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To achieve a compact design for low frequency tunable sound absorption, a membrane sound absorber (MSA) with nonlinear magnetic field is proposed in this paper. By employing a central iron platelet on the membrane, the MSA can be easily tuned by introducing a magnet at a distance from the platelet that can be adjusted. To investigate the low frequency properties of MSA with different magnets and iron-platelets, a series of impedance tube experiments are conducted in detail. The sample absorber has a rear cavity depth of 30 mm, three different magnets were used inside, tested results real that using a strong magnetic field can help broaden the frequency tuning range. Then, results from the MSA with five different sizes of iron plates tuned by one magnet show that the low-frequency tuning range moves to lower with the increase of the area of iron plates.

To construct a smart sound absorber in the low-frequency range with a wide control band, a piezoelectric ceramic (PZT) shunted with multiple resonance circuit is attached onto a micro-perforated panel (MPP) to perform as a smart sound absorber. The absorption can be controlled by the shunt circuit parameters conveniently. This smart micro-perforated panel (MPP) is investigated experimentally to explore the feasibility and design procedure in practical use. Based on the coupling among the acoustical, electrical, and mechanical fields, the proposed broadband sound absorber can achieve good acoustic performance on subwavelength scales. The electrical response of the shunt circuit is tested with a Network Analyzer. The acoustic performance of the smart sound absorber is measured in an impedance tube with the two-microphone transfer function method. The experimental results validate that the shunt circuit can resonate with the PZT patch at multiple frequencies, and hence improve the sound absorption of the smart absorber at these frequencies.

The membrane sound absorber (MSA) with a compact magnet has exhibited excellent tunable properties for low-frequency sound absorption. To further clarify its acoustic properties, this paper presents a theoretical model based on a multi-mechanism coupling impedance method. The model predicts the absorption coefficients and resonant frequencies of the MSA at different tuning magnetic states for three cavity configurations. These parameters are then experimentally measured using an impedance tube for model validation, demonstrating good agreement between the measured and predicted values. Subsequent analysis reveals the iron-platelet-magnet resonance mechanism introduced by the tuned magnetic field is the main factor behind the appearance and shift of absorption peaks in the low-frequency region, which are mostly independent of the back cavity. In other words, the MSA with a back cavity of any size can achieve sound absorption in the low-frequency region. This demonstrates the potential of the structure in achieving an ultra-thin, low-frequency, tunable sound-absorber design that can be adapted to different noise sources.

This paper focuses on the airflow resistivity of a sound absorber. The airflow resistivity is used to predict sound absorption coefficients using mathematical models. As most green materials have unique characteristics, such as fiber curliness and non-circular cross-sections, the effect of these characteristics on acoustical properties needs to be studied. This work firstly discusses the development of an experimental set-up for measuring airflow resistivity and then develops an equation for estimating the airflow resistivity of a green material incorporating its unique characteristics. Samples of fibers with different curliness values are formed, and the effect of curliness on airflow resistivity is measured. The curliness of natural fibers is quantified by defining a unique factor, known as the curliness factor. The equation for airflow resistivity prediction is formed by the application of the Ergun equation and the experimental results obtained, which is then validated for fibers of different curliness. The sound absorption coefficients of the samples with different curliness values are measured. It is observed that an increase in curliness increases the airflow resistivity and sound absorption coefficient. The mathematical model proposed by Miki is used to predict the sound absorption coefficient, and the predicted results are compared with the measured results.

The authors propose a space sound absorber made of a permeable membrane (PM), including woven or non-woven fabrics in three-dimensional shapes, e.g., cylinder, rectangle, etc. The proposed absorbers are examined by experimental measurements and boundary element analyses, and it is found that they can be effectively used especially for middle and high frequencies. In order to develop these absorbers for wider applications, it would be desirable to give them additional values and functions, particularly to elaborate on their design. Supposing that they could also be used for lighting equipment, such as lampshades as one of the applications, pilot studies on pseudo-cylindrical, and pseudo-spherical PM space absorbers with uneven surfaces produced by paper-folding (origami technique) are carried out. The pseudo-cylindrical concave curves (PCCC) shell shape has been proven as a suitable form for a lampshade, and the pseudo-spherical concave curves (PSCC) shell shape is an application of PCCC. In this paper, PCCC and PSCC shell shapes are applied to three-dimensional PM space absorbers, and trials are conducted using PMs selected by flow resistance measurements and preliminary simulations. The sound absorptivity of the specimens is measured in a reverberation chamber, and their absorptivity is 0.6 (PCCC case) and 0.4 (PSCC case) at mid-high frequencies.

