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We propose a novel hybrid acoustic metamaterial that can achieve broadband sound insulation below 452 Hz and almost perfect sound absorption at 864 Hz. The metastructure was fabricated using additive manufacturing. Finite element method simulations were used to study the acoustic properties of the fabricated metamaterials. An equivalent electrical circuit was built using an electro-acoustic analogy to evaluate the sound absorption coefficient. The unique design of this meta-structure possesses two resonant frequencies. Low-frequency sound insulation is found due to the effective negative density at frequencies lower than the cutoff frequency of the elastic membrane. In contrast, a negative effective bulk modulus is the reason behind the broadband sound absorption. The theoretical and simulation results were validated through experiments. Experiments carried out showed an overall average sound transmission loss of 26.31 dB between 50 and 1600 Hz and 24.72 dB in the low-frequency zone (<400 Hz). Furthermore, over 69% of the sound intensity is absorbed in the 500–1000 Hz frequency range. The designed meta-structure exhibits broadband effective negative density below 452 Hz and effective negative bulk modulus from 864 to 1220 Hz. The design specifies a sample thickness of 3.8 cm, corresponding to a subwavelength thickness of approximately λ/10. As a result, the developed meta-structure can potentially be employed for sound insulation and absorption at low frequencies in the aerospace and automotive industries and in-room acoustic applications. [ABSTRACT FROM AUTHOR]

Toward identifying efficient sound absorbers, we have formulated and analyzed the micro-perforated panels (MPPs) comprising cylindrical pores of arbitrary axial profiles for out-of-plane wave propagation. First, the forward problem was solved wherein an exact analytical expression for the absorption coefficient α was developed for these MPPs. The dependence of α was analyzed for various profiles comprising convex and concave-shaped sections including divergent and convergent linear and sinusoidal profiles, conic sections, Kilroy, and chirp shapes. The MPPs with pores having a diverging shape at the entrance were found to have higher sound absorption. The non-symmetric fluctuations in the profile led to fluctuations in the spectrum of α. Aiming to address the long-standing challenge of low-frequency sound absorption, we solved the inverse problem to identify the axial profile of the cylindrical pore for maximum sound absorption for frequencies up to 2500 Hz. Given the advances in additive manufacturing, the results of this comprehensive work help in designing MPPs comprising complex-shaped pores suitable for a particular spectral regime. [ABSTRACT FROM AUTHOR]

Bamboo, which is called "green gold" because of its sustainability characteristics, has many wonderful properties that make it environmentally friendly. In this study, polyurethane (PU) composites used in the automotive industry were produced by adding different amounts of bamboo powders (1, 2.5, 5, 7.5, 10 and 15%, w/v) to PU foams. In this regard, density, light and stereomicroscopic analysis, thermal conductivity, spectroscopic analysis, and mechanical (compression) test have been performed. Further, sound absorption analysis and contact angle analysis have been carried out. Optical and stereomicroscopes showed average pore sizes with range of 258–482 µm. As the bamboo powder increased, the contact angle values decreased from 129 ° to 88 °. Compression force deflection values of the samples declined from 1850 to 360 g/cm2. Similar to the mechanical properties, the thermal conductivities of the PU foams decreased. However, in regards to FT‐IR, no chemical reactions were observed between PU matrix and bamboo powder. The sound absorption test results showed that bamboo powder filled PU foams are sound absorption materials and that rigid PU foam with 2.5% (w/v) (PU‐2.5) bamboo powder sample had better sound absorption properties than the bamboo‐powder‐unfilled rigid PU foam (PU‐0), with an average sound absorption coefficient value of 0.86 at 6300 Hz and 0.71 in the range of 1600–6300 Hz. [ABSTRACT FROM AUTHOR]

In order to achieve broadband sound absorption, we propose a composite structure combining acoustic black holes (ABHs) and micro-perforated panels (MPPs). Here, we adopt both simulant and experimental methods to describe the sound transmission mechanism of the proposed composite ABH. This mechanism allows the ABH to have a sound absorption coefficient of over 0.8 in the range of 400–3200 Hz and above 0.7 between 250 Hz and 400 Hz. The total length of the proposed ABH is 155 mm, in which the thickness of the MPPs is 0.5 mm, the pore size is 0.3 mm and the porosity is 0.1 and 0.16, overcoming the length size and bandwidth limit for sound wave suppression in current ABHs. This work can further progress in elucidating the acoustic characteristics of ABH and open new avenues in ultra-broad-band sound wave control. [ABSTRACT FROM AUTHOR]

Diatomites are well-known mineral materials formed thousands of years ago from the skeletons of diatoms. They are found in many places around the world and have a wide range of applications. This article presents innovative research related to the possibility of using diatomite as a filler in composites to improve their sound absorption properties. The results of the study of the effect of diatomite processing (calcination) and its degree of fineness on the sound absorption coefficient of thermoplastic composites are presented. Three fractions of diatomite (0 ÷ 0.063 mm; 0.5 ÷ 3 mm; 2 ÷ 5 mm) and its variable mass proportion (0, 25, and 50 wt.%) were used. The composites were made with flax fibers as a reinforcement, polylactide as a matrix, and diatomite as an additional filler. This paper also presents the results of oxide chemical composition, diatomite mineral phase composition, morphology, and thermal conductivity coefficient of all diatomite fractions studied. In addition, the average particle size for diatomite powder was also determined. The most important of the studies was the determination of the acoustic properties of the aforementioned composites. As a result of the tests, it was found that the smallest fraction of diatomite particles and a variant without thermal treatment give the best effect in terms of sound absorption. [ABSTRACT FROM AUTHOR]

Material development in acoustic engineering plays a significant role in various applications, such as industrial noise control. It is important and relevant to consider alternative materials capable of reducing noise levels in different frequency ranges. One commonly used material in engineering structures is epoxy adhesive compositions. Favoring the use of adhesive compositions are their main characteristics, including weight reduction in structures, corrosion resistance, relatively low manufacturing costs, and high mechanical strength. This paper aims to discuss the relationship between the mechanical properties of modified epoxy adhesives, their structure, and sound absorption efficiency. The subjects of this study were specimens of an epoxy composition in the cured state. Acoustic absorption coefficients were evaluated using a dual-microphone impedance tube, and tensile, compressive, and bending strength properties were determined using a testing machine. The impact strength of the compositions was also investigated. An analysis of the structure of the adhesives in the cured state was carried out using a scanning electron microscope. The test specimens were made from Epidian 5 epoxy resin cured with a polyamide PAC curing agent. Nanobent ZR2 aluminosilicate in an amount of 1%, CaCO3 calcium carbonate in an amount of 5%, and CWZ-22 activated carbon in an amount of 20% were used as modifiers. The conducted studies revealed that the highest tensile strength was obtained for the adhesive composition with the addition of ZR2 filler. The highest compressive strength was exhibited by the adhesive composition with the addition of CWZ-22 filler. The highest flexural strength was demonstrated by the unmodified composition. For all the tested adhesive compositions, low sound absorption values were achieved, with a maximum of approximately 0.18. From the perspective of the reduction index R, it was observed that these samples performed better in reduction than in absorption. The best values were achieved in the compositions modified with CaCO3. [ABSTRACT FROM AUTHOR]

