimentally. When the period is smaller than a half of wavelength, high-order reflected waves carrying the most energy are prevented from radiating into an acoustic far field at omni-directional incidence, resulting in an enhancement of oblique-incidence sound absorption compared with the uniform porous layer with the same thickness. The study shows that the remarkable enhancement of sound absorption can be obtained even though the phase gradient is not uniformly distributed, making the metaporous layer effective in a specific frequency range. The study provides a method to design non-uniform sound absorbers.

Deep subwavelength split ring neck acoustic resonator

Abstract Here, a novel acoustic resonator is proposed, realizing deep sub-wavelength form. An embedded split ring neck configuration is adopted, which makes the acoustic resonator being flat while lowering the acoustic resonance frequency. This resonant frequency can also be varied through changing the length of split ring neck. The effectiveness of analytical prediction is demonstrated through a case study, and perfect sound absorption is realized in low frequency range. The thickness of the proposed acoustic resonator is 1/79 of the resonant frequency’s wavelength, making such apparatus deep subwavelength.

A kind of tunable acoustic metamaterial for low frequency absorption

Low frequency noise is always an important factor affecting people’s quality of life. At present, the most widely used sound absorbing materials include polyurethane foam, trimeric amine, mineral cotton, textiles, cotton and special sound insulation materials. However, the sizes of these materials are generally large, and the sound absorption efficiencies are often low, especially in a low frequency range (below 2000 Hz). Acoustic metamaterial is a kind of artificial composite material, which is constructed by microunits whose dimensions are much smaller than the working wavelength. The results show that if the strong coupling condition between the resonant scatter and the waveguide is satisfied, the sound energy flowing through the metamaterial will be completely offset by the internal loss of the resonant scatter. Therefore, it is believed that this kind of acoustic metamaterial can solve the absorption problem of low-frequency sound waves. In order to solve this problem, researchers have conducted a lot of exploratory researches. However, most of the structural units that are constructed with acoustic metamaterials are passive, that is, once the material is processed and shaped, its properties are fixed and cannot be changed. This defect greatly limits the development of acoustical metamaterials, so it is urgent to study acoustical metamaterials whose material properties and the working frequency bands are flexibly adjustable. Although tunable acoustic metamaterials have been studied, few people have extended this research to the field of low-frequency tunable sound absorption. In our previous work, we systematically studied the acoustic properties of two kinds of acoustic artificial " meta-atoms”, namely, open hollow sphere model with negative equivalent elastic modulus and hollow tube model with negative equivalent mass density. The research shows that these two kinds of " meta-atoms” both have obvious sound absorption effect. According to our previous studies, in this paper we couple these two kinds of " meta-atoms” into a whole, and design a new nested model of open loop. The model has the advantages of simple structure and easy preparation. Through theoretical analysis, numerical simulation and experimental testing, it is found that the strong coupling resonance effects between these " meta-atoms” can be excited by the low frequency incident acoustic wave in the nested structure, thus achieving nearly perfect sound energy absorption. In addition, the relative impedance of the metamaterial can be changed by simply rotating the inner splitting ring around the axis, therefore the position of the absorption peak can be freely controlled in a wide frequency band. Because of its deep sub-wavelength size, the metamaterial is very useful for miniaturizing and integrating the low-frequency acoustic absorption devices. What is more, this model also lays a foundation for designing the broadband absorbers.

Perfect sound absorption of ultra-thin metasurface based on hybrid resonance and space-coiling

We propose an acoustic metasurface for perfect absorption at dual frequencies within a compact space. Meta-molecules of the metasurface contain four subwavelength meta-atoms whose cavities are coiled by three foldings. The meta-atoms comprising a meta-molecule have slightly different neck sizes to obtain hybrid resonances at desired frequencies. In order to consider an effect of coiled spaces in the meta-atoms, we use equivalent straight cavities with an effective length in an analytical model. By using the analytical model, metasurfaces are optimally designed for perfect absorption at desired frequencies. The experimental results show that a λ/23-metasurface exhibits over 99% energy absorption at 294 Hz and 406 Hz. Furthermore, we define another optimization problem to minimize the thickness of the metasurface for perfect absorption at two desired frequencies. The experimental results show that a λ/32-metasurface exhibits over 91% energy absorption at 281 Hz and 99% sound energy absorption at 403 Hz.

Perfect Absorption of Sound by Rigidly-Backed High-Porous Materials

The authors acknowledge financial support from the Metaudible Project No. ANR-13-BS09-0003, co funded by ANR and FRAE. This article is based upon work from COST Action DENORMS CA 15125, supported by COST (European Cooperation in Science and Technology).

