Acoustic Materials Research Articles

Temperature-tunable topological zero-refraction acoustic metamaterials

Zero-refractive index metamaterials have a wide range of applications in directional transmission and wave-front shaping due to their unusual acoustic properties. However, for most given acoustic topological metamaterials, the operating frequency is relatively fixed and the effect of temperature on their topological properties is rarely considered. Therefore, temperature-controlled tunable topological zero-refraction acoustic metamaterials are proposed in this paper. Firstly, a metamaterial with quadruple degenerate Dirac-like points at the center of the Brillouin zone is constructed, and the influence of temperature on the Dirac-like points is analyzed. The results show that the topological bandgap frequency range is more sensitive to temperature. The existence of pseudospin-polarized edge state is demonstrated by analysing the band structure of supercells with different topological phase phonon crystal. The topological zero-refraction property of the edge states outcoupled into free space is numerically demonstrated, and the non-contact active control of their operating frequencies can be realized by temperature. This study can provide a corresponding reference for the intelligent control of near-zero refractive index acoustic topological materials in elastic wave collimation and acoustic communication.

Broadband low-transmission study of ventilation metasurfaces based on Archimedean spirals

The primary objectives of acoustic material design include ensuring indoor ventilation and isolating external noise. This study introduces a ventilation metamaterial surface based on Archimedean spirals (ASVM) and calculates its energy transmission coefficient using simulation and the Transfer Matrix Method (TMM). Experimental validation confirms the accuracy of the proposed model, highlighting ASVM’s potential as an effective ventilation metamaterial surface. Subsequently, ASVM is integrated with Helmholtz resonators (HR) to create two new metasurfaces: HR-ASVM without a neck and N-HR-ASVM with a neck. The former exhibits a power transmission coefficient below 0.2 across the broadband range of 1300 Hz to 2500 Hz, while the latter demonstrates superior sound insulation performance (power transmission coefficient below 0.1) from 639 Hz to 2500 Hz, with complete transmission suppression at 710 Hz. Notably, the thickness of N-HR-ASVM at this frequency is only 1/13 of the wavelength. Simulations of the acoustic pressure field and energy flow analyze the mechanisms responsible for low transmission. Finally, the energy transmission coefficient of N-HR-ASVM is experimentally measured and found to align closely with simulation results. This study provides valuable insights into the research on spiral ventilation metamaterial surfaces.

Numerical study of thermo-viscous effects in acoustic materials using linearized Navier-Stokes equations

Acoustic materials, characterized by small-scale structures, require a detailed understanding of dissipation effects within the viscous and thermal boundary layers where most of the dissipation takes place. Despite the prevalent assumption of lossless or isentropic conditions in many applications, a comprehensive study of regions where thermal and viscous losses occur is crucial, particularly in scenarios where wall-induced losses are significant. Bridging this gap, the study aims to investigate these losses in various structures to guide the development of innovative acoustic materials. To achieve this objective, the Linearized Navier-Stokes Equations (LNSE) solver from the finite element software COMSOL is employed to determine the absorption coefficients and to identify predominant regions of thermal and viscous losses. The model is validated using experimental and published data. The study effectively reveals specific regions where these losses significantly dominate, highlighting their critical influence on acoustic material performance. Although the structures are simplified to 2D models to balance physical details with computational feasibility, the ability to identify dominant loss regions marks a significant contribution to the field and paves the way to refine the design of acoustic materials.

Low cost modelling tools for the design of additively manufactured acoustic materials

Additive manufacturing (AM) methods have unlocked new capabilities in the design of acoustic materials and their topologies. This has enabled the design of devices with breakthrough performance in sound absorption and insulation while achieving subwavelength characteristics. However, material fabrication through AM presents its own challenges. These challenges include inherent defects which can be specific to the printing method employed. Examples of defects in low-cost AM include void formation, surface roughness and poor bonding between layers. Each of these features of AM have different acoustic impacts which may degrade the performance of a design. To optimize the production of additively manufactured acoustic materials it is imperative that designers can exploit features, such as surface roughness, to enhance rather than degrade the overall material performance. The aim of this paper is to investigate the effects of surface roughness in extrusion-based AM on acoustic material performance. A novel, low-cost numerical methodology is used to capture the influence of surface roughness in a subwavelength acoustic absorber. With this approach the impact of roughness can be captured in the design process. The intention is to develop efficient low-cost design methodologies that can be exploited for industrial development of novel additively manufactured acoustic materials.

