Impedance Tube Experiments Research Articles

Analysis of Low-Frequency Sound Absorption Performance and Optimization of Structural Parameters for Acoustic Metamaterials for Spatial Double Helix Resonators

Low-frequency noise absorbers often require large structural dimensions, constraining their development in practical applications. In order to improve space utilization, an acoustic metamaterial with a spatial double helix, called a spatial double helix resonator (SDHR), is proposed in this paper. An analytical model of the spatial double-helix resonator is established and verified by numerical simulations and impedance tube experiments. By comparing the acoustic absorption coefficients of the spatial double-helix resonator, it is shown that the results of the analytical model, the numerical model, and the experiments are in good agreement, proving the accuracy of the theoretical model. The effects of different structural parameters on the peak sound absorption coefficient and resonance frequency are quantitatively revealed. The impedance variation law of the model is obtained, and the resistance and reactance distributions at the resonance frequency are analyzed. In the optimization model, the Back Propagation (BP) network is used to construct the mapping between the structural parameters and the resonance frequency and sound absorption coefficient, and this is used as the constraints of the equation, which is combined with Wild Horse Optimization (WHO) to establish the BP-WHO optimization model to minimize the volume of the spatial double helix resonator. The results show that, for a given noise frequency, the optimized structural parameters enhance the space utilization without affecting the performance of the space double helix resonator.

Efficient and broadband sound absorption properties of slotted aluminum foam

To enhance the sound absorption performance of aluminum foam, a slotted structure was developed. Firstly, the theoretical model of sound absorption for the slotted aluminum foam was established by the transfer matrix method. Secondly, the finite element model was established using COMSOL software to predict the sound absorption coefficient. The reliability of the theoretical and finite element models was validated through impedance tube experiments. The sound absorption mechanism is investigated by analyzing the internal sound field. Finally, the sound absorption properties of aluminum foam with other slot patterns are investigated. Additionally, the factors that influence sound absorption properties are investigated. The results indicate that the slots alter the sound pressure distribution within the material, inducing a pressure diffusion effect. When sound waves enter the interior of the material through the narrow slots, they are absorbed and dissipated by the matrix material on the sides of the slots. The sound absorption coefficient can be improved by increasing the thickness, slot scale, and slot depth of the slotted aluminum foam. Specifically, when the slot depth is 15 mm, and the slot width is 5 mm, the average sound absorption coefficient of incompletely slotted aluminum foam in the frequency range of 1000 ∼ 6300 Hz is 0.86, which can realize broadband sound absorption. With the increase of slot depth, the sound absorption peak becomes more pronounced.

Experimental study on the acoustic properties of monofilament fabrics

The traditional machining perforated sound absorption structure has high processing cost and inconvenient installation and disassembly, so that how to efficiently and economically manufacture submillimeter micropores still remains a challenge. In this study, a kind of flexible microporous sound absorbing material was obtained by weaving a polymer artificial fiber covered with ethylene polyester fiber, and the relationship between the sound absorption performance of the curtain and the porosity of the pass and fabric was obtained through the finite element simulation model and impedance tube experiment. The mechanism parameters of wide-band flexible microporous material with high sound absorption performance are obtained, which guide the design of a kind of sound absorption material with lower production cost and easy installation and disassembly.

A multifunctional heterogeneous structure inspired by shell brick-mud structure: Balancing noise reduction and mechanical performance

Electric armoured vehicles, which are crucial for strategic operations, must possess superior mechanical properties and noise reduction to maintain normal functionality. However, achieving high energy absorption and noise reduction performance simultaneously is challenging with a single uniform material or structure. This study introduces heterogeneous structures with metal lattice skeletons filled with polymer materials. The skeleton, known as the bionic mantis shrimp structure (BMS), is filled with polyurethane and epoxy. Impedance tube experiments revealed that the sound absorption coefficients of Heterostructure-polyurethane foam (HS-PU) and Heterostructure-epoxy (HS-EP) are significantly higher than BMS. HS-PU achieved an average sound insulation of 33.6 dB, marking a 739 % improvement over BMS. Compression tests show that polymer filling can inhibit the destructive collapse of BMS and enhance its mechanical performance. HS-EP exhibits specific strength and specific energy absorption values of 57 MPa/(g·cm−3) and 30.75 J/g, respectively, representing an increase of 67.6 % and 315 % compared to BMS. Additionally, as the volume fraction of BMS increased in the heterogeneous structure, the elastic modulus and compressive strength increase while the specific energy absorption decreased. This research suggest that the design of metal/polymer heterogeneous structures offers a promising approach for developing high-strength and noise-reducing protective structures.