Dual-frequency sound-absorbing metasurface based on visco-thermal effects with frequency dependence

We investigate theoretically an acoustic metasurface with a high absorption coefficient at two frequencies and design it from subwavelength structures. We propose the use of a two-dimensional periodic array of four Helmholtz resonators in two types to obtain a metasurface with nearly perfect sound absorption at given target frequencies via interactions between waves emanating from different resonators. By considering how fluid viscosity affects acoustic energy dissipation in the narrow necks of the Helmholtz resonators, we obtain effective complex-valued material properties that depend on frequency and on the geometrical parameters of the resonators. We furthermore derive the effective acoustic impedance of the metasurface from the effective material properties and calculate the absorption spectra from the theoretical model, which we compare with the spectra obtained from a finite-element simulation. As a practical application of the theoretical model, we derive empirical formulas for the geometrical parameters of a metasurface which would yield perfect absorption at a given frequency. While previous works on metasurfaces based on Helmholtz resonators aimed to absorb sound at single frequencies, we use optimization to design a metasurface composed of four different Helmholtz resonators to absorb sound at two distinct frequencies.

Tunable acoustic absorbers with periodical micro-perforations having varying pore shapes

Circular pores with sub-millimeter diameters have been widely used to construct micro-perforated panels (MPPs), the acoustical performance of which can be predicted well using the Maa theory (Maa D.-Y., J. Acoust. Soc. Am., 104 (1998) 2861). We present a tunable MPP absorber with periodically arranged cylindrical pores, with their cross-sectional shapes systematically altered around the circle while maintaining their cross-sectional areas unchanged. Numerical analyses based on the viscous-thermal coupled acoustical equations are utilized to investigate the tunable acoustic performance of the proposed absorbers and to reveal the underlying physical mechanisms. We demonstrate that pore morphology significantly affects the sound absorbption of MPPs by modifying the velocity field (and hence viscous dissipation) in the pores. Pore shapes featured as meso-scale circular pores accompanied with micro-scale bulges along the boundaries can lead to perfect sound absorption at relatively low frequencies. This work not only enriches the classical Maa theory on MPPs having circular perforations, but it also opens a new avenue for designing subwavelength acoustic metamaterials of superior sound absorption in target frequency ranges.

Rainbow-trapping absorbers: Broadband, perfect and asymmetric sound absorption by subwavelength panels for transmission problems

Perfect, broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem. The panels are composed of a periodic array of varying crosssection waveguides, each of them being loaded by Helmholtz resonators (HRs) with graded dimensions. The low cut-off frequency of the absorption band is fixed by the resonance frequency of the deepest HR, that reduces drastically the transmission. The preceding HR is designed with a slightly higher resonance frequency with a geometry that allows the impedance matching to the surrounding medium. Therefore, reflection vanishes and the structure is critically coupled. This results in perfect sound absorption at a single frequency. We report perfect absorption at 300 Hz for a structure whose thickness is 40 times smaller than the wavelength. Moreover, this process is repeated by adding HRs to the waveguide, each of them with a higher resonance frequency than the preceding one. Using this frequency cascade effect, we report quasi-perfect sound absorption over almost two frequency octaves ranging from 300 to 1000 Hz for a panel composed of 9 resonators with a total thickness of 11 cm, i.e., 10 times smaller than the wavelength at 300 Hz.

Heterogeneously perforated honeycomb-corrugation hybrid sandwich panel as sound absorber

An ultra-lightweight sandwich panel with perforated honeycomb-corrugation hybrid (PHCH) core is innovated as a novel sound absorber, which demonstrates great sound absorption as well as excellent mechanical performance. Based on the hybrid-cored sandwich panel, small perforations of different diameters distributed heterogeneously on both the top facesheet and the corrugation are introduced to obtain perfect sound absorption at low frequencies with almost no deterioration in mechanical performance. Theoretical models for sound absorption coefficient and bending stiffness are established, then verified by numerical simulations. With specific mass taken into consideration, an integrated index is proposed to further evaluate the acoustic-mechanical property of the hybrid sandwich. It is found that the PHCH with thinner honeycomb always performs better at low frequencies, and the facesheet is more important to the acoustic-mechanical property than the corrugation. This new kind of lightweight sandwich constructions show promising engineering applications, capable of serving as multi-functional structures with great acoustic and mechanical properties at the same time.