Towards topology optimisation of poroelastic materials using field-based heuristics

In passive noise control, it is well known that the shapes of acoustic porous material absorbers can be modified to achieve favourable resonances resulting in high sound absorption. However, finding the optimised shapes for a given sound field environment is challenging as it involves solving an inverse problem. Recently, the use of topology optimisation for designing the shapes of acoustic porous material absorbers has gained interest. Previous studies suggest that conventional gradient methods can often result in local optimal results that may be poorer in overall absorption than what can be achieved by using heuristic optimisation techniques. In this study, the use of field variables such as acoustic pressures and velocities in developing better heuristic optimisation methods are explored with the aim of avoiding cumbersome gradient computations. The acoustic shapes produced by the field-based heuristics are compared with traditional approaches to develop insights as to what patterns in the shape designs contribute to improved sound absorption in various target frequencies.

Experimental study of shooting sound around semi-open shooting ranges

Measurements of shooting sound around semi-open shooting ranges are analyzed. Recorded sound signals show bullet sound and muzzle sound contributions. Time differences between bullet and muzzle sound pulses are explained by calculation of acoustical travel times. In general bullet sound contributions are stronger in forward directions (angles 0 to 90 degrees with respect to the shooting direction) than in backward directions. A representative frequency is defined to characterize the relative importance of bullet sound in the emitted sound spectrum. This is further analyzed by means of silencers on the firearms, as silencers suppress the muzzle sound but not the bullet sound. The effect of acoustical absorption material on the shooting range is also investigated. Differences of the order of 10 dB between different shooting ranges are attributed to acoustical absorption material of the overhead screens (baffles) on the range. For practical applications it would be useful to represent a shooting range by a single point source with a direction-dependent sound emission spectrum. The accuracy of this approximation is investigated.

Structured acoustic materials for mitigating low-frequencybroadband aircraft noise: transfer matrix method

This study investigates, by numerical modeling, the mitigation of low-frequency broadband aircraft noise using a structured assembly of acoustic materials using the Transfer Matrix Method. The prevalence of low-frequency aircraft noise poses challenges to environmental sustainability and passenger comfort. The investigation focuses on an approach that employs strategically arranged structured acoustic materials to attenuate undesirable noise frequencies. The Transfer Matrix Method is used to model the acoustic behavior of the structured assembly, enabling a comprehensive analysis of sound transmission and absorption properties. This method supports the design and optimization of the assembly. The performance of the structured assembly to selectively attenuate low-frequency broadband noise is assessed through numerical simulations. The paper discusses implications for aircraft design, considering factors such as weight and cost. The benefits for environmental impact and passenger experience are also explored. Challenges and limitations in the implementation of structured acoustic materials are examined. The configuration studied presents an assembly of structured metamaterials arranged in series and in parallel which are integrated into a layer of glass wool. Present research contributes to the development of sustainable solutions for low-frequency aircraft noise. The structured assembly analyzed shows promise for significant noise reduction while considering practical constraints.

Acoustic aspects in the construction and upgrading of military firing ranges - Part 2

Military firing ranges are indispensable facilities to train armed forces in handling and use of small arms. In the past, such ranges were outdoor installations. Due to noise problems around some of these shooting ranges they will be replaced more and more by indoor shooting facilities, which increases the noise impact on the personnel. First, the design of shooting ranges must consider the internal and external firing safety. However, acoustics comes next. There are two different but equally important objectives. The facilities should be built in such a way that (1) the noise impact on the hearing of personnel and (2) the noise impact on the neighbourhood is as low as possible. The design of shooting ranges should already take these aspects into account during the planning phase. This article focusses on the topic of acoustic safety (1) and is divided into two parts. Part II: The second part of the article describes the practical application of the new directive, with its acoustic material and measurement requirements, based on a recent tendering procedure in which various companies were offered the opportunity to develop acoustic wall and ceiling systems for indoor shooting ranges as part of a so-called competitive dialog.

Acoustic materials based on the Gosper curve

In this work, slotted acoustic materials based on a space filling curve called the Peano-Gosper curve are proposed and investigated. The slits in such materials form a complex pattern because they are divided by walls built along lines generated by the Gosper curve algorithm. The pattern can be twisted around an axis normal to its surface to increase the tortuosity inside the material, and therefore, modify its acoustic properties, which can be controlled by the turning angle or pitch of the twist. A highly efficient semi-analytical model has been developed to accurately predict the acoustic properties, in particular the sound absorption of such materials. It only requires a representative part of the pattern, or better, scanning the surface of the fabricated material so that the actual geometry and dimensions (in particular slit widths) are well reproduced in a two-dimensional finite element mesh generated on a representative fluid domain. The mesh is used to solve a dedicated Poisson problem and determine a few key parameters, and the rest of the modelling is based on analytical formulas. Material samples with straight and twisted slit patterns were 3D printed and then measured in an impedance tube to confirm semi-analytical sound absorption predictions.

