Normal Incidence Sound Absorption Research Articles

Mycelium-Based Composites as a Sustainable Solution for Waste Management and Circular Economy.

The global population is expected to increase by nearly 2 billion individuals over the next three decades, leading to a significant surge in waste generation and environmental challenges. To mitigate these challenges, there is a need to develop sustainable solutions that can effectively manage waste generation and promote a circular economy. Mycelium-based composites (MBCs) are being developed for various applications, including packaging, architectural designs, sound absorption, and insulation. MBCs are made by combining fungal mycelium with organic substrates, using the mycelium as a natural adhesive. Mycelium, the vegetative part of fungi, can be grown on various organic feedstocks and functionalized into a range of diverse material types that are biobased and thus more sustainable in their production, use, and recycling. This work aims to obtain mycelium-based composites with acoustic absorption properties, using coffee grounds and agricultural waste as raw materials. The topic approached presents a new method of recovering spent coffee grounds that does not involve high production costs and reduces two current environmental problems: noise pollution and abundant waste. Measurements of the normal-incidence sound absorption coefficient were presented and analyzed. Mycelium-based composites offer an innovative, sustainable approach to developing bio-composite sound-absorbing surfaces for interior fittings. The material by Ganoderma lucidum exhibits exceptional sound-absorbing properties at frequencies below 700 Hz, which is a crucial aspect of creating sound-absorbing materials that effectively absorb low-frequency sound waves. The modular construction system allows for a high degree of flexibility to adapt to short-term changes in the workplace.

Metamaterial-based absorbers for simultaneous absorption of air-borne sound and structural vibration

In this work, the idea of metamaterial-based absorbers is further extended to develop the concept of sound and vibration absorbers (SVA) capable of simultaneously absorbing sound waves propagating in air and structure. Each SVA unit cell consists of an outer coating and an inner core. The outer coating is a resilient closed-cell foam, superseding the commonly used open-cell porous materials to significantly improve the structural integrity and service endurance. The inner core embedded in the outer coating provides multidirectional inertia to introduce multi-modal interferences with the outer coating. Utilizing a combination of analytic and numerical techniques, we reveal the underlying physics of the SVA for the simultaneous absorption of sound and vibration. A proof-of-concept SVA sample was fabricated using an expanded polyethylene foam as the outer coating and a polylactide hollow cylinder as the inner core. Good agreements are observed between the simulated and experimental results in terms of the normal incident sound absorption coefficients and the frequency response functions. We find that the SVA exhibits two nearly-perfect sound absorption peaks at 850 Hz and 1210 Hz owing to the resonant-induced impedance matching. Meanwhile, the SVA can effectively attenuate structural vibration across an ultra-wide frequency range relying on the effects of the locally-resonant bandgap and the structural damping dissipation. Finally, the noise reduction feasibility of the SVA sample applied in a plate-cavity coupled system was demonstrated. Our reported findings open alternative perspectives for the construction of practical metamaterial-based absorbers to attenuate noise propagating in complex transmission paths.

High-frequency measurement method of normal-incidence sound absorption coefficient with a square cross-section impedance tube using eight microphones

The authors have proposed a high-frequency measurement method of normal-incidence absorption coefficient with a circular cross-section impedance tube using eight microphones. This method enables measurement at frequencies about three times higher than the conventional two-microphone method. This study investigates the case of using an impedance tube with a square cross-section. Four microphones are placed at intersections of the (2, 2) acoustic mode nodal lines in one cross-section. When the signals of these microphones are summed, the effects of higher-order acoustic modes whose cut-on frequencies are lower than that of the (4, 0) acoustic mode are canceled. In this case, normal-incidence sound absorption coefficients can be measured at four times higher frequency than the conventional one. Numerical and experimental results are presented to demonstrate the validity and feasibility of the method.

Sound Absorption and Material Properties of Asphalt Rubber Concrete Specimens

The acoustic and material properties of asphalt crumb rubber (ARC) pavement have been measured and correlated. Laboratory measurements revealed nonlinear relationships between the sound absorption coefficients and material properties of compacted specimens as a function of the percentage of crumb rubber content in conventional and modified binders. The acoustic performance of conventional asphalt and asphalt rubber concrete specimens are compared to their material properties as a function of crumb rubber content to determine how crumb rubber content influences sound absorption. The normal incidence sound absorption coefficients, porosity, percentage of voids in mineral aggregate, and absolute gas permeability are measured for specimens containing 0, 5, 10, 15, and 20% crumb rubber in the asphalt binder. The random incidence sound absorption coefficient and material property data share nonlinear distributions within some frequency ranges, as a function of crumb rubber content in the conventional and modified asphalt binders.