Laminated acoustic metamaterials for low-frequency broadband ultra-strong sound insulation

In this paper, a kind of laminated acoustic metamaterials (LAMS), which can realize the bandwidth widening and amplitude improvement through the parallel coupling of in-plane gradient parameters and the series laminated design in the thickness direction of the local resonance units, is proposed. The designed LAMS consists of two different layers of single-layer acoustic metamaterials (SLAMs) laminated in series in the thickness direction. During the design process of the two layers of SLAMs, non-uniform mass distribution and non-consistent frame coupling layouts are respectively introduced to broaden the sound insulation frequency band of the acoustic metamaterials. Through careful design, the two layers of SLAMs have distinct working frequency bands, effectively preventing the occurrence of sound insulation valleys with low amplitude. Based on numerical analysis of two layers of SLAMs, this study discusses the results of transmission loss (TL) and the working mechanism of interlayer superposition coupling sound insulation. The analysis of TL for LAMS reveals its capability to achieve ultra-strong sound insulation effects across a low-frequency broadband range. The accuracy of the numerical analysis in this study is verified through semi-analytical methods and small-scale impedance tube experiments. The LAMS proposed in this paper holds significant potential for wide-band noise reduction control in various high-noise mechanical equipment or workplaces.

Numerical analysis and experimental study of membrane-type acoustic metamaterial plate with X-shaped pendulum arm and cylindrical mass blocks

Membrane-type acoustic metamaterials have a favorable noise suppression effect. Hence, a membrane-type acoustic metamaterial plate (MAML) with an X-shaped pendulum arm and cylindrical mass blocks (CMB) was proposed in this paper. The theoretical model based on spring–mass systems and numerical simulation models of the membrane-type acoustic metamaterial cell (MAMC) were established under fixed and periodic boundary conditions to reveal the noise attenuation mechanism quantitatively, and the necessity of the CMB is discussed. Based on the results, it can be observed that the normal displacement of the membrane is nearly zero when the noise frequency is at the peak of the sound transmission loss (STL) curve. However, when the frequency is at the valley of the STL curve, the displacement is nonzero and fluctuates significantly. Meanwhile, by comparing the STL curves with and without the CMB, it was found that the MAMC performance is improved effectively by the CMB at low frequencies. The effective mass density of MAMC was found to be negative. To verify the accuracy of numerical calculations, an impedance tube experiment was conducted. Finally, orthogonal experiments were designed to describe the effects of the structural parameters a, l, and t on the effective bandwidth [Formula: see text], peak frequency [Formula: see text], and comprehensive index [Formula: see text] and to obtain the optimal structural parameter combinations for different indexes. This work further contributes to applying and developing the membrane-type acoustic metamaterial.

3D Printed Nature Pattern Metastructure for The Sound Absorption Coefficient

This research investigates the influence of the shape of 3D printed metastructures on sound absorption performance, focusing on three distinct configurations inspired by nature: honeycomb, spiderweb, and gyroid. The objectives involve designing and producing sound-absorbing structures using the 3D printing Fused Deposition Modelling (FDM) method. Simultaneously, the study analyzes and compares their sound absorption performance by considering the sound absorption coefficient and the noise reduction coefficient (NRC) for each pore shape. Using computer-aided software, the three different pore shapes were designed and produced using Polylactic Acid (PLA) material through FDM 3D printing. Each design was replicated at three different thicknesses: 10mm, 20mm, and 30mm, all with a diameter of 94mm. The sound absorption coefficient of the samples was evaluated through impedance tube experiments, collecting data for alpha (α) from 250 Hz to 2000 Hz. Subsequently, NRC values were calculated for four different frequencies: 250Hz, 500Hz, 1000Hz, and 2000Hz, ensuring a comprehensive analysis. Results indicate that the gyroid structure exhibited the highest overall sound absorption coefficient across the tested frequency range, followed by the spider-web and honeycomb structures. Additionally, the 30mm thickness demonstrated greater sound-absorbing performance than the 20mm and 10mm thicknesses. These outcomes provide valuable insights into the sound absorption capabilities of 3D printed metastructures, highlighting the superior performance of the gyroid structure. Understanding the impact of pore shape and thickness on sound absorption performance contributes to the development of acoustically optimized materials for various applications

Exploring acoustic properties of banana fiber composites with elastomeric filler through a computational approach