The construction and building sector is responsible for a large share of energy and material used during the life cycle of a building. It is therefore crucial to apply a circular economy model within the process wherever possible to minimize the impact on the environment. In this paper, the possibility of producing thermal and acoustic boards from industrial nonwoven waste textile is studied and presented. The nonwoven polyester textile obtained directly from the production line in the form of strips and bales was first shredded into smaller fractions and then in the form of pile compressed with a hot press to form compact thermal insulation boards. The first set of specimens was prepared only from waste polyester nonwoven textile, whereas the second set was treated with sodium silicate in order to check the material’s reaction to fire performance. The experimental work was conducted to define the acoustic properties, reaction to fire behavior and thermal conductivity of the produced specimens. The obtained results show that the thermal conductivity coefficient of specimens without added water glass dissolution is near to the values of conventional materials used as thermal insulation in buildings. The reaction to fire testing proved that the addition of water glass actually propagates the progressive flame over the entire product. It can be concluded that the presented thermal insulation can be used as an adequate and sustainable solution for building construction purposes.

The effects of the density and thickness of samples made from used cigarette butts on acoustic characteristics were analyzed in this study. All the analyzed samples showed high acoustic performance, indicating that the fabrication of acoustic absorbing material may be a good use for this problematic waste (due to its toxicity, continuous generation, lack of recycling method, etc.). An increase in either density or thickness shifted the absorption characteristics of the samples to lower frequencies and increased the overall absorption. The relationships of the frequency and value of the maximum absorption coefficient with thickness and/or density were analyzed. The shift of the maximum absorption coefficient value due to varying thickness is in good agreement with previous studies.

An investigation of sound absorption in 6 species of hardwoods according to the pore structure of cross-sections revealed that higher gas permeability was associated with greater sound absorption at high frequencies. In addition, diffuse-porous wood exhibited superior sound absorption performance compared with ring-porous wood. Through-pore porosity was associated with improved sound absorption in the cross-sections of all 6 species, while closed-pore porosity was associated with poor sound absorption. The results of these studies could lead to development of hardwood products with superior sound absorption and help elucidate the roles of porosity in drying, impregnation, and thermal insulation properties of wood.

For many years, copper and copper alloys materials have been recognized as an efficient antimicrobial. Experimental studies have reported that solid copper surfaces can kill 99.9% of microorganisms within two hours of contact, mainly attributed to the release of ions. However, copper materials in acoustical applications have not been usual, mainly because of their costs. The COVID-19 outbreak has boosted the use of copper in the most frequently touched surfaces to help reduce the spread of the virus. In addition to being malleable, ductile, and an extremely good conductor of electricity, copper exhibits other interesting material properties. The characteristic impedance of copper is 80% of the characteristic impedance of stainless steel and it is slightly twice the characteristic impedance of aluminum. Meanwhile, the critical frequency by unit thickness (Hz-m) of a copper panel is about 50% greater than that of an aluminum or steel plate. In this communication, a sound-absorbing device completely made of copper is presented, which makes use of a copper perforated panel and a copper fibrous infill material. The device would be useful to provide sound absorption in those environments where the protection of people's health is important.

The present work analyses the acoustic properties of acoustic absorbent textile products obtained from the thermo-compression of recycled End-of-Life Tyres. The objective is to obtain products of greater added value from the waste of End-of-Life Tyres, which can become a substitute for the fibrous materials currently used as acoustic absorbents. Experimental design was used to determine the number of samples to be prepared and the manufacturing conditions to maximise acoustic absorption. Five factors were taken into account: material type, temperature, time, weight and compaction factor. The obtained results demonstrate the validity of the experimental design to select the manufacturing factors in order to improve the sound absorption of these textile waste products.

