Acoustic Absorption Research Articles

Initial exploration on potential fire hazards detection from coal spontaneous combustion applied by acoustic wave

Coal spontaneous combustion often wastes resources and causes environmental pollution. Rapid and accurate identification of high temperature areas in coal is essential to reducing such combustion and environmental pollution. The acoustic thermometry method has the benefits of large temperature measurement space, non-contact, and high interference resistance. Determining the attenuation characteristics of acoustic waves in loose coal is the basis and premise for realizing acoustic temperature measurement. Four types of bituminous coal were scanned by computer tomography equipment. A self-designed acoustic attenuation test device was used to test coal samples under different temperatures and particle sizes. The study result demonstrates that the distribution characteristics of loose coal voids are mainly related to the particle size. The smaller the particle size range, the more uniform the void distribution. As the size of the coal particles increases, the voids become larger. The acoustic attenuation coefficients of four coal samples showed an increasing trend as frequency increased. The influence of coal particle size distribution on the acoustic attenuation coefficient was greater than that of temperature and metamorphic degree. The peak values of coal sound attenuation for different particle sizes were around 400, 700, 1100, and 1600 Hz. This indicated that the distribution of voids was the main factor affecting the propagation of acoustic waves. By analysing the attenuation mechanism of the acoustic wave in loose coal, the attenuation of acoustic temperature measurement signal was caused by the combined effect of loose coal on acoustic wave absorption and scattering. The study results provide theoretical support for the realization of acoustic wave detection of high temperature point in loose coal spontaneous combustion.

A general and stable approach to modeling and coupling multilayered acoustical systems with various types of layers

In this article, a general method is proposed to model layered acoustical systems with two-by-two transfer matrices, and further, to solve for their acoustic absorption, reflection, and transmission coefficients. Since the proposed method uses the matrix representation of various layers and interfaces, as in the Transfer Matrix Method (TMM), the equation system can be established efficiently. However, the traditional TMM can become unstable when there is a large disparity between the magnitudes of the waves traveling in opposite directions within the layers (i.e., at higher frequencies, for a thick layer, or for extreme parameter values). In such cases, the contribution of the most attenuated wave can be masked by numerical errors and can induce instability when solving the system. Therefore, in the proposed method, the accuracy of the wave attenuation terms is ensured by decomposing each layer’s transfer matrix and reformulating the equation system. This method can be used to couple different layer types in a general way and is easy to assemble and implement with numerical code. The predicted acoustic properties of layered systems calculated using the proposed method have been validated by comparison with those predicted by previously-existing methods.

Frozen sound: An ultra-low frequency and ultra-broadband non-reciprocal acoustic absorber

The absorption of airborne sound is still a subject of active research, and even more since the emergence of acoustic metamaterials. Although being subwavelength, the screen barriers developed so far cannot absorb more than 50% of an incident wave at very low frequencies (<100 Hz). Here, we explore the design of a subwavelength and broadband absorbing screen based on thermoacoustic energy conversion. The system consists of a porous layer kept at room temperature on one side while the other side is cooled down to a very low temperature using liquid nitrogen. At the absorbing screen, the sound wave experiences both a pressure jump caused by viscous drag, and a velocity jump caused by thermoacoustic energy conversion breaking reciprocity and allowing a one-sided absorption up to 95 % even in the infrasound regime. By overcoming the ordinary low frequency absorption limit, thermoacoustic effects open the door to the design of innovative devices.

Topological bound state in the continuum induced unidirectional acoustic perfect absorption

Topological bound state in the continuum induced unidirectional acoustic perfect absorption

Assessing Enhanced Acoustic Absorption From Sonosensitive Perfluorocarbon Emulsion With Magnetic Resonance-Guided High-Intensity Focused Ultrasound and a Percolated Tissue-Mimicking Flow Phantom.