In recent years, there has been a significant increase in the use of natural fibers as reinforcements in composites. This trend is mainly driven by their eco-friendly nature and wide availability. The objective of this research is to explore the extensive potential of combining Banana Fiber with Tyre Rubber Particles (TRP) as elastomeric fillers in natural fiber composites, focusing primarily on their applications in the automotive and aeronautical industries, particularly in terms of acoustic response. The composite sequences were named as NFC-I to NFC-V, where NFC represented Natural Fiber Composites. To thoroughly investigate the acoustic properties of these innovative composites, experimental analysis on the impedance tube was conducted along with numerical simulations on the Ansys Harmonic Acoustics® module with Transmission Loss (TL) as the key parameter. An orthotropic material model based on the chopped fiber RVE (Representative Volume Element) was developed on the Ansys 2022 R1® Material Designer Module which could be deemed as the novelty of the current work. From the results, it was seen that within the designated frequency bands, specifically the low (67–400 Hz), medium (400–1900 Hz), and high (1900–6300 Hz) ranges, the NFC-4 (15 wt% of TRP) composite consistently demonstrated the lowest average TL values. Conversely, NFC-3 (27 wt% of TRP) consistently showcased the highest TL values, reaching a peak of 27.13 dB in the low-frequency range and 46.73 dB in the high-frequency range. The results obtained from the impedance tube experiments align remarkably well with the numerical simulation outputs, demonstrating the reliability of the numerical model in estimating the transmission loss of composite laminatesa crucial parameter for evaluating acoustic performance.

Fiber metal laminates for high strain rate applications with layerwise shock impedance tuning

Novel materials such as fiber-metal laminates (FMLs) have demonstrated significant potential in a variety of applications. They must contend with problems such fatigue, creep, high-speed projectile impact, and deformation at high strain rates while in use. When employed as structural materials in aircraft, especially when exposed to shock wave impact and high velocity impact, fiber-metal laminates’ high strain rate characteristics become crucial. Shock impedance matching is a revolutionary approach used for shock-tuning the separate layers. The novelty of the current work is in developing custom shielding laminates, with in-depth analysis on the response of the shock impedance tuning of individual layers on the laminate behaviour at high strain rates. In the current study, five stackups of FMLs comprising metallic (AA 6061-T6) and fiber-reinforced polymer (FRP) plies, were formulated, incorporating shock impedance matching. The fiber-polymer plies used in the FMLs include ultra-high molecular weight polyethylene (UHMWPE), p-aramid for supplementing the impact resistance. Transmission loss functions (TL) estimated from the impedance tube experiments were used to indicate the shock tuning of the various laminates. The laminates underwent testing using a Split Hopkinson Pressure Bar (SHPB) apparatus to determine their properties at high strain rates (350, textrm{s}^{-1} to 460, textrm{s}^{-1}). The variation in the Shock Energy (SE) absorbed by the laminates at various strain rates was analyzed as a function of the corresponding Transmission Loss employing regression. The dynamic stress-strain curves showed an increase in shock energy absorption at higher strain rates. The sequence SSP-IV and SSP-II showed the highest values of energy absorption as well as Transmission Loss.

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.

Sound absorption mechanism of sonic black hole with labyrinthine units and micro perforated panel

Acoustic black hole (ABH) is a technique capable of manipulating the propagation of flexural wave, and the sonic black hole (SBH) is a kind of ABH which is used to manipulate sound wave in a fluid medium. In this paper, we propose an SBH structure with labyrinthine units and combine with micro-perforated panel (MPP) to form a composite sound absorption structure. The sound absorption mechanism of the absorption structure is deeply investigated using numerical and simulation methods. The simulation reveals the sound absorption mechanism by acoustic streaming effects of composite sound absorption structure. We analyze the flow characteristics of the acoustic medium under acoustic excitation, and the effect of the flow field on the distribution of the acoustic field, and the energy dissipation distribution. Our theoretical results show that the sound absorption is attributed to the effects of sound energy focusing of ABH, the local resonance of MPP, and the acoustic energy localization and dissipation effect of labyrinthine units caused by large flow velocity gradients. Finally, the proposed composite sound absorption structure has good sound absorption performance, which is also confirmed by impedance tube experiments. It can provide a new way of thinking for the design and optimization of the SBH structure.

A composite acoustic black hole for ultra-low-frequency and ultra-broad-band sound wave control

Achieving ultra-low and ultra-broad-band sound absorption and full-band sound insulation is a major challenge. Here, we propose a composite structure of a multilayer micro-perforated plate and acoustic black holes to achieve this purpose. Combining the stable sound absorption effect of the multilayer micro-perforated plate in the full frequency band and the sound insulation effect of the acoustic black hole in the low frequency and the excellent sound absorption effect in the high frequency, the excellent sound control effect of 600–3150 Hz absorption coefficient greater than 0.8 and 100–3150 Hz sound transmission loss greater than 50 dB is achieved. The acoustic properties of different components and different acoustic black hole outlet were evaluated by finite element method, and the principles of sound absorption and insulation of the composite structure were elaborated. Finally, the results of finite element method are verified by impedance tube experiments. This work can make further progress in elucidating the acoustic properties of the ABH and open up new avenues in the control of ultra-low and ultra-wide frequency acoustic waves.