In modernen energieeffizienten Heizungsanlagen mit geringer Abgastemperatur kondensiert der beim Verbrennungsvorgang entstandene Wasserdampf im Abgasstrang und bildet in Verbindung mit den Abgasen korrosives Kondensat. Das Absorbermaterial für Abgasschalldämpfer muss somit akustische Eigenschaften mit chemischer Beständigkeit verbinden. Gemeinsam mit einem Hersteller von Abgasschalldämpfern und einem Hersteller von PTFE‐Produkten wurde ein Schallabsorber entwickelt, der über die integralen Eigenschaften Kondensatbeständigkeit, Schallabsorption mittlerer und hoher Frequenzen, Reinigbarkeit und Wiederverwendbarkeit verfügt. Dieser Beitrag stellt die Ergebnisse des hierzu durchgeführten Forschungsvorhabens [1, 2] vor. Ausgehend von der Materialauswahl wird der Ansatz für eine nachhaltige Lösung des akustischen, ökologischen und letztlich auch ökonomischen Problems dargestellt. Im Labormaßstab wurden Muster von Absorbern erstellt und chemische sowie akustische Eigenschaften ermittelt. Anhand eines Materialmodells werden die Eingangsdaten für ein Absorbermodell abgeleitet. Modellrechnungen werden Messungen gegenübergestellt und diskutiert. Möglichkeiten zur Reinigbarkeit und Wiederverwertung des Schallabsorbers zeigt das Beispiel eines Schalldämpfers für Heizungsanlagen. Über den dargestellten Projektansatz hinaus weisen reinigbare und chemisch beständige Schallabsorber ein erhebliches Transferpotential in andere Marktbereiche wie der Medizintechnik und Lebensmittelindustrie auf.

The main objective of this study is to maximize the rate of sound absorption by applying the parameters of micro-perforated plate (MPP) such as the holes diameter, holes spacing, thickness of MPP, and air cavity depth of MPP. In this study, an optimization algorithm – Firefly algorithm is adopted to determine an optimum set of four parameters for MPP. There are four pieces of MPP with different holes of diameter and spacing was used as specimens. The two-microphone impedance tube method was used to measure sound absorption coefficient (SAC) of MPP sound absorber according to ASTM E1050-12 standard. From the experiments, MPP C (hole diameter = 0.5 mm, hole spacing = 7 mm) for both air cavity depth (30 & 60 mm) score the highest SAC which is 1.00 while MPP B (hole diameter = 0.9 mm, hole spacing = 5 mm) obtain the lowest SAC for air cavity of 30 and 60 mm, which are 0.62 and 0.54 respectively. Then, the firefly algorithm is applied to obtain the optimal solution the set of parameters for MPP sound absorber to reduce the noise level. Hence, it is concluded that by increasing the air cavity depth, holes spacing, and decreasing holes diameter size can increase the rate of sound absorption for MPP. The optimal set of parameters obtained from this study for MPP sound absorber for air cavity, hole diameter and holes spacing are 30, 0.71 mm and 0.5 respectively.

Traditional porous media such as melamine foam absorb sound due to their three-dimensional porous struts. However, the acoustic properties at low frequencies are greatly related to its thickness. In this paper, a novel type of thin and lightweight sound absorber composed of melamine foam and hollow perforated spherical structure with extended tubes (HPSET) is introduced to enhance the sound absorption performance at low frequencies. A theoretical model for the normal absorption coefficient of the HPSET with melamine foam is established. Good agreements are observed between the simulated and the experimental results. Compared with the virgin melamine foam, the proposed absorber can greatly improve the low-frequency sound absorption and retain the mid- to high-frequency sound absorption, while the thickness of the proposed absorber is less than 1/28 of the wavelength.

A novel membrane sound absorber based on the magnetorheological membrane (MRM) is developed for active sound absorption. Due to its good flexibility and stretch-ability, the MRM consisting of carbonyl iron particles (CIPs) and polydimethylsiloxane (PDMS) elastomer matrix can be easily deformed. The deformation of MRM is investigated by the digital image correlation method. The off-plane displacement is positively related to both of the CIPs mass fractions and magnetic field strength. The sound absorption performance of MRM sound absorber is investigated and it is found that the thickness of the membrane, CIPs mass fractions, and air cavity length behind the membrane significantly influence the sound absorption performance. The magnetic field can adjust the sound absorption frequency without affecting the high acoustical absorption coefficients. The double-layer membrane structure has two sound absorption peaks within the testing frequency range and further improves the frequency adjustability of the sound absorber. Therefore, it is expected to be a particularly promising alternative for active sound absorption. Finally, the numerical solutions calculated by the equivalent circuit method also support the experimental results.