Effect of scatterer shape, opening angle and periodicity on sound attenuation by local resonating sonic crystal

Noise Control has become critical and demanding task for acoustic engineers due to continuous Technological developments in construction, transportation, and industries. These developments have raised the level of low frequency noise in densely populated areas. Unfortunately, there cannot be sufficient noise reduction with traditional acoustic materials. However, new research has shown that the manipulation of sound waves and drastic reduction in sound transmission can be possible using periodic structure known as Sonic Crystals (SnC). This study describes the eigenfrequency analysis to investigate the effect of different shapes, periodicities, and opening angles of scatterers in a Local resonating Sonic Crystal (LRSnC). The Block-Floquet boundary condition is applied to the unit cell of the LRSnC, using the Finite Element Method to study the complex acoustic wave interactions throughout the structure. An acoustic module has been applied to find the band gap and to simulate transmission loss spectra in COMSOL Multiphysics. Results show that shape, opening angle and periodicity effect the band gap and transmission loss significantly. Based on these results, it is possible to develop advanced SnC with a wider band gap and higher sound attenuation in the lower sound frequency range by carefully considering these factors.

Investigating the acoustic performance of metamaterials with internal simple expansion chamber structure

The tunable acoustic properties are possessed by acoustic metamaterials (AMMs) with internal configurable structures. Nine samples(B1-B9) were designed in SOLIDWORKS and then fabricated using 3D-printed photopolymer resin featuring an internal Simple Expansion Chamber (SEC) structure. SEC length varied from 2 to 18 mm, maintaining a constant area expansion ratio (m) of 16. Sound Absorption Coefficient (SAC) and Sound Transmission Loss (STL) were measured through the Impedance Tube Apparatus (ITA). The sample with a 4 mm SEC length exhibited a maximum SAC of 0.99 at 4.8 kHz. As SEC length increased peak SAC and peak absorption frequency linearly decreased. Across all samples, STL ranged from 1- 41 dB (0.5-2.4 kHz) and 20-36 dB (2.4-6 kHz) revealing a periodic pattern in the tested frequency range. With SEC increasing from 2 to 18 mm Peak STL decreased from 41.2 to 38.8 dB. This research contributes valuable insights into the design of acoustic materials for effective noise attenuation showcasing the potential for achieving at least a 20 dB reduction in the 2.4-6 kHz range.

The density effect on disposable mask waste as an acoustic absorber material

Abstract Mask ability in absorbing sound has been proven as it increases in line with thickness adding. Another characteristic that can optimize the absorption is density. This article aims to study the effect of mask density on the sound-absorption capability. This research was conducted by making absorber material models made of masks from two different brands. In the model making, this research uses two types of masks, the folded peach-colored folded face mask black duckbill type which are arranged up to 2.5 cm in thickness. In that thickness, the folded peach mask has 254 kg/m3 density and the black mask has 305 kg/m3 density. The analysis is carried out using the tube method. The research results show that at a frequency of 400 Hz, the folded peach mask has the highest absorption coefficient of 0.70, whereas the black mask has an absorption coefficient of 0.48. There is a significantly reduced absorption coefficient at the 600 Hz frequency that makes the mask not meet the standard. It can be concluded that the mask material can be used as an acoustic material as it meets the acoustic standard on absorption coefficient of at least 0.3. These findings support previous research on the potency of mask waste as an acoustic material. Therefore, the absorber material from used mask could be a solution to reduce non-degradable waste.

Lightweight and hydrophobic silica aerogel/glass fiber composites with hierarchical networks for outstanding thermal and acoustic insulation

Silica aerogels are regarded as potential thermal and acoustic insulation materials with low density and porous structure. However, the hydrophilicity due to surface hydroxyl groups and the low mechanical intensity have limited its further application. Generally, it can be achieved by adjusting the process parameters of the sol-gel to refine the structure of silica aerogels and optimize performance. In this study, we adopted an appropriate modification strategy of incorporating glass fiber felts as the reinforcing phase and trimethylchlorosilane (TMCS) as the hydrophobic modifier. By adjusting the solvent exchange time, the micro-nano structure, i.e. hierarchical networks, was optimized. The TMCS-modified silica aerogel/glass fiber/hot-melt fiber composites (TSGMs) exhibited great sound insulation, and the maximum sound transmission loss (STL) was up to 34 dB at 6300 Hz. Compared to unmodified, STL improved by 8 dB in simulated practical applications. In addition, TSGMs also exhibited high hydrophobicity with a water contact angle of 147° and excellent thermal insulation with a thermal conductivity of 0.0345 W (m k)-1. The results could contribute to the refinement in the preparation process of silica aerogel composites and use in the fields of thermal and acoustic insulation.