Three-dimensional acoustic characteristic study of porous metasurface

We propose a two-dimensional (2D) acoustic metasurface fabricated using porous material. An analytical model is developed to predict the three-dimensional sound field produced by an incident acoustic wave interacting with acoustic porous metasurface. The validity of analytical model is validated by comparing with numerical simulations. The directivities of reflected waves can be fully predicted by the analytical model, which overcomes the limitations imposed by using a model based on the three-dimensional generalized Snell’s law. The proposed porous metasurface allows to provide quasi-omnidirectionally perfect sound absorption at the designated frequency and a fixed periodic length. Such porous metasurfaces also display better broadband sound absorption properties than that constructed by a homogeneously porous layer.

Hybrid acoustic metamaterial as super absorber for broadband low-frequency sound

A hybrid acoustic metamaterial is proposed as a new class of sound absorber, which exhibits superior broadband low-frequency sound absorption as well as excellent mechanical stiffness/strength. Based on the honeycomb-corrugation hybrid core (H-C hybrid core), we introduce perforations on both top facesheet and corrugation, forming perforated honeycomb-corrugation hybrid (PHCH) to gain super broadband low-frequency sound absorption. Applying the theory of micro-perforated panel (MPP), we establish a theoretical method to calculate the sound absorption coefficient of this new kind of metamaterial. Perfect sound absorption is found at just a few hundreds hertz with two-octave 0.5 absorption bandwidth. To verify this model, a finite element model is developed to calculate the absorption coefficient and analyze the viscous-thermal energy dissipation. It is found that viscous energy dissipation at perforation regions dominates the total energy consumed. This new kind of acoustic metamaterials show promising engineering applications, which can serve as multiple functional materials with extraordinary low-frequency sound absorption, excellent stiffness/strength and impact energy absorption.

Ultra-thin metamaterial for perfect and quasi-omnidirectional sound absorption

Using the concepts of slow sound and critical coupling, an ultra-thin acoustic metamaterial panel for perfect and quasi-omnidirectional absorption is theoretically and experimentally conceived in this work. The system is made of a rigid panel with a periodic distribution of thin closed slits, the upper wall of which is loaded by Helmholtz Resonators (HRs). The presence of resonators produces a slow sound propagation shifting the resonance frequency of the slit to the deep sub-wavelength regime (λ/88). By controlling the geometry of the slit and the HRs, the intrinsic visco-thermal losses can be tuned in order to exactly compensate the energy leakage of the system and fulfill the critical coupling condition to create the perfect absorption of sound in a large range of incidence angles due to the deep subwavelength behavior.

Subwavelength total acoustic absorption with degenerate resonators

We report the experimental realization of perfect sound absorption by sub-wavelength monopole and dipole resonators that exhibit degenerate resonant frequencies. This is achieved through the destructive interference of two resonators' transmission responses, while the matching of their averaged impedances to that of air implies no backscattering, thereby leading to total absorption. Two examples, both using decorated membrane resonators (DMRs) as the basic units, are presented. The first is a flat panel comprising a DMR and a pair of coupled DMRs, while the second one is a ventilated short tube containing a DMR in conjunction with a sidewall DMR backed by a cavity. In both examples, near perfect absorption, up to 99.7%, has been observed with the airborne wavelength up to 1.2 m, which is at least an order of magnitude larger than the composite absorber. Excellent agreement between theory and experiment is obtained.

Sound reflection in an acoustic impedance tube terminated with a loudspeaker shunted by a negative impedance converter

A system for the absorption of sound at the termination of an acoustic tube using an electrodynamic loudspeaker connected to a shunt circuit is presented. A theoretical model of the electrodynamic loudspeaker in the acoustic tube is used for the calculation of the frequency dependence of the ideal shunt circuit impedance, which yields perfect sound absorption in broad frequency range. It is shown that both the real and imaginary parts of the shunt circuit impedance must be negative in the considered system. The required negative values of the shunt circuit impedance are achieved using a negative impedance converter. Frequency dependences of the reflection coefficient were measured in the acoustic impedance tube using the two-microphone transfer function method. Greatly reduced values of sound absorption coefficient were achieved in a narrow frequency range. The stability, applicability of the sound absorption system, and broadening its frequency range are discussed.

119 消音ホイールカバーによるタイヤ騒音低減に関する研究(その2)(騒音振動対策,騒音・振動評価・改善技術)

This paper studied a new concept of a quieting wheel cover which can attenuate tire road noise. BEM simulations predicted that ρ c boundary at the circumferential surface could reduce radiated noise from a tire to 90°direction by 2〜4dB and acoustically soft boundary could reduce it by more than 10dB. These results were proved by speaker tests. A sound absorbing wheel cover could attenuate tire road noise by 2〜4dB at the frequency higher than 630Hz in which almost perfect sound absorption is established. And a resonator wheel cover could attenuate it by 6dB at 1250Hz band. Finally a drum test showed that a sound absorbing wheel cover could attenuate it by 5dB at any speed.