A neural network-augmented two-microphone impedance tube method to estimate sound propagation characterisics

Two-microphone impedance tubes are a popular method to measure the normal incidence sound absorption coefficient. However, a single measurement with this method does not offer enough information to identify the sound propagation characteristics, i.e. the characteristic impedance and the wave number of absorbent samples. Instead, more elaborate measurement techniques that require more time, more equipment and enhanced user knowledge need to be applied. This contribution presents a technique combining neural networks and the two-microphone impedance tube method to estimate the propagation characteristics of a highly porous sample from a single two-microphone impedance tube measurement. The measured surface impedance serves as input to a U-net architecture, which preserves the high frequency resolution of the data and returns an estimate of the surface impedance mimicking an air cavity attached behind the sample. Then, the two cavity method is used to calculate the propagation characteristics from the measured and the networks predicted surface impedances. The presented approach gives precise estimates of the propagation characteristics of simulated highly porous samples and for two different, physically measured melamine foam samples, while drastically reducing the required resources and expenses. In this way, it provides an efficient way to estimate the propagation characteristics of absorbent samples.

Development of Sustainable Plant-Based Sound-Absorbing Boards to Reduce Noise in Interior Spaces

The reduction in CO2 emissions has become an important issue as global environmental problems have become more serious. The replacement of conventional petroleum- and mineral-derived raw materials for building materials with local plant-based resources is expected to reduce CO2 emissions. This study examined the possibility of using compression-molded boards made from plant-based resources as sound-absorbing materials in rooms. Among plant resources, few studies have conducted detailed measurements of the sound absorption properties of boards compressed from reeds. When measuring the normal incidence sound absorption coefficient, a material layered with a reed compressed board, wood fiber insulation, and an air layer showed a peak in the sound absorption rate at approximately 850 Hz. This indicates the potential to effectively absorb noise in the frequency band of human voices (500–1000 Hz). By changing the layering of multiple sound-absorbing materials, the presence or absence of an air layer behind them, and the installation conditions of the sound-absorbing materials, and then measuring the sound absorption rate, variations were observed in the sound absorption rate and the frequency at which the peaks were observed. This provides guidelines for material configurations that exhibit sound absorption at specific frequencies.

Design Of aesthetic acoustic metamaterials window panel based on Sierpiński triangle for sound-silencing with free airflow

Design of high-efficiency, low frequency (<1000Hz) soundproof window or wall absorber which is transparent to airflow is presented. Due to the massive rise in human population and modernization, environmental noise has significantly risen globally. Prolonged noise exposure can cause severe physiological and psychological symptoms like nausea, headaches, fatigue, and insomnia. There has been continuous growth in building construction and infrastructure like offices, bus stops, and airports due to urban population. Generally, a ventilated window is used for getting fresh air into the room, but at the same time, unwanted noise comes along. Researchers used traditional approaches like noise barrier mats in front of the window or designed the entire window using sound-absorbing materials. However, this solution is not aesthetically pleasing, and at the same time, it is heavy and not adequate for low-frequency noise shielding. To address this challenge, we design a transparent hexagonal panel based on Sierpiński fractal triangle which is aesthetically pleasing, demonstrates normal incident sound absorption coefficient more than 0.96 around 700 Hz and transmission loss around 23 dB, while maintaining air circulation through triangular cutout. Next, we present a concept of fabrication of large acoustic panel for large-scale applications which lead to suppressing the urban noise pollution

Theoretical and Experimental Analyses on the Sound Absorption Coefficient of Rice and Buckwheat Husks Based on Micro-CT Scan Data.

In this study, the sound absorption coefficients of rice and buckwheat husks were estimated. Computed tomography (CT) images were processed to determine the circumference and surface area of voids in the granular material, and the normal incident sound absorption coefficients were derived. In addition, the tortuosity, which expresses the complexity of the sound wave propagation through the structure, was measured for each material. The theoretical sound absorption coefficients were then compared to the measured sound absorption coefficients with and without consideration of the tortuosity. A correction factor was used to bring the surface area of the granular material closer to the actual surface area and observed that the tortuosity obtained theoretical values that matched the trend of the measured values. These results indicate that using CT images to estimate the sound absorption coefficient is a viable approach.