Experimental characterization of acoustic damping materials

AbstractDue to their ease of use and low cost, passive damping methods are a preferred mean for the reduction of noise in many engineering applications. This applies in particular to foam materials, which exhibit good acoustic and mechanical damping properties. The selection of suitable materials is usually carried out experimentally and can be very labor‐intensive and time‐consuming. For this reason, it is helpful to develop qualified numerical methodsthat can be used for material design and selection. Hence, foam materials must be characterized experimentally in order to enable a later comparison with vibroacoustic simulations. This contribution, therefore, aims at providing suitable parameters for the use in numerical analyses and their validation. Firstly, the microstructure of a foam specimen is captured by means of a CT scan. In addition to providing the geometry for the multi‐physics simulations, this measurement is also used to determine characteristic foam features, for example, strut thickness and pore size distribution. In the second step, the frequency‐dependent stiffness and damping properties of the material are determined by a special experimental setup utilizing an electrodynamic shaker. Here, the dynamic system is approximated as a single‐mass oscillator, which is sufficiently accurate for low frequencies. These properties will later be used in the numerical model to evaluate different parameter identification approaches. In the third and last step of the experimental campaign, measurements with an impedance tube are conducted to obtain the coefficient of absorption. This material parameter is particularly suitable for comparing experiments and simulations. Finally, the correlation between the experimental results is examined to provide a deeper understanding of the foam materials.

Optimization of acoustic porous material absorbers modeled as rigid multiple microducts networks: Metamaterial design using additive manufacturing

This research presents a strategy to enhancing sound absorption in porous acoustic metamaterials through the optimization of micro-duct networks. The study employs a combination of analytical and numerical methods, systematically adjusting micro-duct diameters within a 2D grid to optimize absorption coefficients at specific frequency bands. The Finite Element Transfer Method (FETM) is utilized for modeling, supported by a multi-objective function influenced by the surface impedance of the network. This approach ensures practical applicability and manufacturability of the designed metamaterial. A significant aspect of the study is the development of an analytical mobility matrix for each microduct, integrated through the Transfer Matrix Method using the visco-thermal dissipation theory. This integration results in a physically coherent model, for which a semi-analytical sensitivity analysis related to the microduct diameters can be directly performed. The optimization employs the Method of Moving Asymptotes (MMA), effectively managing the complexity associated with a large number of design variables. Subsequently, the optimized structure is adapted into a 3D grid, facilitating prototype creation using Additive Manufacturing Technology (AMT) with a specific polymer material. The research methodology is validated through four distinct test cases, each demonstrating the efficacy and adaptability of the optimization tools and techniques. Experimental validation, conducted using an impedance tube, indicates a significant shift in the first absorption maximum from 2500 Hz to 1000 Hz, achieved without altering the material thickness. Additional validation through 3D Finite Element Method (FEM) modeling, which includes visco-thermal effects, further confirms the acoustic efficiency of the final designs. While the potential for achieving lower sub-wavelength conditions is recognized, the study opts for simpler structures, considering the current limitations of 3D printing technology. This study contributes to both theoretical understanding and practical application in acoustic material design, emphasizing the potential for customizing materials to specific frequency ranges. The integration of FETM modeling with MMA provides a systematic and effective approach for optimizing acoustic materials, particularly in enhancing absorption at lower frequencies.

An automatic simulation pipeline for coupled simulations of acoustic damping materials

AbstractFoamed materials are widely used to reduce noise due to their comparably good acoustic damping behavior. However, out of a large variety of these materials a suitable candidate has to be identified for each application. This is a challenging process that is typically guided by experiments and experience. While numerical simulations could support these experiments and reduce the effort to a great extent, no suitable discretization approach has yet been established that can fully capture the complex geometry of the foam. A fully resolved model is desirable in order to yield reliable predictions that can then be used to establish homogenized models. We established a monolithic coupling approach based on the finite cell method (FCM) that realizes a vibroacoustic simulation in this sense. The fluid and the structure domain are discretized by Cartesian grids and the geometry defined based on computed tomography scans is accounted for during the quadrature of the weak form. Our simulation in the time domain makes use of explicit time marching schemes and is therefore limited by a critical time step size. This is known to be arbitrarily low for discretizations with the FCM containing cells with arbitrarily small support. As a remedy against this we use the classical ‐stabilization technique and investigate its potentials and limitations.

High frequency venting of MEMS ultrasonic transducers and sensors: materials solutions.