We report the first work on the development of biodegradable acoustic foams from self‐grown bacterial cellulose (BC) from kombucha solution and green processing techniques. Kombucha BC‐agar and kombucha BC‐polyvinyl alcohol foams were fabricated by using slurries of BC pellicles mixed with the additives, froze for 24 h, and recast into foams using the freeze‐drying method. The foams were aerogel‐like with a density 20 kg/m3, thermally stable upto 220 °C, and crystallinity ~74%. Morphological analysis revealed macropores with coarse walls in agar‐based foams, whereas the PVA‐based foams comprised a polydisperse pore network with coarse walls. A broadband absorption was observed with a maximum of 0.4 for the 2.5‐mm‐thick single‐layer foams and 0.9 for the 12‐mm‐thick multilayer foams. The sound absorption was found to be on par with the biodegradable foams of agar and other cellulose‐rich materials such as agricultural residue, yet with a lightweight. Being largely porous, the foams had little transmission loss. As the densities and the crystallinities of these foams are tunable by the addition of other polymers, the results from this study provide scope to explore BC‐based lightweight materials for large‐scale fabrication suitable for sound absorption over specific regimes. [ABSTRACT FROM AUTHOR]

In practical engineering, noise and impact hazards are pervasive, indicating the pressing demand for materials that can absorb both sound and stress wave energy simultaneously. However, the rational design of such multifunctional materials remains a challenge. Herein, inspired by cuttlebone, we present bioinspired architected metamaterials with unprecedented sound-absorbing and mechanical properties engineered via a weakly-coupled design. The acoustic elements feature heterogeneous multilayered resonators, whereas the mechanical responses are based on asymmetric cambered cell walls. These metamaterials experimentally demonstrated an average absorption coefficient of 0.80 from 1.0 to 6.0 kHz, with 77% of the data points exceeding the desired 0.75 threshold, all with a compact 21 mm thickness. An absorptance-thickness map is devised for assessing the sound-absorption efficiency. The high-fidelity microstructure-based model reveals the air friction damping mechanism, with broadband behavior attributed to multimodal hybrid resonance. Empowered by the cambered design of cell walls, metamaterials shift catastrophic failure toward a progressive deformation mode characterized by stable stress plateaus and ultrahigh specific energy absorption of 50.7 J/g—a 558.4% increase over the straight-wall design. After the deformation mechanisms are elucidated, a comprehensive research framework for burgeoning acousto-mechanical metamaterials is proposed. Overall, our study broadens the horizon for multifunctional material design. Noise and impact hazards are pervasive in engineering, necessitating materials capable of absorbing both sound and stress wave energy. Here, we present bioinspired metamaterials with exceptional sound-absorbing and mechanical properties using a weakly-coupled design strategy. These materials incorporate multi-layered resonators for superior acoustic performance and cambered cell walls for enhanced structural strength. They achieve an average absorption coefficient of 0.80 across the 1.0 to 6.0 kHz range, all within a sleek 21 mm thickness. Furthermore, the design transitions failure modes from catastrophic to progressive, resulting in a remarkable 558.4% increase in energy absorption compared to conventional designs. [ABSTRACT FROM AUTHOR]

A simplified model of the absorption coefficient of traditional Helmholtz resonators (THR) was established, and the influence of different geometric parameters on the absorption coefficient of THR was analyzed. To realize the low-frequency broadband acoustic structure design, an accurate theoretical model for the sound absorption coefficient of the curled acoustic metasurface (CAM) unit was established. Based on the complex frequency plane method (CFPM), the CAM units with perfect sound absorption at four discrete frequencies were designed. The low-frequency broadband acoustic metasurfaces in parallel under decoupled and coupled conditions were studied, and the thickness is only 12 mm. The high efficiency of sound absorption above 0.8 was achieved in the frequency range of 758 Hz–940 Hz. The experiment verifies the efficient sound absorption effect of the CAM unit and the broadband sound absorption effect under coupled conditions. The research in this paper has a certain potential applications for low-frequency broadband noise control technology. [ABSTRACT FROM AUTHOR]

Noise pollution is ranked as the third environmental pollution that can interfere with communication, recreation, or concentration. Hence, choosing the best materials could be one way to resolve a sustainable solution for noise pollution problems. Materials like bagasse are suitable composite materials for construction. Its natural fiber properties exhibit a good absorber characteristi c that would be used for acoustics barriers. Thus, this study aimed to investigate the sound absorption of bagasse using the twomicrophone transfer function method. Using two different thicknesses for the samples, 2.0 cm and 4.0 cm, the measured sound absorption coefficients were considerably higher than the conventional concrete. The measurements revealed that the 4.0 cm samples exhibited better sound absorption behavior between the two thicknesses, having a noise reduction coefficient (NRC) of 40% to 80%. It can be noted that it has shown consistency in the energy absorption throughout the frequency range of 250 Hz - 3000 Hz. Meanwhile, for the 2.0 cm-thickness samples, bagasse's performance is less absorptive from 250 Hz-1000 Hz but becomes more absorptive as it goes to higher frequencies with a peak value of approximately 95%. Also, the material's density and the sample's thickness influence the measured sound absorption coefficients. The result implies that bagasse, a green waste, can be a suitable candidate for acoustic building applications. Its advantages included low-cost materials from renewable sources, non-toxicity, and comparably high performance compared to standard or commercial products. These types of barriers can positively impact the noise level in the areas. [ABSTRACT FROM AUTHOR]