Plane-wave-enriched finite element method (FEM), an efficient wave-based prediction method, uses shape functions incorporating a set of plane waves propagating in various directions to enhance the sound field approximation capability. The method can be an efficient wave-based acoustic solver for room acoustic simulations because it can simulate wave phenomena in rooms accurately with markedly fewer finite elements in spatial discretization than those used for standard FEM. However, several aspects remain to be addressed before its use for practical room acoustic simulations. Accurate sound absorber modelings able to address their frequency and incident angle dependence of absorption characteristics present extremely important issues. This paper presents a proposal of a method of implementing an extended-reaction model of microperforated panel (MPP) and permeable membrane (PM) sound absorbers into a room acoustic solver with plane-wave-enriched FEM. First, we demonstrate the validity of the proposed method in comparison with theoretical values in which impedance tube problems including three sound absorbers composed of MPP and PM are used. Then, the effectiveness over standard FEM is demonstrated via 2D real-scale office problems with the three MPP-PM sound absorbers.

Noise is an unpleasant sound generated from high level sound source, which is a major concern on the residential areas and disturbed comfort, safe activity performance and human health. Now a days, most people are interested on light weight, having good mechanical strength and biologically degradable as well as compostable products. The aim of this research work was characterized the acoustic properties of new alternative ecological composite materials using low cost and light weight agro waste resources. The sound absorption performance of enset fabric and its composite were determined using two microphone transfer mechanism with impedance tube method. The porosity and void structure of enset fibers were studied by scanning electron microscope (SEM) for analysis of enset fabric/enset fabric composite sound dissipation mechanism. The sound absorption coefficient of enset fabric reinforced green composite had a value of 0.9 at critical frequency of 4500 Hz. The test result revealed that, enset fabric had better sound absorption capacity than commercially used glass fabric and had a promising potential to substitute flax in ecofriendly automobile and construction sectors with a wide range of acoustic absorbing frequencies, 3000 Hz - 6000 Hz.

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For most acoustic metamaterials, once they have been fabricated, their operating frequencies and functions cannot be adjusted, which is an intrinsic barrier for development of realistic applications. The study to overcome this limit has become a significant issue in acoustic metamaterial engineering. Although with the advance of metamaterials in the past two decades, a series of methods such as electric or magnetic control have been proposed, most of them can only work in the condition of no fluid passage. Some metamaterials with large transmission losses have been proposed, but the sounds are essentially reflected rather than absorbed. Here, to overcome this intrinsic difficulty, we propose a ventilated sound absorber that can be manually tuned in a large range after being manufactured. During the tuning which is achieved through an intricately designed slider, high-performance absorption and ventilation are both ensured. The tunable ventilated sound absorber is demonstrated experimentally and the effective model of coupled lossy oscillators can be employed to understand its mechanism. The manually tunable ventilated metamaterial has the potential application values in various complicated pipe systems that require frequency adjustment and it also establishes the foundation for future development of active tunable ventilated acoustic metamaterials.

The environmental issue becomes the central topic in the discussion for the last decade. Scientific works to overcome the problems are thus progressing including finding the alternative natural materials to replace the conventional synthetic ones. This chapter discusses the use of natural fibers extracted from pineapple leaf as natural acoustic absorber. The preparation for absorber samples is discussed, and the effects of fiber density, sample thickness, and introduction of backed air gap were measured using normal field incidence method in an impedance tube. The measured results reveal that the pineapple fibers can have good sound absorption above 500 Hz for thickness of 30 mm and density of 117 kg/m3. Almost the same performance can be achieved for thickness of 20 mm by introducing backed air gap of 20 mm. The effect of the quarter-wavelength with the presence of backed air gap can be clearly observed where this can be used as the design guide to determine the required thickness of the absorber.