Sonosensitive high-boiling point perfluorocarbon F8TAC18-PFOB emulsions previously exhibited thermal enhancement during focused ultrasound heating in ex vivo pig livers, kidneys and a laminar flow phantom. The main objectives of this study were to evaluate heating under turbulent conditions, observe perfusion effects, quantify heating in terms of acoustic absorption and model the experimental data. In this study, similar perfluorocarbon emulsions were circulated at incremental concentrations of 0.07, 0.13, 0.19 and 0.25% v:v through a percolated turbulent flow phantom, more representative of the biological tissue than a laminar flow phantom. The concentrations represent the droplet content in only the perfused fluid, rather than the droplet concentration throughout the entire cross-section. The temperature was measured with magnetic resonance thermometry, during focused ultrasound sonications of 67 W, 95% duty cycle and 33 s duration. These were used in Bioheat equation simulations to investigate in silico the thermal phenomena. The temperature change was compared with the control condition by circulating de-gassed and de-ionized water through the flow phantom without droplets. With these 1.24 µm diameter droplets at 0.25% v:v, the acoustic absorption coefficient increased from 0.93 ± 0.05 at 0.0% v:v to 1.82 ± 0.22 m-1 at 0.25% v:v using a 0.1 mL s-1 flow rate. Without perfusion at 0.25% v:v, an increase was observed from 1.23 ± 0.07 m-1 at 0.0% v:v to 1.65 ± 0.17 m-1. The results further support previously reported thermal enhancement with F8TAC18-PFOB emulsion, quantified the increased absorption at small concentration intervals, illustrated that the effects can be observed in a variety of visceral tissue models and provided a method to simulate untested scenarios.

Experimental evaluation and numerical interpretation of various noise mitigation strategies for in-service elevated suburban rail

This research evaluates the efficiency of five noise mitigation measures including rubber floating slab track, straight noise barrier, track acoustic absorber, track-side noise barrier, semi-closed noise barrier, and a combined strategy on an elevated railway through in-situ measurements. In-situ experiments were conducted by sequentially installing various mitigation measures for comparative evaluation. A numerical model was then developed to interpret the noise control characteristics of the mitigation measures. The experimental results indicate that the rubber floating slab track can mitigate bridge-borne noise by 0-4 dB sound pressure level (SPL); the track acoustic absorber can mitigate the railway noise by 3–5 dB(A) and its combination with track-side noise barrier boosts the insertion losses of SPL by 2–7 dB(A). The combined control strategy shows overall better performance than individual mitigation measures within the efficient noise reduction regions. The experimental and numerical results can serve as a guide on the design of noise control strategies for elevated railways.

Enhancement of the hygroscopic and acoustic properties of indoor plasters with a Super Adsorbent Calcium Alginate BioPolymer

The present study introduces a novel-composition lime plaster that contains a Super Adsorbent BioPolymer (SABP) based on Calcium Alginate (CaAlg) for indoor applications, to improve hygrothermal and acoustic comfort. The sorption isotherm of the plaster, the moisture buffering value (MBV), the acoustic absorption, and the thermal conductivity have been assessed. The hygrothermal and acoustic properties of the modified plaster have been compared with those of the original composition to evaluate its performance under different concentrations of CaAlg (between 10 and 30% wt). Adding the CaAlg SABP to a conventional lime plaster consistently enhanced its hygroscopic properties. The results show a substantial increase in the equilibrium moisture uptake (between 10 and 23 fold) and MBV, from 0.9 to 9.3 g/(m2⋅%RH). The sound absorption coefficient improved by 0.1–0.2 at higher frequencies than 500 Hz. This paper provides detailed information on the characterization of the material. It includes details about using a dynamic vapor sorption (DVS) analyzer to evaluate the sorption isotherm and MBV. It also contains information on the impedance tube method used to measure the acoustic absorption of the material.

Looking to block out noise? Try metamaterials

The small frequency ranges of acoustic metamaterial absorbers is an advantage in commercial sound blocking applications.

Synthesis and characterization of fiber-reinforced lightweight foamed phosphogypsum-based composite