A design method for accurate acoustic impedance matching under coherent coupling of sound-absorbing subunits

Acoustic metamaterials have garnered significant attention as an effective means to control low-frequency noise. However, the accurate design of complex structures composed of multiple subunits is still a challenge due to local coupling effects. To address this issue, in this work, a new design method is proposed that accurately achieves impedance matching at the target frequency when subunits are coupled in parallel. The method is demonstrated using six Fabry–Pérot (F–P) tubes to achieve perfect sound absorption in the continuous band of 405–445 Hz and the discontinuous bands of 400–410 and 430–440 Hz. Theoretical results show an average absorption coefficient of 99.3% in the target frequency band, which is verified through an impedance tube experiment. In addition, this paper explores the stability of this method under complex design conditions and discusses the mechanism of the influence of subunit parameters on sound-absorption performance from the perspective of impedance matching. Overall, the proposed design method offers a promising approach to achieving broadband sound absorption using multiple coupled subunits. The results of this study provide valuable insights for future research and the design of acoustic metamaterials.

Experimental study of a slit in the presence of a bias flow under medium- and high-level acoustic excitations

This work presents an experimental investigation of the acoustic properties of a slit in the presence of a bias flow under moderate- and high-acoustic excitations. Impedance tube experiments are discussed for a geometry inspired by deep punching resulting in a cut in the plate. The acoustic transfer impedance of the plate is discussed for several bias flow velocities, acoustic excitation, and different frequencies. In the range considered for this study, a bias flow appears to have two main effects, globally enhancing the sound absorption of the plate and creating a protective layer downstream of the plate due to the interaction between the slits. A maximum of the enhancement factor is found at a specific ratio between the acoustic velocity and the bias flow velocity. Two simple asymptotic behaviors are found, dominated by the flow or by the acoustic excitation, respectively. The behavior of the inertance is complex. Globally the inertance decreases with decreasing flow Strouhal number while its dependency on the amplitude of the acoustic velocity is less obvious.

Ventilated acoustic meta-barrier based on layered Helmholtz resonators

The performance of a multilayer sound attenuating metamaterial with ventilation capacity based on arrays of wall-embedded Helmholtz resonators is investigated analytically, numerically and experimentally. Each cell of the barrier front wall is a Euclidian polygon, such as square or hexagon. One thickness layer of the barrier cell contains a parallel array of 4 (for square cell) or 6 (for hexagon) Helmholtz resonators connected via the axial ventilation duct element. Multiple layers of resonators can be connected in sequence with the extension of the ventilation channel. The sound attenuation performance of the barriers is investigated first using the lumped parameter theory and transfer matrix method (TMM), and numerically using the finite element (FE) simulations solving the Helmholtz equations in frequency domain. These results indicate that the sound attenuation in audible range (300–2000 Hz) can be meaningfully improved using several layers with a unique peak resonance frequency assigned to each corresponding layer, with each additional layer improving the total attenuation. However, it is acknowledged that the previously studied fundamental trade-offs between the total barrier thickness, ventilation capacity, operational frequency range and integral sound attenuation do apply to the proposed design as well. Nevertheless, it is suggested that the simplicity of both structural design and theoretical methods of performance estimations make the suggested barrier a good candidate for applications with a well known desired attenuation spectrum. The impedance tube experiments validate the sound blocking performance and show a reasonable agreement with the numerical predictions.

Membrane sound absorber with a granular activated carbon infill

An impervious membrane sound absorber with an air cavity partially filled with a porous material is a viable alternative to perforated panels when impermeable surfaces are required. On the other hand, due to its unusually high sound absorption properties at low frequencies, using activated carbon (AC) in granular form as an absorbing material has recently received attention. This paper reports the sound absorption performance of a novel system consisting of a thin stretched impervious circular membrane made of silicone, backed by an air cavity partially filled with granular AC. The system is analyzed theoretically using the impedance translation theorem. The normal-incidence sound absorption was measured in an impedance tube for combinations of cavity depth, membrane tension, and membrane surface density. The results of using an AC and a fiberglass infill material of the same thickness in the air cavity were also compared. The findings validated the advantage of AC as porous infill material of a membrane sound absorber in the low-frequency range. Furthermore, the outcomes of the impedance tube experiments were in good agreement with theoretical predictions. The findings reported in this work could be used to design new innovating sound-absorbing metamaterials based on activated carbon and stretched membranes.