Aiming at achieving low‐frequency and broadband sound absorption under the premise of light and thin layers, in this paper, polyvinyl butyral (PVB) nanofiber membranes were micro‐perforated and then combined sequentially to prepare multi‐layer micro‐perforated nanofiber membrane (MPNM) for acoustic noise reduction. It was demonstrated that the multi‐layer MPNM exhibited a high absorption (constantly over 50%) in the frequency of 480–2500 Hz. In addition, the established theoretical model of the sound absorbing coefficient can accurately predict the sound absorption performance of the structure with different layers, which can provide a theoretical foundation for the design of the structure of the nanofibrous membrane acoustic absorber. Based on the proposed acoustic model, the relationships between the absorption properties and the parameters were investigated, and it was found that the effective acoustic absorption frequency range and acoustic absorption coefficient curve of the multi‐layer MPNM were closely related to the size and arrangement of hole diameter, perforation rate, fiber membrane thickness, and cavity depth. Optimization of the structural parameters utilizing algorithms can achieve superior sound absorption performance, with an average absorption coefficient of 0.81 in the frequency of 100–2500 Hz. This study provides a theoretical and experimental basis for the development of low‐frequency sound‐absorbing materials and is of great significance for optimizing the acoustic performance of nanofiber membranes and expanding their applications in various acoustic engineering applications. [ABSTRACT FROM AUTHOR]

Effective sound absorption is crucial in environments like schools and hospitals. This study evaluates open-pore polyurethane foam and perforated onyx panels, which attenuate noise via distinct mechanisms: porous materials convert sound energy to heat through viscous and thermal losses, while perforated panels use resonant behaviour for energy dissipation. The impact of hole geometries and panel orientations on the sound absorption coefficient and noise reduction coefficient was investigated using COMSOL Multiphysics 6.0 for finite element analysis and ISO 10534-2 compliant impedance tube experiments. Six perforated panel configurations were 3D-printed with varying hole diameters and backed by a 24 mm polyurethane foam layer. Both 'forward' and 'reverse' configurations were assessed. A tapered hole from 4 mm to 2 mm showed the highest sound absorption across the 100–4000 Hz range, with a noise reduction coefficient of 0.444, excelling in both orientations. Reverse designs generally performed less, underscoring the importance of hole geometry and orientation. Experimental results aligned with FEA simulations, validating the computational model. This study elucidates sound absorption mechanisms of porous and perforated materials, providing a validated framework for material selection in noise-sensitive settings and highlighting 3D-printing's potential in noise control. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Broadband adjustable sound absorbers are desired for controlling the acoustic conditions within enclosed spaces. Existing studies on acoustic absorbers, either passive or active, aim to maximize the sound absorption coefficients over an extended frequency band. By contrast, this paper introduces a tunable acoustic absorber, whose working frequency band and sound absorption characteristics can be defined by users for different applications. The approach leverages an error signal that can be synthesized using a standing wave separation technique. The error signal encodes different target reflection coefficients, leading to arbitrary absorption coefficients between 0 and 1. Experimental validation is conducted in a one-dimensional standing wave tube, demonstrating that the proposed active absorber achieves near-perfect absorption within the 150–1600 Hz frequency range, boasting an average absorption coefficient of 0.98. Adjustable absorption is demonstrated across three octave bands, aligning closely with theoretical predictions. Furthermore, when coupled with a shaping filter, the absorber exhibits spectrally tunable broadband absorption capabilities, selectively reflecting specific frequency bands while effectively absorbing others. These outcomes underscore the versatile tunability of the proposed active acoustic absorber, which is expected to pave the way for personalized regulating of the indoor acoustic environment. [ABSTRACT FROM AUTHOR]

Currently, FEA software such as ABAQUS uses empirical models to predict the sound absorption coefficient of poroelastic materials. However, based on a recent review of the literature it was found that the current sound absorption empirical models are inadequate for accurate prediction of thin (t < 20 mm), low-density materials (ρB < 50 kg/m³). Therefore, the predictions of the sound pressure levels in vehicle cabins, using such software, will be inaccurate since the trim materials are thin and have a low density. Thus, this research aimed to develop an empirical model that can accurately predict the sound absorption coefficient of these materials. Hence, polypropylene fibres consisting of four different diameters were manufactured and converted into nonwovens. Thereafter, airflow resistivity and impedance tube experimental testing were performed on the specimens. Subsequently, statistical analysis of the data was performed using SAS software. SAS was used to identify which independent variables should be included in the models to be developed. The empirical models were developed using the regression analysis toolbox in Microsoft Excel. Once the models were developed, various checks were performed to validate the assumptions of linear regression. The software NumXL was used to perform Cook's distance tests. Thereafter, the models were validated against the validation dataset, where it was found that the developed exponential model performed best. Finally, the exponential model was compared to existing models using two data sets i.e. an internal dataset, and an external dataset derived from the literature. The developed model outperformed all the historic models on both datasets. [ABSTRACT FROM AUTHOR]

In order to partially replace jute sound absorption material, sound absorption performance and influencing factors of the palm kapok fiber felt/poly (3 hydroxybutyrate-valerate (PHBV) hot pressed composite material were studied. The effcts of palm fiber, kapok fiber and PHBV mass ratio, fiber felt surface density, palm fiber linear density, the thickness of the composite material on the average sound absorption coefficient at low frequency (200 Hz-1 600 Hz) were analyzed. The results showed that the 2 pieces of fiber felt was adopted and the mass ratio of palm, kapok, PHBV was 10:30:60, surface density of the fiber felt was 185 g/m2 and 145 g/m2 respectively, linear density of kapok fiber was 0.25 tex, linear density of palm fiber was 0.60 tex, thickness of hot pressed composite materials was 4 mm, the average low frequency sound absorption coefficient of hot pressed composite material was 0.42, which could be used as absorption material in actual operation. [ABSTRACT FROM AUTHOR]