Исследование акустических характеристик строительных материалов и конструкций необходимо для комфортной жизни людей в городских условиях. Несмотря на то что в нормативных документах развитых государств есть ряд методов их определения, единого понимания самих характеристик и путей их адекватного определения до сих пор нет. В настоящей работе мы попытались систематизировать и дать критический обзор нормативных документов, содержащих методы определения звукопоглощающих свойств материалов и звукоизоляционных характеристик конструкций. Впервые выявлены присущие разным методам закономерности определения характеристик звукопоглощения и звукоизоляции. Определены наиболее характерные показатели звукопоглощения. Произведено ранжирование по коэффициенту уменьшения шума NRC наиболее часто применяемых строительных материалов. Названы области применения рассматриваемых методик, проанализированы их преимущества и ограничения. Ключевые слова: коэффициент уменьшения шума, звукопоглотитель, импедансная труба, реверберационная камера The study of the acoustic characteristics of building materials and structures is necessary for provision of comfortable life of people in urban conditions. Despite the fact that in the normative documents of developed countries there are a number of methods for their determination, there is still no common understanding of the characteristics themselves and the ways of their adequate determination. In this work we tried to systematize and give a critical review of regulatory documents containing the methods for determining the sound-absorbing properties of materials and the sound-insulating characteristics of structures. For the first time the regularities in determination the characteristics of sound absorption and sound insulation inherent in different methods have been revealed. The most characteristic indicators of sound absorption have been determined. The ranking of the most commonly used building materials was made according to the noise reduction factor (NRC). The areas of application of the considered methods are presented, their advantages and limitations are analyzed. Keywords: noise reduction factor, sound absorber, impedance tube, reverberation chamber

This study discussed an alternative material known as pineapple leaf fibres (PALF) as a replacement of synthetic fibres in sound absorber production. The samples were fabricated from two different sizes of PALF, with and without binder of different thicknesses and densities to determine their effects on the sound absorption coefficient (SAC). The performance of SAC was measured by using an impedance tube instrument according to ISO 10534-2. The resulting frequency peak value of PALF obtained was in the range of 1–2 kHz. The results demonstrated PALF as a new hope for environmentally-friendly sound absorption material in replacing synthetic fibres. The PALF was capable of achieving SAC of more than 0.9 on average above 1 kHz by keeping the densities and thicknesses of the fibres under control. Additionally, the acoustic performance of PALF specimens was better than that of synthetic absorbers available in the market. Therefore, PALF is a promising natural fibre that can be potentially used as a sound absorber material.

This paper reports the utilisation of fibres from the pineapple leaf (PALF) to be an alternative natural acoustic material. We fabricated samples from raw pineapple leaf fibres with different densities and thicknesses to observe their effects on the sound absorption characteristic. Measurement was conducted for the normal incidence sound absorption coefficient in an impedance tube based on ISO 10534-2. It reveals that the pineapple leaf fibres can achieve sound absorption coefficient of 0.9 on average above 1 kHz by controlling the densities of the fibres and/or by introducing the air gap behind the samples. It is also demonstrated that the sound absorption performance is similar to that of the commercial rock wool fibres and synthetic polyurethane foam.

In this study, the effects of physical parameters on the acoustic absorption properties of woven fabrics were studied. The acoustic absorption properties of 24 kinds of woven fabrics with different structural parameters were measured and analyzed. Multiple linear regression models have been established to characterize the relationship between acoustic behavior and various physical parameters. It has been found that the acoustic absorption properties were mainly determined by the perforation ratio and air permeability. The decrease in perforation ratio and air permeability results in an increase of acoustic absorption properties of woven fabrics. Furthermore, three woven fabrics were used to validate the proposed multiple regression models. The established models could well predict the acoustic absorption properties of woven fabrics where the correlation coefficient is higher than 0.77 with air gaps of 1, 2, and 3 cm, respectively.

The average sound absorption coefficient of a composite absorber from 0 to 1.6 kHz was maximized using a teaching–learning-based optimization algorithm, where the geometric parameters were used as optimization variables. The effective-medium model and transfer-matrix method were used to calculate the sound absorption coefficient, and optimized sound absorption coefficient was maximized at 0.938. The specific acoustic impedance of the material is almost perfectly matched with air thanks to the optimized size of the lateral plates, micro-perforated plates, and cavities. Experimental results confirm the theoretical calculation, showing the investigated passive acoustic design method of designing sound absorber.

To reduce the noise in a ventilated system, three-dimensional (3D)-printed sound absorbers, based on multiple spiral waveguides, are proposed in this letter. A waveguide unit consists of a fixed-length cylindrical straight tube and a certain number of spiral tubes. The geometrical parameters of the spiral waveguides can be flexibly regulated, aided by the 3D digital modeling method to control the phase of the sound wave. Experimental results show that the proposed sound absorber can obtain the sound transmission loss (STL) about 9.6 dB in a frequency range of 300-1250 Hz with an 18% ventilation ratio.