A foamed phosphogypsum-based composite (FPGC) was synthesized in this study via chemical foaming and the effect of hydrogen peroxide (H2O2) (0.5% to 2.5%) on its properties was investigated. With the increasing dosage of H2O2, more foamed pores were produced, causing a decline in the mechanical strength and water resistance of mixtures. Nonetheless, the increased pores resulted in an improvement in thermal insulation and acoustic absorption. Fiber addition resulted in slight enhancement in the acoustic absorption and thermal conductivity but significantly improved the mechanical strength and water resistance of fiber-reinforced FPGC, particularly polyvinyl alcohol (PVA) fiber. The calcium stearate (CS) (5% to 25%) as foam stabilizer was also incorporated to ascertain its effect on the properties of FPGC. CS content exceeding 15% had a positive effect on the properties of fiber-reinforced FPGC. Finally, the fiber-reinforced FPGC with 1% PVA fiber, 1.5% H2O2, and 20% CS attained a bulk density of 826 kg/m3, compressive strength of 5.08 MPa, softening coefficient of 0.69, and improved sound absorption and thermal conductivity of 0.483, and 0.221 W/m·K, respectively. This study presents a promising approach for the utilization of phosphogypsum (PG), and further works can be explored to evaluate the long-term performance and durability of FPGC in various environmental conditions.

Acoustic metamaterial absorbers: The path to commercialization

Acoustic metamaterial represents the synergism between wave physics and designed geometric structures, aimed at novel acoustic properties. For the important functionality of acoustic absorption, metamaterials face the challenges posed by a mature, existing group of absorbing materials that have proven their effectiveness. Can the metamaterials do better? If so, at what price? This article aims to answer these important questions. In particular, it is shown that the critical advantage offered by metamaterial absorbers lies in the tunability of the absorption spectrum to fit the needs of the client, in conjunction with a minimum absorber thickness set by the causal constraint. By overcoming the obstacles set by the mass production cost, acoustic metamaterial absorbers have now entered the commercialization stage. We give three examples of the commercialized applications together with their underlying working principles and conclude with some observations.

Additive manufactured acoustic absorbers made of wood-fiber composites with modified infill patterns

ABSTRACT The rapid urbanization and economic expansion have increased the negative consequences of environmental noise pollution. The sound that originates from various sources comes from distinct spectrums. To address the aforementioned issue, acoustic absorbers were 3D printed with varying infill patterns to absorb sound across a wide range of frequencies. Acoustic absorbers made of polylactic acid/polyhydroxyalkanoates-wood fibers (PLA/PHA-WF) were fabricated using Fused Deposition Modelling (FDM) technology, and their physical, mechanical, water absorption, biodegradation, and acoustic characteristics were measured and discussed. Acoustic absorbers printed with different infill patterns have a slight difference in density due to the size of the pores and the contact sites exhibited by the infill pattern. As a result, acoustic absorbers printed with varying infill patterns allow sound waves to enter and absorb sound at distinct spectrum levels. Modifying the infill pattern also had an effect on mechanical, water absorption, and biodegradation properties with minor deviations. The proposed biodegradable acoustic absorbers made of natural fiber composites and manufactured by FDM can be installed on building walls or roofs depending on the required acoustic absorption spectrum.

Acoustic absorption by the atmosphere explained by random propagation times

Acoustic and ultrasonic propagation results in a Linear Invariant Filter (LIF). Its complex gain is most often described by a ‘frequency power law’. Equivalently, the complex gain is the characteristic function (c.f.) of a ‘stable probability law’. This strong property justifies a modelization by ‘random propagation times’, which together predict the measured attenuations and obey the principle of energy balance. Except for a Gaussian component, propagation through the atmosphere has no connection with ‘frequency power laws’. In this paper, we show that other components are c.f. of probability laws linked to Poisson and exponential random variables (r.v). Consequently, random propagation times are able to explain propagation losses in the atmosphere.

Low-frequency broadband sound absorption based on Cantor fractal porosity

Proposals for new absorber designs for broadband sound absorption are of great interest due to their wide applicability in sound energy control. In this sense, the behavior of an acoustic absorber composed of a panel with slit-type perforations based on Cantor’s fractal is presented. The analytical model for the fractal porosity of the absorber as a function of the initial geometric parameters, the number of iterations, and the fractal dimension was established. The behavior of broadband sound absorption was evaluated theoretically, numerically, and experimentally, in which the predominant total thermal-viscous dissipation in the region of perforations increases as the fractal porosity of the absorber decreases. Furthermore, an experimental broadband sound absorption of 692 Hz (from 382 to 1074 Hz) with a peak amplitude greater than 80% is obtained with the proposed structure presenting a sub-wavelength scale, i.e., λ/27. Finally, this work contributes to the understanding of the use of Cantor’s fractal porosity in the proposal of new absorbers that allow broadband sound absorption at low frequency.