This paper demonstrates an acoustic metasurface for energy harvesting at tunable frequencies. The support structure of the metasurface was fabricated by additive manufacturing. The acoustic absorption coefficient and sound transmission loss of the metasurface can be tuned by optimization of various structural parameters, such as mass, mass size, prestress, membrane thickness, and array arrangement. The impedance tube was used to test the sound absorption ability of the fabricated metasurface, and the numerical simulation agreed well with the experiment, with a minimum acoustic absorption coefficient at ∼400 Hz. A PZT structure was designed and integrated into the acoustic metasurface, transforming the absorbed acoustic energy into electrical energy. Real-time acoustic energy harvesting was realized, and the peak voltage of 1.469 V was successfully monitored under the excitation of 410 Hz. For future applications, the energy harvesting efficiency can be further increased by improving the isolation components of the whole system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Porous sound absorption ceramic is one of the most promising materials for effectively eliminating noise pollution. However, its high production cost and low mechanical strength limit its practical applications. In this work, low-cost and in situ mullite whisker-reinforced porous sound-absorbing ceramics were prepared using recyclable construction waste and Al2O3 powder as the main raw materials, and AlF3 and CeO2 as the additives, respectively. The effects of CeO2 content, AlF3 content, and sintering temperature on the microstructure and properties of the porous ceramics were systematically investigated. The results showed that a small amount of CeO2 significantly promoted the growth of elongated mullite crystals in the resultant porous ceramics, decreased the growth temperature of the mullite whiskers, and significantly increased the biaxial flexural strength. When 2 wt.% CeO2 and 12 wt.% AlF3 were added to the system, mullite whiskers were successfully obtained at a sintering temperature of 1300 °C for 1 h, which exhibited excellent properties, including an open porosity of 56.4 ± 0.6%, an average pore size of 1.32–2.54 μm, a biaxial flexural strength of 23.7 ± 0.9 MPa, and a sound absorption coefficient of >0.8 at 800–4000 Hz. [ABSTRACT FROM AUTHOR]

Today, municipal solid waste (MSW), noise pollution and fire attack are three ongoing issues for inhabitants of urban including in Malaysia. To solve these issues, an eco-friendly autoclaved aerated concrete (AAC) containing recycled ceramic and gypsum waste (CGW) as partial replacement for sand with different ratios (0%, 5%, 10%, 15%, 20% and 25% wt) have been prepared. The work density (ρw) and mechanical properties of samples such as compressive strength (fc) and specific strength (S) have been investigated. The functional properties of samples such as sound absorption coefficient (SAC) and direct fire resistance also have been carried out. All samples showed normal color behavior i.e. grey and free crack with a ρw was in the range of 593.71 kg/m³ to 605.29 kg/m³. The fc increased in the range of 6.10% to 29.88%. The maximum value of fc was 2.13 MPa for 15% wt of CGW. The SAC of samples at low frequency (500 Hz) is higher than reference sample (RS). All AAC-CCW samples are categorized as material class B and C absorbers at frequency 500 Hz. Except for RS, the fire resistance results showed the physical surface of the samples had free crack and not burned during direct fire at 950°C for 300 s. The results showed that CGW have positive effect on the mechanical and functional properties of AAC. [ABSTRACT FROM AUTHOR]

The growing amount of used tires presents many environmental challenges around the world. Therefore, new ways to reuse used tires are being sought. One of the uses of waste tires is in sound-insulating constructions. Waste tires can be shredded into granules, which can be further devulcanized to increase their porosity. These granules can then be glued to panels and used in sound-insulating structures. Acoustic louvers were investigated in this study, with the louvers’ plates covered with rubber granule panels. Sound absorption parameters of rubber granule panels were tested across frequency bands ranging from 160 Hz to 5000 Hz. The results showed the normal incidence sound absorption coefficient reached 0.87–0.96 at 3150 Hz for 12 mm rubber granule plates. Measurements were conducted in a semi-anechoic chamber. The study has shown that rubber louvers can reduce the sound pressure level by 8 dB–12 dB, depending on the composite of the rubber granule panels and the tilt angle of the louvers’ plates. [ABSTRACT FROM AUTHOR]

This study was conducted to investigate some characteristics of thermally treated Scots pine (Pinus sylvestris L.) wood specimens such as air-dried density, color change, average surface roughness, and sound absorption capacity. Heat treatment of Scots pine wood was performed at atmospheric pressure at 140, 160, 180, and 200 °C for 2 h. As a result, the air-dried density values of the thermally treated wood decreased as the temperature of the thermal treatment increased. With the increase of thermal treatment temperature, an increase in total color change values was detected on the surfaces of the samples and the color of the samples became darker. The average surface roughness (Ra) value of samples improved due to thermal treatment conditions and the highest value was determined in thermally treated samples at 200 °C as 3.59 µm. At 140 °C the value of maximum sound absorption coefficient was observed to be 0.48 at 2500 Hz and the highest sound transmission loss value, which was 36.7 dB, was measured at 6300 Hz and at 200 °C. [ABSTRACT FROM AUTHOR]

A stepped-type multi-cavity combined sound absorber (SMCSA) was developed to improve the low and mid-frequency sound absorption capabilities of micro-perforated panels. The sound absorption characteristics of the SMCSA were thoroughly investigated using experimental methods, theoretical calculations, and numerical simulation. The particle swarm optimization algorithm was employed to optimize the SMCSA parameters. The findings revealed that the board thickness, hole size, and cavity depth significantly influenced the sound absorption properties of the SMCSA. When the hole size ranged from 0.4–0.6 mm, the sound absorption coefficient of the SMCSA reached 0.9, and the resonance frequency was 600–800 Hz. After optimization utilizing the hybrid particle swarm algorithm, the maximum sound absorption coefficients at the resonance frequency of the two-step, four-step, and eight-step structures were all 0.99. [ABSTRACT FROM AUTHOR]

The advancement of acoustic metamaterials enables the highly efficient absorption of low-frequency noise with a subwavelength structure thickness, but the complexity of these structures often hinders their large-scale practical applications. Here, we propose a straightforward and compact acoustic metamaterial structure composed of Helmholtz resonators with side slits (HRSS) for low-frequency noise absorption. The introduction of side slits not only simplifies the overall structure but also allows for easy adjustment of acoustic characteristics. By adjusting the depth of the resonator within the slit across 25 distinct units, an absorption coefficient above 0.8 is realized from 470 to 930 Hz. This work demonstrates the extensive low-frequency sound absorption capability of HRSS, providing valuable insights into the design of future practical acoustic materials. [ABSTRACT FROM AUTHOR]

The microperforated panel (MPP) is a resonant-type sound-absorbing material considered the next-generation sound-absorption material due to its attractive characteristics. However, the drawback of narrow bandwidth sound absorption remains a matter of concern. This study explored the impact of varying backing cavity depths and different combinations of multi-section perforation sizes on the sound absorption performance of microperforated panels (MPP). A two-microphone impedance tube was used to obtain the sound absorption coefficient (SAC) of MPP following the ISO 10534-2 standard. MPP with single-size perforation diameters of 0.5, 0.7, and 1.0 mm which were compared to multi-section MPP with combinations perforation sizes of 0.5 & 0.7, 0.7 & 1.0, and 0.5 & 1.0 mm. The investigation reveals that increasing the backing cavity depth from 1.0 cm to 3.0 cm generally shifts the sound absorption coefficient (SAC) peak to a lower frequency range and decreases the SAC peak across all MPP samples. A higher SAC peak and wider absorption bandwidth were achieved from 0.5 mm single-size perforation diameter MPP and its combination in multi-section perforation MPP. [ABSTRACT FROM AUTHOR]