Exploratory Acoustic Investigation of Customizable 3D-Printed Hybrid Acoustic Materials (HAMs) through Interlaboratory Impedance Tube Measurements

Acoustic materials are widely used for improving interior acoustics based on their sound absorptive or sound diffusive properties. However, common acoustic materials only offer limited options for customizable geometrical features, performance, and aesthetics. This paper focuses on the sound absorption performance of highly customizable 3D-printed Hybrid Acoustic Materials (HAMs) by means of parametric stepped thickness, which is used for sound absorption and diffusion. HAMs were parametrically designed and produced using computational design, 3D-printing technology, and feedstock material with adjustable porosity, allowing for the advanced control of acoustic performance through geometry-related sound absorbing/diffusing strategies. The proposed design methodology paves the way to a customizable large-scale cumulative acoustic performance by varying the parametric stepped thickness. The present study explores the challenges posed by the testing of the sound absorption performance of HAMs in an impedance tube. The representativeness of the test samples (i.e., cylindrical sections) with respect to the original (i.e., rectangular) panel samples is contextually limited by the respective impedance tube’s geometrical features (i.e., cylindrical cross-section) and dimensional requirements (i.e., diameter size). To this aim, an interlaboratory comparison was carried out by testing the normal incidence sound absorption of ten samples in two independent laboratories with two different impedance tubes. The results obtained demonstrate a good level of agreement, with HAMs performing better at lower frequencies than expected and behaving like Helmholtz absorbers, as well as demonstrating a frequency shift pattern related to superficial geometric features.

Additively manufactured spinodoid sound absorbers

Spinodoid structures, also called spinodoid metamaterials, are non-periodic cellular structures that mimic spinodal topologies that are observed during diffusion-driven phase separation processes. Computationally efficient to model, spinodoid structures can be fabricated using additive techniques and offer an attractive route to the design of multifunctional structures. In this paper, we investigate the normal incidence sound absorption behavior of four distinct spinodoid topologies: isotropic, cubic, columnar, and lamellar. We fabricate the test samples using the fused filament fabrication process and systematically study the effect of the Gaussian Random Field (GRF) parameters on their underlying open pore network and sound absorption behavior. We employ a watershed segmentation-based image processing approach to correlate their pore and throat radii distributions to the GRF parameters. The normal incidence sound absorption properties are experimentally measured using the two-microphone impedance tube test method. Finally, we use a particle-swarm-based inverse characterization approach to extract the bulk properties necessary to model their acoustical behavior by using the Johnson-Champoux-Allard formulation. Our results show that the open pore network and acoustical properties of spinodoid structures are primarily a function of their relative density and wavenumber. Further, while the absorption behavior of the isotropic, cubic, and columnar spinodoids is similar, the lamellar spinodoids display unique low-frequency sound absorption behavior. Overall, all four spinodoid topologies provide favorable sound absorption characteristics, which may be tuned by varying the GRF parameters. The presented work advances the state-of-the art by establishing the feasibility of using additive manufacturing to enable non-periodic porous structures for that can be tuned to simultaneously provide mechanical stiffness and noise dampening capabilities.

Evaluation of metal foam liners for broadband noise reduction of turbofan engines

Previous research shows that open-celled metal foam liners have the potential to provide substantial aircraft noise reduction benefits. Our recent results show that their normal incidence sound absorption performance can be further improved by compressing them uniformly; this compression reduces their effective pore size and increases the thermoviscous losses. Further, their performance may be optimized by stacking foams with varying degrees of compression and creating a step-wise effective relative density gradient. Here, we present the results from our recent experiments, evaluating the attenuation performance of such a step-wise gradient metal foam liner using the Advanced Noise Control Fan (ANCF). Liner spools with two different step-wise configurations were fabricated and tested using the ANCF test rig at the Notre Dame Turbomachinery Laboratory. Farfield data was gathered with the liner spool mounted at the inlet and aft locations. In this paper, we present our results comparing the attenuation performance under both test conditions as compared to measurements obtained using a hard wall baseline configuration. Our results show that step-wise metal foam configurations can provide robust acoustical performance for aircraft noise reduction applications.