Microperforated panel (MPP) absorber progress has continued with various configurations for a relatively narrow bandwidth. Serial, parallel, and a combination of serial and parallel (hybrid) configurations of MPPs have been proposed to address the bandwidth problem, and experimental results agree well with theoretical ones. However, considerable space is still required, particularly at low frequencies, because of the presence of a backing air cavity. This drawback has limited MPP applications for low-frequency noise control, where longer cavity depths are required. Therefore, in this study, a coiling structure was introduced on the backing air cavity to obtain a thinner MPP for the same targeted operating frequency. Moreover, a parallel configuration consisting of several sub-MPPs was utilized to excite multiple resonances to obtain an overall wider absorption bandwidth. Minute holes were applied to avoid the considerable dips that are generally present in such a configuration. Thin and broadband MPPs were developed, and theoretical and experimental results are provided.

In this study, an acoustic structure consisting of cellular and perforated panel absorbers has been designed to noise control. Then, the sound absorption behaviour of designed acoustic structure has been investigated experimentally by reinforcing with STFs having different rheological properties. In addition, the sound absorption performance of the designed acoustic structure has been compared with the Gyroid which is a type of cellular sound absorber. The findings have indicated that the perforated panel activates significantly the sound absorption performance of the designed acoustic structure especially in the low ( f 1500 H z ) and high ( f > 3500 H z ) frequency ranges. In addition, it has been also observed that this effect becomes stronger with STF reinforcement and significantly improves the sound absorption behaviour of the designed acoustic structure. Besides, it has been noticed that the sound absorption performance of the designed acoustic structure increases with increasing molecular weight of PEG for low frequencies (i.e. f 1500 H z ). On the other hand, it has been also observed that the effect of PEG molecular weight on sound absorption behaviour remains relatively limited level for high frequencies (i.e. f > 3500 H z ). Finally, it has been detected that the effectiveness of STF on the sound absorption performance of designed acoustics structure improves remarkably with increasing silica-nanoparticle ratio in the case of low and high frequencies (i.e. f 1500 H z and f > 3500 H z ).

Porous sound absorptive material (PSAM) and micro-perforated plate (MPP) limited by space are weak in absorbing low- to mid- frequency noise. In order to improve sound absorption in 100–1600 Hz, a compound sound absorber comprised of perforated plates with extended tubes (PPET) and a PSAM is proposed in this paper. A theoretical model is described to predict the sound absorption coefficients of two combinations: one has three parallel-arranged PPETs and a PSAM layer; the other one has two parallel-arranged PPETs, an MPP and a PSAM layer. The calculated results are then validated by the results measured in an impedance tube. The proposed combinations demonstrate superior sound absorption performance over more than three octaves in the targeted frequency range.

An ultra-lightweight sandwich panel with perforated honeycomb-corrugation hybrid (PHCH) core is innovated as a novel sound absorber, which demonstrates great sound absorption as well as excellent mechanical performance. Based on the hybrid-cored sandwich panel, small perforations of different diameters distributed heterogeneously on both the top facesheet and the corrugation are introduced to obtain perfect sound absorption at low frequencies with almost no deterioration in mechanical performance. Theoretical models for sound absorption coefficient and bending stiffness are established, then verified by numerical simulations. With specific mass taken into consideration, an integrated index is proposed to further evaluate the acoustic-mechanical property of the hybrid sandwich. It is found that the PHCH with thinner honeycomb always performs better at low frequencies, and the facesheet is more important to the acoustic-mechanical property than the corrugation. This new kind of lightweight sandwich constructions show promising engineering applications, capable of serving as multi-functional structures with great acoustic and mechanical properties at the same time. Keywords: Perforated honeycomb-corrugation hybrid, Sound absorption, Bending stiffness, Multi-functional structure

Satisfactory sound absorption using a low mass density foam is an intriguing desire for achieving high fuel efficiency of vehicles. This issue has been dealt with a microcellular geometry manipulation. In this study, we demonstrate the relationship between cell openness of polyurethane (PU) foam and sound absorption behaviors, both theoretically and experimentally. The objective of this work is to mitigate a threshold of mass density by rendering a sound absorber which shows a satisfactory performance. The cell openness, which causes the best sound absorption performance in all cases considered, was estimated as 15% by numerical simulation. Cell openness of PU foam was experimentally manipulated into desired ranges by adjusting rheological properties in a foaming reaction. Microcellular structures of the fabricated PU foams were observed and sound absorption coefficients were measured using a B&K impedance tube. The fabricated PU foam with the best cell openness showed better sound absorption performance than the foam with double mass density. We envisage that this study can help the manufacture of low mass density sound absorbing foams more efficiently and economically.