A technique for estimating the sound absorption of materials under oblique incidence plane wave and diffuse field excitations is proposed. It requires a mobile loudspeaker and a pair of fixed microphones above a layer of absorbing material. Starting from sound pressure measurements made above the material surface for multiple source positions and by inverting Allard's propagation model, the proposed method allows for the identification of two effective (or equivalent) parameters, namely, the complex effective density and the complex wave number. Obtaining these parameters makes it possible to estimate the sound absorption coefficient for a plane wave with any angle of incidence or under a diffuse acoustic field by summation over the angles. The results are compared to theoretical values calculated using the Johnson-Champoux-Allard model and to reference measurements obtained using an impedance tube, a small cabin, a large reverberant room, and a sound field synthesis method. One of the limitations of this method lies in the assumptions associated with the Allard model to describe the sound field above the material (assumed to be isotropic, homogeneous, of constant thickness, and to behave like an equivalent fluid). The main advantages are (1) that the sound absorption coefficient can be estimated under oblique incidence plane wave and diffuse acoustic field with values that are always in a physical range (between 0 and 1), and (2) that samples of the order of the square meter without specific preparation can be tested which reduces the constraints associated with impedance tube or reverberant room measurements. [ABSTRACT FROM AUTHOR]

Since symmetrical geometry makes it easier to arrange pyramid absorbers on anechoic chamber walls compared to wedge absorbers, the goal of this paper is to present an optimal pyramid-shaped absorber which, in addition to the optimal performance in sound absorption, has other advantages such as less thickness, lower material consumption and cost, easier construction, arrangement and maintenance and occupying less space compared to wedges. The COMSOL Multiphysics software was used to simulate the optimized sound absorber by using the genetic algorithm programed in MATLAB software, with the goal of a 100 Hz cut-off frequency. The results show that the optimized chamfered pyramid at the top performs better at sound absorption compared to the complete pyramid and wedge with 18% less thickness. Furthermore, the cut-off frequency decreased to 91 Hz when a gap of up to 15 mm was created between the adjacent absorbers mounted on the surface, which is desirable. [ABSTRACT FROM AUTHOR]

The purpose of this study is to evaluate the effect of the number of diffusers placed in the same room, as well as the effect of diffusers with a flat area (8.4, 13.2, and 18 m2) on the calculated sound absorption coefficient in a reverberation room, when an iterative procedure based on the evaluation of the sound power level is applied (this is done by using the sound power equation and substituting with different values of equivalent absorption area on both sides of the transformed equation to reach the correct value for absorption area, which achieves equality on both sides of the equation in order to achieve the goal). The sound absorption coefficient at random incident of three materials having different degrees of absorption—has been measured in a reverberation room according to ISO 345: a highly absorbent material (sponge) a medium absorbent material (rubber) and a low absorbent material (wood).There is a noticeable convergence between the calculated and measured sound absorption coefficient from the usual ISO 354 procedure and from the sound power level equation from ISO 3741,when diffusers are present. The difference in sound power level is close to 1.0 dB depending on the position of the source with the diffuser area. Thanks to the diffusers effect, the difference between the measured and the calculated sound absorption coefficient can be considerably reduced. [ABSTRACT FROM AUTHOR]

This paper presents the design and characterization of DENORMS (Designs for Noise‐Reducing Materials and Structures) cell‐based acoustic metamaterial developed using a 3D printing technique. The metamaterial's acoustic absorption properties were investigated through experimental testing and numerical simulations. The experimental testing of the acoustic metamaterial was conducted in an impedance tube using a two‐microphone method. The numerical simulations were carried out using a thermoviscous module. The simulations allowed for a deeper understanding of the influence of geometric parameters on the absorption coefficient, providing valuable insights for optimizing the design. It was observed that upon increasing the spherical diameter while maintaining the same cylindrical diameter and cylindrical length resulted in an increase in the absorption coefficient throughout the frequency range. It was also observed that upon increasing the cylindrical diameter, there was a significant decrement of absorption coefficient, and upon increasing the length of the cylinder, there was a shift of response toward lower frequency. The augmentation in cell count from 9 to 24 led to a rise in the absorption coefficient and a shift of the response toward lower frequencies. The findings highlight the significance of photopolymer 3D printing in tailoring complex geometries for enhanced performance of the acoustic metamaterial. Highlights: Photopolymeric resin used for DENORMS cell‐based acoustic metamaterial.High cleanability and accuracy of digital light processing 3D printing technique.Usage of thermoviscous model for simulation of sound absorption behavior.Use of two microphone impedance tube to determine sound absorption coefficient.Geometric parameters significantly affect the absorption coefficient. [ABSTRACT FROM AUTHOR]

Down-polyethylene film material has been introduced for the first time as an excellent non-frame sound absorber, showing a distinctively outstanding performance. It contains down fiber adjacent to each other without firm connection in between, forming a structure of elastic fiber network. The unique structure has broadband response to sound wave, showing non-synchronous vibration in low and middle frequency and synchronous vibration in middle and high frequency. The broadband resonance in middle and high frequency allows the structure to achieve complete sound absorption in resonance frequency band. Moreover, down-polyethylene film material possesses forced vibration, corresponding sound absorption coefficient has been obtained based on vibration theory. The down-film sound absorption material has the characteristics of light weight, soft, environment-friendly, and has excellent broadband sound absorption performance. [ABSTRACT FROM AUTHOR]

Recycled fibers obtained from pre-textile waste present advantages and challenges in acoustic absorber design. This study examines the potential of incorporating textile waste into acoustic absorption systems, with a focus on pre-used waste, to increase the percentage of recycled fibers in the textile industry using the quantitative method Used to evaluate the efficiency of panels manufactured in different sound frequencies, from up to 2000 Hz, and, in some cases, up to 3000 Hz Observations indicate that surface cleaning products, pneumafil waste a twist, comb bat residue and waste grinders exhibit the highest sound absorption coefficients in and using waste samples In addition, a procedure has been developed to evaluate the sound absorption reduction coefficient, which exposes high correlations with values obtained by other methods Sound contour mapping further confirms the homogeneity of the sample and reveals the effectiveness of specific sound absorption systems, such as air condition station filters, spinning pneumafil waste, and weaving waste (samples 18, 20, 21, and 23). Nevertheless, the study of integration emphasizes the ability to reuse waste fibers for sound insulation that can absorb adequate sound, reduce reflected sound, and offer a promising solution for dealing with noise pollution. A thorough examination of sound contour maps across absorber surfaces is undertaken to account for the material structure of the absorber, ensuring an accurate representation of how sound is absorbed and distributed within the designated area. [ABSTRACT FROM AUTHOR]