Superior broadband sound absorption in hierarchical ultralight graphene oxide aerogels achieved through emulsion freeze-casting

Ultra-lightweight aerogels have profound applications as advanced multi-functional sound-absorbing materials. At present, conventional aerogels are associated with pores with sizes too small for effective sound absorption in the audible range. In this work, we fabricated a hierarchically porous ultralight graphene oxide (GO) aerogel through a novel emulsion freeze-casting process of an air-in-water emulsion. Microstructural analysis of the aerogels reveals a hierarchical pore morphology consisting of highly inter-connected pores produced from the emulsion bubbles and the micro-pores produced by freeze-casting. Sound absorption measurements revealed that an aerogel with a surfactant-to-GO weight ratio of 1:1 displays an average absorption coefficient of up to 0.86 at a broadband frequency range between 250 Hz and 6300 Hz, a vast improvement by up to 58% as compared to conventional GO aerogels. Numerical microstructural models were developed to model the acoustic absorption phenomenon and understand the mechanisms behind enhanced sound absorption. These models identified the enhanced acoustic absorption as attributed to the increased air-wetting area as derived from the high reticulation rate in this hierarchical microstructure. Overall, we demonstrate the potential of leveraging the highly inter-dependent material-process-structure relationships of GO aerogel production to achieve hierarchical GO aerogels as advanced ultralight sound-absorbing materials.

A thin-film acoustic metamaterial absorber with tunable sound absorption characteristics.

A thin-film absorber with tunable acoustic properties over a wideband is designed based on the acoustic metamaterial theory. The thin-film acoustic metamaterial absorber (TFAMA) consists of a frame made of piezoelectric material and several flexible films with attached mass blocks (mass-spring vibration system). Based on the vibration mechanism of the mass-spring vibration system, a cellular model of local resonance form is established, and the material properties of negative effective mass are discussed. Combined with the vibration modal analysis of the coupling of mass block, elastic film, and piezoelectric material, the acoustic characteristics of the TFAMA under alternating voltage excitation are studied by finite element and experimental methods. The simulation and experimental results show that the sound wave can be well absorbed when it is incident on TFAMA to cause the membrane-cavity coupling resonance. By applying an alternating voltage to the TFAMA to excite the mass-spring vibration system to generate local resonance, the absorption of sound waves can be further enhanced in a relatively wide band near the excitation frequency. In view of the convenience of voltage parameter adjustment, the sound absorption band can be flexibly tuned in a wide range, including low frequency.

The potential of date palm midribs-based fabric acoustic panels for sustainable interior design

This paper presents an experimental study of acoustic absorption of novel fabric acoustic panels utilizing non-woven fabric cores and fabric facings made of date palm midribs-fibers; to be used as absorbent panels for spaces with privacy needs. Normal acoustic absorption coefficients of various configurations; different densities, thicknesses and facings; were measured using impedance tube. Findings showed that the highest absorption was exhibited by 50 mm- thick midribs-fibers cores. Additionally, 150 kg/m3 cores balanced between high tortuosity and porosity. Results of full assembly tests showed that 30 mm-150 kg/m3 and 40 mm-100 kg/m3 cores with woven midribs-fibers facings enjoyed significantly high absorption at low frequencies and high frequencies. Additionally, 40 mm-100 kg/m3 and 50 mm-200 kg/m3 cores with cotton facings provided competitive overall performances to commercial acoustic panels while providing flexible interior design options. Thus, interior designers can incorporate midribs-based fabric acoustic panels into their designs as local sustainable absorbent panels to enhance acoustical quality and complement space aesthetics.