Abstract This paper introduces the concept of ultrasonic venting. Similar to acoustic vents, ultrasonic vents refer to the aperture in air-coupled MEMS ultrasonic transducers (either PMUT or CMUT) intended to allow the equalization of internal and external pressures, the transfer of heat and the pass of ultrasonic waves, while impeding the penetration of fluids or particles that can affect the transducer membrane. To that end, vents are covered with a porous membrane whose properties are tuned to meet the afore mentioned requirements. The main difficulty in ultrasonic venting, compared with acoustic venting, is that the required “transparency” to ultrasonic waves is much more difficult to achieve. This involves two main problems as both transmission loss and frequency distortion are much larger at ultrasonic frequencies than in the audio range. The objectives of this paper are: to measure the response of acoustic venting materials in the ultrasonic frequency range, to determine the usability of these materials in ultrasonic vents, and to extract useful information for the design of efficient ultrasonic venting materials. Transmission coefficient spectra of different acoustic venting materials is measured in the frequency range 0.2 –2.7 MHz. The origin of the ultrasonic losses and frequency distortion are analysed as well as the role of mode conversion, internal interferences, modes interference, etc. Results reveal that none of the acoustic venting materials analysed can be used in ultrasonic venting applications, but the obtained knowledge about the response of these materials in the ultrasonic frequency range permit to advance in the selection of successful candidate materials for this application.

Broadband acoustic waves in plate-like structures for acoustic material characterisation

Abstract Guided acoustic waves can be used for a multitude of different testing and material characterisation purposes. The options for evaluation are especially plentiful if a method for broadband excitation, e.g. thermoelastic excitation using focused, pulsed laser radiation, is used, and if the distance between excitation and detection of the acoustic waves can be varied. In this contribution, methods to infer different quantitative material properties from the recorded spatiotemporal measurement data are presented, ranging from isotropic elastic properties to parameters describing absorption for complex absorption models.

Temperature and porosity effect on Rayleigh velocity in superconducting material

Our work's objective is to study the temperature effect and the porosity on ultrasonic wave velocity (Longitudinal wave velocity (VL), transverse wave velocity (VT), and Rayleigh wave velocity (VR)) of superconductor material type Y123 (or YBa2Cu3O7-x). A new model rational of temperature-porosity depending on ultrasonic wave velocities of surface and volume for an Y123 superconducting material was proposed, with each parameter having a distinct physical meaning. The study allows us to deduct the Rayleigh velocity VR following their evolution as temperature and porosity functions, bringing into focus the modelling of both the reflection coefficient R(θ), By analysing it, we were able to determine the variations of the longitudinal wave velocity and transverse wave velocity of the superconducting material considered as a function of the porosity and the temperature, and attenuating oscillations of the acoustic material signature, V(z) curves. The vibration working frequency of 570 MHz at a sensor, according to the porosity. This quantitative investigation is based on the experimental results obtained on porous and non-porous Y123 superconducting materials in the 5 to 300 K temperature range, This enables the estimated evolution of the Rayleigh velocity, VR, variation on the temperature-porosity depending. Since the model can well predict the ultrasonic wave velocities of porous superconducting materials from extreme low temperature to ultrahigh temperature under different porosity rates. In addition, the models obtained could aid well understand the behavior and performance of porous superconducting materials, and offer a new method to describe the temperature-porosity dependent ultrasonic wave’s velocities of superconducting materials.

An overview of patents of noise reduction in buildings

Noise is an unwanted sound in urban areas. Natural and or recycled acoustic absorbing or insulation materials are invented to reduce air and or impact sounds in buildings. Yet, they are not widely and commercially available. This review presented an overview of relevant studies on natural materials particularly on cotton and rubber using Scopus search engine and of the patents registered and granted using Espacenet and Google Patents search engines on noise reduction in buildings from the years 1996 to 2023. The 18% and 68% (out of 32 articles) relevant articles, respectively, were written on cotton and rubber and other recycled natural material e.g., consumed teabags, rubber crumb, indicating a clear vision for experimentation and modeling of natural materials e.g., to investigate their sound absorption coefficients at lower frequencies (below 250 Hz) for (recycled) cotton sound absorbing materials and for producing ultralight density with high sound absorption materials (0.56) using recycled rubber material. There were found 31 patents registered and granted, in total. Natural materials, namely, cotton, rubber, glass wool and rock wool were applied in 29% of patents registered and granted from 1996 to 2022. Yet, 93% of patents registered and granted lacked data on sound absorption index of materials they invented. LDA models estimated 29% of Espacenet and Google Patents patents registered and granted having an increased trend of patent innovations for example, in method, structure and building materials in the last decade.