We prepared ultrafine MnCo 2 O 4 particles using a facile sol–gel autocombustion method. The structural and vibrational characterizations confirmed the formation of a pure cubic spinel phase in as-synthesized MnCo 2 O 4 particles. The surface morphology analyses revealed the porous structure with an average particle size of 5 nm and uniform distribution of manganese (Mn), cobalt (Co), and oxygen (O) in the particles. We observed the presence of multiple valence states of Mn and Co, occupying both tetrahedral and octahedral sites, enhancing our insight into the material's electronic properties and structural configuration. Our magnetization studies unveiled a Curie Temperature (T C) of 185 K with ferrimagnetic ordering below T C , resulting in substantial magnetization (9.8 emu/g) and coercivity (3.46 kOe) at 2 K and weak magnetic behavior above T C. This is attributed to the finite size and surface effects. Furthermore, the incorporation of MnCo 2 O 4 nanoparticles and graphene oxide into a polyurethane foam matrix resulted in an excellent sound absorption coefficient of 90 % at 2000 Hz. This outstanding acoustic performance underscores the versatile applications of MnCo 2 O 4 nanoparticles in diverse noise mitigation applications. [Display omitted] [ABSTRACT FROM AUTHOR]

To further design and develop broadband and efficient underwater sound absorption structure, we propose a hybrid acoustic structure composed of a rubber matrix layer, cylindrical cavity, pentamode metamaterial (PM) layer, impedance matching layer, and steel backing plate. Thanks to the special properties of the PMs, the hybrid acoustic structure obtains excellent broadband sound absorption performance (SAP) in the frequency range of 500–10000 Hz than that of the traditional cavity acoustic structure. To this end, we study the design principle of PM structure and its metal–water characteristics and analyze the sound absorption mechanism at the working frequency. In addition, the research shows that the material and geometric parameters of the structure have a significant impact on the SAP. By the optimization of structural material parameters, the average absorption coefficient at 500–10000 Hz can further increase to above 0.781. Finally, we also demonstrate that the structure still has good SAP under a wide range of oblique incidence angles. The research in this paper opens up bright perspectives for the design of underwater anechoic coatings for multifunctional applications. [ABSTRACT FROM AUTHOR]

Purpose: Single-layer micro-perforated panel (MPP) absorbers with limited air back cavities are unable to provide appropriate broadband sound absorption capabilities at low frequencies. To improve wideband sound absorption in the low-frequency region, this research suggests a composite layer MPP (CMPP) absorber with the least cavity depth that combines with a thin layer of jute felt instead of the conventional porous absorbers. To get the best absorption characteristics, the position of jute felt layer is adjusted between the MPP layer and the rigid wall. Methods: The absorption characteristics of CMPP absorbers can be predicted by employing Maa's MPP model, the equivalent fluid model of jute felt layer, and the transfer matrix method. The effect of the structural parameter of MPP (perforation ratio) and the jute felt position of the composite absorber on acoustic absorption are studied through a factorial statistical analysis. The finite element method (FEM) is implemented to simulate the composite MPP absorber and estimate its acoustic properties to corroborate the predicted outcomes. Moreover, measurements are carried out for selected configurations of composite absorbers in a two-microphone impedance tube to approve the predicted results. Results and Conclusion: The CMPP absorber exhibited excellent wideband absorption characteristics at low frequencies compared to the sole MPP or jute felt of the same dimensions. When placed exactly in the middle of the absorber, the jute felt provided a maximized sound absorption average of 53% compared to its other positions. Its half absorption band is also found to be the maximum (2670 Hz) among the proposed structures. The addition of jute felt layer widens the absorption curve due to the extra damping provided by it. The absorption peak shifted toward a higher frequency and got wider with the increase in perforation ratio of MPP layer. The jute felt position was found to be a dominating factor over the perforation ratio of the MPP layer. The predicted absorption coefficients of CMPP absorbers show excellent consistency with the result obtained through FEM and fairly agree with the experimental outcomes. [ABSTRACT FROM AUTHOR]

In this article, the acoustic properties of 3D printed glycol-modified polyethylene terephthalate (PETG) reinforced with organically modified montmorillonite (OMMT) nanoclay/short carbon fiber (SCF) nanocomposites are experimentally investigated using ASTM E1050-08 standard. To this end, the sound absorption coefficient (SAC) of different PETG composites was calculated with the aid of two microphone impedance tube, working in the frequency range of 50-6300 Hz. The effect of different weight percentages (wt%) of OMMT nanoclay, SCFs, and 3D printing infill density on the acoustic behavior of PETG nanocomposites is studied. The experimental results reveal that higher wt% of OMMT nanoclay and SCFs has a beneficial effect on sound absorption. Further, the trend of variation of SAC is justified with morphological studies. Also, an artificial neural network (ANN) based prediction methodology to predict SAC is developed using the datasets obtained from the experimentation. Levenberg-Marquardt backward propagation algorithm with 20 neurons trains the ANN model. Using the trained ANN model, the acoustic properties of PETG/OMMT/SCF nanocomposites with different operating frequencies, infill density and wt% of reinforcements are predicted with less than 5% average error. This can be beneficial in eliminating the fabrication and experimentation costs incurred while assessing the acoustic properties of the PETG composites. Highlights • PETG/OMMT nanoclay/SCF composite filaments are fabricated. • Acoustic absorption of 3D printed PETG composites are experimentally studied. • ANN based predictive tool is developed using experimental data. • ANN tool predicts the sound absorption and reduces experimentation cost. • Microstructural studies justify the trend of sound absorption. [ABSTRACT FROM AUTHOR]