Effect of Crumb Rubber Particles on Antisliding and Noise-Reduction Performance of Asphalt Pavement

Traffic noise has gradually become the primary source of noise in urban areas. The risk of health issues such as cardiovascular diseases and sleep disturbances associated with noise exposure are receiving widespread attention. The skid resistance of asphalt pavements is important for safe driving. However, its antisliding performance deteriorates owing to automobile driving, and the antisliding index cannot satisfy requirements of long-term safe driving. Crumb rubber-modified asphalt (CRMA) has been widely used owing to its characteristics, such as excellent noise reduction, antisliding performance, and durability. This study aims to investigate the effect of crumb rubber (CR) particle content (0%, 2%, 3%, and 4% by weight of fine aggregates in asphalt mixture) on the antisliding and noise reduction characteristics of asphalt pavements. The functional performances of three different types of asphalt mixtures were tested to obtain the best gradation. Then, an accelerated wear tester was used to analyze the deterioration of the antisliding performance under different cumulative load times. A British pendulum number (BPN) tester and 3D laser technology were used to measure friction properties and texture depths of asphalt mixtures with varying CR particle contents after wheel-load repetitions. The tire-drop and standing wave tube methods were employed to evaluate the noise-reduction characteristics and acoustic absorption performance of asphalt mixtures with varying CR particle contents. Finally, the optimal CR particle content was determined using grey-target decision-making based on the entropy weight method. The results indicate that adding rubber particles to asphalt mixtures slows down the attenuation rate of BPN and texture depth, which shows a clear enhancement of the antisliding performance. The addition of rubber particles to asphalt mixtures improves the noise-reduction performance in tire-pavement systems by increasing damping properties and the sound absorption coefficient. The asphalt mixture with 3% rubber particles exhibited the optimal performance.

Characterization and Optimization of the Angle-Dependent Acoustic Absorption of 2D Infinite Periodic Surfaces of Helmholtz Resonators

Acoustic metamaterials have emerged as alternative solutions to achieve useful physical effects that differ from the ones obtained with traditional materials. In terms of sound absorption, previous works have addressed their potential as compact surfaces with high performance. Nevertheless, studies on their angle-dependent behavior are scarce. In this work, an analytic model and a numerical model to estimate the performance of periodic surfaces with unit cells composed of 2D Helmholtz resonators are presented. By making use of these modeling tools, the absorption of surfaces with one and three different resonators is studied as a function of both incidence angle and frequency. Changes in the incidence angle can cause variation of the maximum absorption coefficient, the frequencies at which the maximum performance is observed, and the frequency range of significant absorption. Furthermore, the rate at which the performance changes as a function of the incidence angle is larger as the angle increases. Given the angle dependency of these absorbers, a strategy to optimize the dimensions of the surface elements to maximize the absorption performance for predefined ranges of incidence angles and frequencies is presented.

Design Construction and Performance of Porous and Rubbrised Asphalt Pavement

Moreover, porous asphalts have volumetric and surface characteristics (e.g., acoustic absorption, drain ability, texture, and friction). These characteristics are influenced by both intrinsic (such as gradation and bitumen content) and extrinsic (such as traffic load) variables, and the complex phenomena and processes that give rise to them determine how they change over time. Deterioration, which consequently impacts budget, noise, and safety. Despite the deterioration of so many different and intricate qualities over time, there aren’t any standards for synergistically improving the pavement system. So, the goal of this study is to establish and validate a design methodology that, when applied to the most important characteristics of friction courses as a component of a pavement structure, works in concert. The aforementioned method is based on extensive literature analyses as well as experiment conducted in a lab and on-site over a period of time to assess how the major surface and volumetric qualities degrade over time.

A multibody dynamics approach on a tree-shaped acoustic black hole vibration absorber

Acoustic black hole (ABH) has shown great potential and aroused tremendous interest for vibration and sound control in the past two decades. However, efficient computational methods are still lacking for the design of ABH structures with practically required properties. Herein, a multibody system transfer matrix method is proposed to deal with a system comprising a host structure with an attached ABH cluster, exemplified by a novel tree-shaped ABH dynamic vibration absorber (ABH-DVA). Capitalizing on the flexibility offered by the method, the eigen-solutions and the steady-state vibration response of the combined system are derived and validated by finite element results. Observed vibration reduction and the underlying mechanisms are elucidated in terms of the vibrational energy ratio and vibration spatial distribution in the system. Owing to its rich modal dynamics and energy trapping capability, the proposed absorber demonstrates extraordinary and robust low-frequency and broadband vibration reduction performance from 25 Hz to 2500 Hz for different host structures. This can be ensured by designing its threshold frequency as the lower limit of the targeted frequency range. The proposed method and the vibration-reduction-effect are experimentally validated. This study offers a novel yet general approach for addressing complex ABH structures with various structural components and topologies.