The average sound-absorption coefficient of an ultra-broadband parallel sound absorber, between 0 and 2000 Hz, is optimized in certain conditions. This is accomplished using a teaching–learning-based optimization algorithm, where the geometric parameters are used as optimization variables. The optimized ultra-broadband parallel sound absorber has an average sound-absorption coefficient of 0.9255. The surface impedance of its structure is almost perfectly matched with the surrounding air – thanks to the optimized lengths of both the lateral and micro-perforated plates. The effective-medium model, the transfer-matrix method, and the finite-element method are applied and show consistent results. The acoustic-impedance tube measurements confirm that the optimized ultra-broadband parallel sound absorber shows high sound-absorption from 200 to 1715 Hz. The investigated passive acoustic design opens new possibilities for new absorber types in the future.

We develop a hierarchical acoustic metamaterial with ultra-broadband sound absorption and promising applications at high temperatures as a super sound absorber. The proposed metamaterial is constructed through tailoring structural configurations by vertex-based hierarchy. The hierarchical design enables the acoustic metamaterial to achieve an excellent successive absorption spectrum in the low- to mid-frequency range from 0.58 kHz to 2.04 kHz in the first order and 0.64 kHz to 3.42 kHz in the second order, respectively. The sound absorption performance at high temperatures is predicted by means of theoretical and numerical approaches, and the models are validated by experimental measurements at room temperature. This work provides insight and opportunities into the role of structural hierarchy in designing acoustic metamaterials equipped with extraordinary absorption properties, illustrating an unprecedented capability of the hierarchical acoustic metamaterial.

This paper discusses the utilisation of fibres from the oil palm empty fruit bunch (OPEFB) to be an alternative natural acoustic material. The study was carried out by fabricating samples from raw OPEFB fibres with different densities and thicknesses to observe their effects on the sound absorption performance. It has been demonstrated that the sound absorption performance can be improved by increasing the thickness of the sample and also by having optimum densities of fibres. In particular for lower frequencies, this can be achieved by introducing air cavity gap behind the fibre samples. Measurement of the normal incidence absorption coefficient in an impedance tube based on ISO 10534-2 found that the OPEFB fibres can have absorption coefficient of 0.9 on average above 1 kHz. The sound absorption performance of OPEFB fibres is also shown to be comparable to that of the commercial synthetic rock wools.

This study investigates the acoustical and nonacoustical properties of composites using corn husk fiber (CHF) and unsaturated polyester as the sound-absorbing materials. The influence of the volume fraction of CHF on acoustic performance was experimentally investigated. In addition, the nonacoustical properties, such as air-flow resistivity, porosity, and mechanical properties of composites have been analyzed. The results show that the sound absorptions at low frequencies are determined by the number of lumens in fiber, particularly the absorption coefficient, which increases the amount of fiber. For high-frequency sound, the absorption coefficient is determined by the arrangement of fibers in the composite. An absorption coefficient is close to zero when the fibers are arranged in a conventional pattern; however, when they are arranged in a random pattern, a high absorption coefficient can be obtained. The bond interface between the fiber and resin enhances its mechanical properties, which increases the longevity of the composite panel.

Natural fibres are widely used in different applications and one of the important applications is sound insulation. The research trends in the improvement of thermal and sound insulation of the jute-based composite materials filled with polymer materials are studied by the researchers. Jute fibres exhibit admirable heat and sound insulation properties but this has not been completely researched as yet. A detailed information related to enrich the insulation properties of jute non-woven-reinforced polymer composites based on its design of manufacturing, density of composites, morphology, structural parameters of fibre batting, number of layers and recent strategies are discussed for its high-performance potential applications.