The micro-perforated panel structure has good sound absorption performance in medium and high frequencies, and the labyrinth metamaterial structure has good low-frequency sound absorption performance. Both the two structures are relatively simple, and can be obtained by simple processing with resin material, which is convenient for batch production. In order to enhance sound absorption ability for the sound absorption structure within low frequency, this research work has put forward one coupled structure via the micro-perforated panel and three second-order maze metamaterials structure, resulting in a coupled structure with a low-frequency sound absorption performance. Combination of equivalent circuit method calculates acoustic resistance and absorption coefficient for the sound absorption structure, and genetic algorithm has been employed to optimize the parameters of this sound absorption structure. Finally, the theoretical calculations analysis results have been verified by finite element simulation, which shows the trend of the curve of sound absorption coefficient of the two structures was basically consistent. And the sound absorption coefficient of the three peaks is all above 0.7, and the mean value of the sound absorption coefficient is above 0.6 at the acoustic frequency of 0–1500 Hz, indicating that the coupled structure has a good performance of absorbing medium- and low-frequency sound waves. [ABSTRACT FROM AUTHOR]

This study investigates the acoustic absorption properties of pistachio shell samples as eco-friendly alternatives to synthetic sound-absorbing materials. The samples were measured for thickness (3 cm and 5 cm) and forms (whole, semi crushed, fully crushed) using impedance tube measurements in the laboratory, followed by analysis using the Dunn–Davern (DD) model and optimal inverse method to predict the sound absorption performance. Additionally, the morphological tendencies of the samples were examined using the FESEM technique. The sound absorption mechanism in pistachio shell samples, akin to other porous materials, encompasses frictional dissipation between air molecules and material walls, along with viscosity-related losses due to air movement entrance to smaller voids. These processes facilitate the conversion of acoustic energy into heat. The findings reveal that increasing the thickness and density of the pistachio shell samples leads to higher absorption coefficients and a shift of the absorption peak toward lower frequencies. Furthermore, an increase in airflow resistivity resulting from enhanced tortuosity contributes to higher absorption coefficients and a similar peak shift toward lower frequencies. The degree of sample crushing positively influences the absorption coefficient by increasing the collision surface and component entanglement. However, it should be noted that the one-dimensional nature of the DD model restricts its accuracy, particularly for materials with non-uniform porosity. Conversely, the inverse DD model employing the NMS method proved highly effective in predicting the absorption coefficient of the samples. Overall, the results suggest that pistachio shell samples offer a viable alternative to conventional synthetic absorbers, demonstrating suitable absorption coefficients in middle and high frequencies, as well as possessing rigid and stable surface properties. [ABSTRACT FROM AUTHOR]

In recent years, noise pollution has been recognized as a significant environmental issue, and using sustainable materials as sound-absorbing building materials have drawn considerable attention. The influence of graded perforations on the acoustic characteristics of a 3D printed bio-degradable thick micro-perforated panel (MPP) having graded perforation and made of Poly Lactic Acid is presented. Thicker panels are considered owing to the mechanical strength required for practical wall applications. A fused deposition modeling based 3D printer is used to fabricate the MPPs with graded cylindrical perforations and different patterning of perforations. The sound absorption coefficient is measured using the impedance tube technique and compared with theoretical results obtained using an equivalent electro-acoustic model. Results revealed that for normal incidence, the absorption coefficient is only dependent on the overall perforation ratio of the panel, irrespective of the perforation gradation and patterning of perforation. This gives the freedom to distribute the perforation aesthetically for interior wall application. This work also proposes the effective perforation ratio approach to predict the sound absorption coefficient (SAC) of MPPs with graded perforation. For multi-thickness MPPs and MPPs with linearly graded thickness, improved sound absorption characteristics were observed both in terms of bandwidth of absorption and peak value of SAC compared to the conventional constant thickness MPPs. [ABSTRACT FROM AUTHOR]

Variable acoustics systems are promising engineering developments for multi-purpose rooms and workspaces in many buildings. However, due to space requirements associated with most of the tuning devices used for that purpose, these solutions are hardly adopted in practice. In this work, two innovative tunable sound absorbers that cope with this drawback are proposed, one consisting of rotating perforated panels and the other being a panel with an iris-type aperture. Compared with conventional perforated panel sound absorbers, the designed solutions yield a variable open area ratio system, whose configuration allows tuning the absorption bandwidth without misusing space. To assess their sound absorption coefficient, impedance tube experiments were carried out following the standardized method described in ISO 10534-2 over specimens fabricated for this purpose using laser cutting and additive manufacturing technology. The results not only show their good sound absorption performance but also highlight their tuning capabilities. Complementarily, a model based on the ray tracing method was developed to evaluate the performance of these solutions in a case study room, for different occupancy levels, with the results supporting the previous assertions and revealing the improved intelligibility features when used in such scenarios. The proposed solutions, together with the prediction model, provide a feasible approach for the design and development of tunable sound absorbers in variable room acoustics. [ABSTRACT FROM AUTHOR]

ETFE cushions are used as cladding in predominantly mid to large size architectural projects. Smaller cushions are about 10 m2 in size and the biggest single cushions can be several hundreds of square metres large. They are composed of ETFE membranes a few hundred of micrometres thick. From a point of view of room acoustics, their acoustic transparency, which increases with decreasing frequency, makes them to effectively act like sound absorbers. Their voluminous nature and transparency make it challenging to measure the sound absorption coefficient of ETFE cushions in a laboratory. Using the genetic material optimizer of Odeon® software, the effective sound absorption coefficient of ETFE cushions in four rooms was extracted from room acoustic parameters, which were in turn derived from measured room impulse response data. The results were compared and interpreted by the help of theoretical modelling and measurements on small cushions in a reverberant room. There is a relatively good agreement between the three types of results, possible causes for the differences are explained in the paper. [ABSTRACT FROM AUTHOR]

A series of low‐carbon fly ash cenosphere plates was prepared using solid waste based cementitious material as binder through a simple process; meanwhile, effects of cenosphere's particle size and size distribution on microstructure and properties were investigated. In the range of 20–200 mesh, as particle size decreases, sound absorption performance decreases while strength improves. Appropriate particle grading can effectively improve the compressive strength, but it will lead to a significant decrease in sound absorption performance. Samples prepared from fly ash cenosphere with a particle size of 20–40 mesh show good sound absorption performance: when the cavity size is 0 mm, the maximum sound absorption coefficient is 0.64 and the average value is 0.36 in the range of 50–1 600 Hz; when the cavity size is 100 mm, sound absorption coefficient at 100 Hz is 0.47, the maximum sound absorption coefficient in the range of 50–1 600 Hz is 0.90, and the average value is 0.57 in the range of 100–500 Hz. The prepared low‐carbon fly ash cenosphere plates show excellent sound absorption performance at low‐frequency and are expected to have broad application prospects in low‐frequency noise reduction and absorption. [ABSTRACT FROM AUTHOR]