Optimization of sound-absorbing and insulating structures with 3D printed recycled plastic and tyre rubber using the TOPSIS approach

Studies on recycled materials emerged during recent years. This paper investigates the properties of sound absorption and transmission loss of the structures of the perforated plastic panel (PPP) backed with the rubber granule panel (RGP). Under production conditions, such a structure could be produced using only waste. The normal incidence sound absorption and transmission loss of the structure was measured using ISO 10534-2 and ASTM E2611-17 standard methods, respectively. In this study, the plastic panel perforation holes were tilted in steps of 15°–45° compared to the vertical axis of the sample. Different thickness rubber panels were used as backing (12 mm, 22 mm and 27 mm). The results showed that the sound absorption and transmission loss increased with the angle of the perforation pore. To compare the different structures, the multi-criteria analysis using the technique for order of preference by similarity to ideal solution (TOPSIS) approach was done. The RGP thickness, sound absorption coefficient (SAC) and sound transmission loss (STL) were weighted as a criterion. The results of the multi-criteria analysis showed that the optimal structure for a possible application is PPP45°+ RGP 22 mm, which was ranked highest of all alternatives. The first peak of SAC reached 0.97 at 1000 Hz. In the frequency range of 250–1000 Hz, STL values reached up to 10.4 dB and in the frequency range of 1–4 kHz – 20.1 dB. The results obtained using RGP and PPP show promising development potential for sound absorption and insulation applications.

Sustainable Materials from Waste Paper: Thermal and Acoustical Characterization

A growing research interest currently exists in the use of paper as a building material. This work aims to present the results of a measurement campaign developed on innovative waste paper-based building components. The research was carried out in Southern Italy and used some local by-product aggregates. Three different mixture designs were developed in the laboratory by adding three kinds of biomass to a pulp paper blend: fava bean residues (FB), sawdust powder (SP) and coffee grains (CG) extracted from exhausted chaffs. A physical characterization was carried out measuring the bulk density and bulk porosity. Scanning Electron Microscopy (SEM) analysis of the single aggregates was followed by a microstructure analysis of the final components. Bulk density evaluation showed a range between 200 and 348 kg·m−3. Furthermore, thermal performances were measured; the thermal conductivity of the experimented samples ranged from 0.071 to 0.093 W·m−1·K−1, thus it is possible to classify the tested materials as thermal insulators. Moreover, the acoustic properties were evaluated and tested. The normal incidence sound absorption coefficient was measured by the impedance tube on cylindrical specimens. In general, a different behavior was observed between the upper and lower base of each specimen due to the manufacturing process and the shrinkage caused by the different interactions occurring between the aggregates and the pulp paper waste; for example, the presence of sawdust reduced shrinkage in the final specimens, with consequent smaller physical variations among the two faces. The correlation existing between the manufacturing process and the microstructural properties was also investigated by the estimation of the non-acoustical parameters using the inverse method and taking into account the JCA (Johnson, Champoux and Allard) model as a reference.

Experimental Analysis and Simulation of a Porous Absorbing Layer for Noise Barriers

Noise barriers are common noise mitigation measures usually implemented near roads or railways, with proven efficiency. This work presents a study of a porous concrete material incorporating expanded clay as aggregate, to be used on the sound-absorption layer of noise barriers. A theoretical material model is calibrated using experimental data and then used to estimate the diffuse field sound absorption from the normal incidence sound absorption estimation/measurement. Validation of such estimation is performed by comparing to reverberant room measurements. Numerical simulations are carried out using the boundary element method (BEM) and the CNOSSOS-EU calculation method to assess the performance of different types of barriers incorporating this material. L-shaped and vertical barriers are tested, as well as low-height and conventional (taller) barriers, employed in the context of a railway noise scenario. Different results are obtained by the two methods, mainly due to the different underlying physical principles. Good insertion loss values may be obtained using both conventional and low-height noise barriers together with the porous concrete material if a careful choice of its location within the barrier is made.

Acoustic properties of commercial thermal insulators

Material manufacturers give very limited or no information of the acoustic properties of thermal insulators in broad perspective. The purpose of our study was to compare the acoustic performance of commercially available thermal insulators. Thirteen insulator types produced by several manufacturers were studied. The acoustic performance was determined holistically by measuring six acoustic quantities: sound reduction index of bare insulator, sound reduction index of encapsulated insulator (insulator between two boards), normal incidence sound absorption coefficient, airflow resistivity, dynamic stiffness per unit area, and reduction of impact sound pressure level in a floating floor. All measurements were conducted according to prevailing ISO standards in an experienced test laboratory. The acoustic performance differed significantly between insulator types for each acoustic quantity. The ranges were 1027 dB, 3352 dB, 0.200.78, 3.02700 kPas/m2, 1.5730 MN/m3, and 1536 dB, when the thickness of the insulator was 100 mm. Closed-pore insulator types usually carried worse acoustic properties than open-pore insulator types. Lower thermal conductivity was associated with worse acoustic performance of two acoustic quantities. Insulator manufacturers should consider to better declare the acoustic properties of insulator products, because this knowledge is needed in the acoustic design of building constructions and other applications.