Labyrinthine Acoustic Metamaterials with a Subwavelength Bandgap Inspired by Topology‐Optimized Structural Design

Herein, a structural design for labyrinthine acoustic metamaterial is proposed using a topology optimization method to realize a subwavelength bandgap preventing the transmission of low‐frequency sounds. The optimized design is composed of air‐filled channels of different widths, which exhibit the subwavelength bandgap based on the monopole resonance induced in the meta‐atom. In addition to the optimized meta‐atom design, a simplified meta‐atom design with a small coiling coefficient is proposed by extracting structural features of the optimized design. The bandwidth and central frequency of the subwavelength bandgap can be controlled by adjusting the size parameters of the simplified meta‐atom. Numerical analysis for acoustic waves based on the finite‐element method is conducted to demonstrate the low transmission coefficients of the obtained designs for a wide‐ and low‐frequency range. Furthermore, using a 3D printer, experimental specimens of optimized and simplified meta‐atoms are constructed, and impedance tube experiments are performed to show their functionality.

Vibration and sound pressure reduction over a wide frequency range with an optimal dynamic vibration absorber

In this study, an optimized dynamic vibration absorber (DVA) was applied to improve the sound absorption performance and simultaneously attenuate vibrations and structure-borne noise at multiple frequencies. As the size parameters and eigenfrequencies of a DVA demonstrate a nonlinear relationship, tuning the eigenfrequencies by heuristically modifying the geometric parameters of the DVA is difficult. To avoid this intricate process, geometric parameters were optimized using a gradient method solver to tune the eigenvalues of the DVA to the target frequencies. Post-processing was performed for the sake of manufacturing. To confirm the validity of the performance of the sound absorption and vibration/noise attenuation, impedance tube experiments and impact experiments were conducted using the manufactured DVA. The impedance tube experiment verified that the values of the sound absorption coefficient increased with the application of the DVA. From the impact experiments, it was verified that the values of the frequency response function and sound pressure level decreased with the application of the DVA. The present study validates the notion that optimized DVAs improve the sound absorption performance of structures and simultaneously reduce vibrations and sound pressures at multiple desired frequencies.

Acoustic properties of honeycomb like sandwich acoustic metamaterials

Low frequency noise has strong penetrability and has always been an important part of environmental pollution. Limited by the law of mass, traditional lightweight porous materials are difficult to effectively control low-frequency noise. Based on the characteristics of honeycomb structure, Helmholtz resonator and acoustic metamaterials, a kind of honeycomb like sandwich acoustic metamaterials is proposed. This structure combines the lightweight characteristics of honeycomb sandwich structure and Helmholtz resonance characteristics. The finite element models of honeycomb and Helmholtz resonators element are established. The transmission loss (TL) characteristic shows that Helmholtz resonators element will be a TL peak in a narrow frequency band than honeycomb element. The effects of neck diameter, neck length and cavity volume of Helmholtz resonator on its TL are analyzed. The TL of single and multiple Helmholtz resonators are compared. Furthermore, The TL of honeycomb panel and honeycomb like sandwich acoustic metamaterials are analyzed. The results show that the different TL peaks can be introduced into honeycomb like sandwich acoustic metamaterials with different neck diameters. Each Helmholtz resonators element plays an active role in its own frequency band. The sound insulation performance of the honeycomb like sandwich acoustic metamaterials is tested by impedance tube experiment. Excellent sound insulation ability is confirmed. The reasonable design of honeycomb like sandwich acoustic metamaterials structural parameters can solve the problem of low-frequency noise of thin-layer lightweight structure.

A novel membrane-cavity-grating (MCG) meta-structure for enhancing low-frequency sound absorption

For resonant-type absorbers with a back cavity, the improvement of low-frequency sound absorption always depends on increasing the back cavity depth, which hinders its practical application. To realize satisfactory sound absorption performance in low-frequency range (<500 Hz) while keeping the depth of the back cavity unchanged, a membrane-cavity-grate (MCG) meta-structure is proposed. The specific physical mechanisms for sound absorption are analyzed from acoustic impedance and energy dissipate perspectives. Furthermore, some key parameters of the MCG unit cell are investigated to optimize the sound absorption performance. On this basis, a meta-structure with six-unit-cell is proposed. Compared with the structure without the grating, the meta-structure can realize 20% improvement of the average sound absorption coefficient in the range from 300 Hz to 700 Hz with the back cavity depth and the structure size unchanged, which is verified by the high consistency between finite element simulation and acoustic impedance tube experiment results. This work provides a potential avenue for low-frequency noise reduction structure design.