The coupling effects between the stage house and the auditorium in opera houses make the sound field very complex. In the present study, the effects of stage absorption on the primary monaural objective room acoustic parameters in the seating area of an opera house in China were investigated by means of computer simulation and scaled model measurement. In computer simulations, the absorption coefficient of the ceiling and walls of the stage house was changed across several steps, and then a series of experiments were conducted in a 1:20 scaled hall with and without curtains and wall absorption on the stage to verify the simulation results. It was found that stage absorption has a significant influence on early decay time (EDT), reverberation time (RT) and clarity (C80) in the seating area, while the effect on sound strength (G) is much smaller. The values of EDT and RT decrease with an increase in sound absorption on the stage, while C80 displays an opposite trend. In addition, when the average absorption coefficient of the stage walls and ceiling is over 0.5, the variations in these four parameters due to the increase in stage absorption can be constrained within an acceptable range. [ABSTRACT FROM AUTHOR]

Purpose: The low-frequency noise radiated by ships has a significant impact on marine animals; however, traditional ship sound-absorbing materials are not optimal for low-frequency noise control. Therefore, an acoustic metasurface suitable for low-frequency noise control is devised using a spatial folding structure, which is composed of folded channels and perforated structural units. Methods: Numerical methods are used to investigate the acoustic characteristics and absorption mechanism of the metasurface, and the impact of common structural factors on the acoustic performance of the metasurface is studied. An experimental model is produced with 3D printing technology, and the structure's absorption coefficient is examined. Results: A composite acoustic metasurface is designed by coupling multiple cells, and the designed metasurface structure achieves continuous broadband sound absorption in the range of 125–200 Hz. The structure exhibits subwavelength absorption characteristics and has an average absorption coefficient of 0.886 in the target frequency range, with a thickness of 1/27 of the wavelength. Conclusion: Folded channels extend the propagation path of acoustic waves, lead to lower absorption frequencies, and the coupled multicell design broadens the absorption bandwidth. The proposed coupled multicell folded low-frequency acoustic metasurface structure has excellent sound absorption performance and provides new ideas for the design of marine low-frequency broadband acoustic absorbers. [ABSTRACT FROM AUTHOR]

Purpose: Development a new combined material, nanomembrane and melamine foam, for noise attenuation at mid-range frequency to reduce noise inside an airplane cabin. Methods: To be able to investigate how acoustic properties of acoustic foams are affected by nanomaterials, a new type of Helmholtz resonator based on a combination melamine foam and nanomembranes is proposed. Its performance was evaluated experimentally based on ASTM C384 and ASTM C522. Results: The Helmholtz resonator based on an association of nanomembranes doped with carbon nanotubes and melamine foams provided an effective sound absorption coefficient peak (α), ranging from 0.94 to 0.99, in the range of 1200–1600 Hz, a drastic shift from the high frequency range (5500–6000 Hz) commonly detected for melamine foam. The new system sound absorption peak performance at mid-range frequencies (1200–1600 Hz) was obtained with an increase in thickness between 0.05 and 0.09%, a virtual no weight addition to the soundproof material. Moreover, the analytical model based on Langmuir air flow resistivity and the Delany–Bazley equations captured the experimental trend with accuracy. Conclusion: The new material proposed is a valuable option for noise attenuation inside airplane cabin, in special, regional jets. [ABSTRACT FROM AUTHOR]

Sound-absorbing panels are widely used in the acoustic design of aircraft parts, buildings and vehicles as well as in sound insulation and absorption in areas with heavy traffic. This paper studied the acoustic properties of sound-absorbing panels manufactured with three nozzle diameters (0.4 mm, 0.6 mm and 0.8 mm) by 3D printing from three types of polylactic acid filaments (Grey Tough PLA; Black PLA Pro; Natural PLA) and with six internal configurations with labyrinthine zigzag channels (Z1 and Z2). The absorption coefficient of the sample with the Z2 pattern, a 5.33 mm height, a 0.6 mm nozzle diameter and with Black PLA Pro showed the maximum value (α = 0.93) for the nozzle diameter of 0.6 mm. Next in position were the three samples with the Z1 pattern (4 mm height) made from all three materials used and printed with a nozzle diameter of 0.4 mm with a sound absorption coefficient value (α = 0.91) at 500 Hz. The highest value of the sound transmission loss (56 dB) was found for the sample printed with a nozzle size of 0.8 mm with the Z2 pattern (8 mm height) and with Black PLA Pro. The extruded material, the nozzle diameter and the internal configuration had a significant impact on the acoustic performance of the 3D-printed samples. [ABSTRACT FROM AUTHOR]

The area-averaged effective sound absorption coefficient (SAC) of a rigid-backed homogeneous porous material subjected to a monopole excitation is calculated as the absorbed-to-incident sound power ratio. Using Allard's model to describe the sound propagation above the porous material, an analytical model for this power-based SAC is proposed and proves to give a good approximation of the sound absorption performance under monopole excitation of sufficiently large areas of material. The impact of factors on the power-based SAC, such as monopole height, material radial dimension used to calculate the sound powers, and material properties is discussed. The power-based SAC frequency-dependent behavior is analyzed through sound intensity field assessments at the material surface and is compared to normal incident plane wave and diffuse field SACs. The sound absorption behavior of sound absorbers under monopole excitation exhibits notable distinctions and peculiar results compared to those observed under plane wave and diffuse fields, particularly at low frequencies and for sources close to the material. [ABSTRACT FROM AUTHOR]

Porous materials have emerged as one of the most widely employed sound-absorbing materials in practical applications, owing to their excellent sound absorption properties and lightweight nature. Unsaturated porous materials have the potential to achieve superior sound absorption effects and significantly reduce mass. However, current research on the sound absorption performance of unsaturated porous materials is limited. This paper used a modified transfer matrix method (MTMM), which relies on matrix dimensionality reduction, to analyze the sound absorption characteristics of unsaturated porous materials under various boundary conditions. The study also examines the impact of different parameters, such as material thickness and saturation, on sound absorption performance and compares the results. The findings indicate that when a solid elastic plate is attached to an unsaturated porous elastic plate, the absorption coefficient decreases while the transmission loss increases. Additionally, reducing the thickness of the plate and increasing its porosity and saturation levels lead to an increase in the absorption coefficient and a decrease in the transmission loss. [ABSTRACT FROM AUTHOR]