A study on normal incident sound absorption characteristics of Japanese traditional cedar board, yakisugi

Yakisugi is made by charring the surface of a cedar board to form a thin carbonized layer, which is said to improve durability, weatherability and fire resistance. It is a traditional Japanese exterior material that has become popular overseas due to its performance and visual texture. Recently, it has been attracting attention for its use as an interior material, but its basic performance is still unknown. In this study, we focus on the sound absorption characteristics of yakisugi as one of the investigations to understand its performance as an interior material. The carbonized layer of yakisugi is expected to have sound absorbing performance because of its porous nature. As a basic investigation, the sound absorption characteristics at normal incidence are measured using an impedance tube and compared with the carbonized layer conditions. It is found that yakisugi has sound absorption performance in the high frequency range. Moreover, the characteristics are depending on the thickness and surface properties of the carbonized layer.

Estimation of the Acoustic Properties of the Random Packing Structures of Granular Materials: Estimation of the Sound Absorption Coefficient Based on Micro-CT Scan Data

In this study, the sound absorption properties of randomly packed granular materials were estimated. Generally, it is difficult to construct a general mathematical model for the arrangement of randomly packed granular materials. Therefore, in this study, an attempt was made to estimate the sound absorption coefficient using a theoretical analysis by introducing data from computed tomography (CT) scans, as the tomographic images of CT scans correspond to the slicing and elemental division of packing structures. In the theoretical analysis, the propagation constants and characteristic impedances in the voids were obtained by approximating each tomographic image as a void between two parallel planes. The derived propagation constants and characteristic impedances were then treated as a one-dimensional transfer matrix in the propagation of sound waves, and the transfer matrix method was used to calculate the normal incident sound absorption coefficient. The theoretical value of the sound absorption coefficient was derived using the effective density to which the measured tortuosity was applied. As a result, for the theoretical values considering the tortuosity, in many cases, the theoretical values were close to the measured values. For the theoretical values, when both the surface area and tortuosity were considered, the peak sound absorption frequency moved to a lower frequency and was in general agreement with the measured values.

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.

Mathematical Models and Experiments on the Acoustic Properties of Granular Packing Structures (Measurement of Tortuosity in Hexagonal Close-Packed and Face-Centered Cubic Lattices).

In this study, the sound absorption characteristics of hexagonal close-packed and face-centered cubic lattices were estimated by theoretical analysis. Propagation constants and characteristic impedances were obtained by dividing each structure into elements perpendicular to the incident direction of sound waves and by approximating each element to a clearance between two parallel planes. Consequently, the propagation constant and the characteristic impedance were treated as a one-dimensional transfer matrix in the propagation of sound waves, and the normal incident sound absorption coefficient was calculated by the transfer matrix method. The theoretical value of the sound absorption coefficient was derived by using the effective density applied to the measured tortuosity. As a result, the theoretical value was becoming closer to the measured value. Therefore, the measured tortuosity is reasonable.

Normal incidence sound absorption of an acoustic labyrinthine metal-fibers-based porous metamaterial at high temperature

This work studies the sound absorption performance of acoustic labyrinthine porous metamaterials (ALPM) at high temperature. The proposed metamaterial is constructed by perforating a homogeneous metal-fibers-based porous matrix with folded slits. These macroscopic perforations provide long channels for sound waves to enter the structure, allowing sound waves to enter the porous material due to the pressure diffusion effect, resulting in excellent subwavelength absorption at high temperatures. Based on the double porosity theory and temperature-dependent parameters, we develop a high-temperature theoretical model and validate it by numerical method. The results show that with the increase of temperature, the peak frequency of the ALPM moves to higher frequencies and the half-absorption bandwidth becomes wider. The discussion of sound absorption mechanism based on the pressure distribution results shows that with the increase of temperature, the pressure diffusion effect exists in a wider frequency range, which helps to suppress the propagation of long-wavelength sound waves at high temperature. The study of particle vibration velocity and energy dissipation density shows that the higher the temperature, the stronger the motion of air particles, and more sound energy is dissipated in the porous material, resulting in enhanced sound absorption. The ALPM can be regarded as a combination of homogeneous porous material and space-coiled resonator. Compared to these two components, the ALPM exhibits excellent sound absorption in the frequency range of 200Hz to 600Hz, especially at high temperatures. The proposed porous metamaterials show promising applications in low-frequency sound absorption at high temperature.