In this work, we have developed an analytical model of a multilayer porous material based on the transfer matrix method to predict the absorption behavior at plane wave incidence. The aim of this study is to modify/tune the sound absorption coefficient of a felt to obtain an improved absorbing performance in the mid frequency range without increasing its weight. To achieve this target, the developed model has been used to find the best combination of each layer type and thickness. The analytical results were validated by test results.

A model of an efficient sound absorber with an internal structure consisting of several compact resonators with different parameters is considered. Maximum efficiency is achieved by matching the impedance characteristics of the sound absorber and the acoustic environment. Experimental results on determining the maximum absorption coefficient in the 100–1000 Hz frequency range for test samples made with 3D printing technology are reported.

There has been increasing interest in new sustainable materials that can be used as construction materials. Among them, sound-absorbing materials have an important role in both acoustical room conditioning and in room insulation. As a proposal for recycling, one of the most common residues in the world, cigarette butts, is studied. Samples were prepared with used cigarette butts as acoustical absorbent materials. Several samples were prepared and grouped by similarity. Variability analyses of the samples prepared in each group were performed. Moreover, the analysis of some possible influences on absorption properties, such as the length of butts, presence of burnt regions, presence of wrapping paper, etc., were analyzed. The results show the potentiality of this residue to be used as an acoustical absorbent since the absorption coefficients found are greater than 0.8 for frequencies over 2000 Hz. The observed variability in the study group and samples can be considered low, as it was below 2% for the major part of frequencies. Influences on the absorption coefficient, for both the length and status of the butts, were statistically confirmed.

Sub-wavelength sound absorbers are attractive for dealing with noise control at low-frequency (long-wavelength) sounds. To be efficient in absorbing the sound energy, resonator based absorbers are preferable over fibrous porous ones. In this paper, a coiling up space approach is introduced to a tube resonator system in order to realize a sub-wavelength absorber structure. In this way, the air channel of the tube resonator is a coplanar coiled up channel rather than a straight channel as found in conventional tube resonators. The effect of the geometrical properties of the aperture and the air channel were studied further to look at their relationship to impedance mismatch, which coiling up systems typically suffer from. It was found that the proposed approach could realize a sub-wavelength absorber system up to 1/32 wavelength of peak sound absorption. Selection of the shape and dimensions of the aperture must be done with great care as indicated by the measurement results. Moreover, the behavior of the coiled up tube resonator deviates from that of the straight tube as the reflection factor is increased, although the target resonance frequency is close to the target. It was also found that a squared aperture shape as well as increasing the cavity thickness is useful to deal with impedance mismatch.

Microperforated panel (MPP) absorbers have been widely used in noise reduction and are regarded as a promising alternative to the traditional porous materials. However, the absorption bandwidth of a single-layer MPP is insufficient to compete with the porous materials. In order to improve the sound absorption ability of the single-layer MPP, a composite MPP sound absorber with membrane cells (MPPM) is introduced. Sound absorption properties of the MPPM are studied by the impedance tube experiment. Results show that the membranes have a significant influence on the sound impedance. The sound absorption performance of MPPM gradually increases with the increase of the membrane area. The single-layer MPP with some small area membrane cells may have the same effect and single large area membranes. By adjusting the size of the membrane cells, one can implement a sound absorber with a wider absorption bandwidth and higher absorption peaks than the single-layer MPP.

Studies to find alternative low environmental-impact materials for acoustic absorbers are still progressing, particularly those originated from natural materials. However, most of the established works are mainly focused on the fibrous-type absorbers. Discussion on the non-fibrous-type absorbers is still lacking and this therefore becomes the objective of this paper. Use of bamboo by utilizing its hollow structure to absorb sound energy is discussed here. The normal incidence absorption coefficient was measured based on the length and diameter of the bamboo, as well as different arrangement of the bamboo structure subjected to the incidence sound, namely, axial, transverse, and crossed-transverse arrangements. The trend of absorption coefficient appears in peaks and dips at equally spacing frequencies. For all arrangements the peak of absorption can reach above 0.8. Introducing an air gap behind the bamboo shifts the peak absorption to lower frequency. Covering the front surface of the absorber improves the sound absorption coefficient for axial arrangement by widening the frequency range of absorption also towards lower frequency range. The transverse arrangement is found to have average absorption coefficient peaks of 0.7 above 1.5 kHz. By arranging the bamboo structure with crossed-transverse arrangement, the suppressed absorption peaks in normal transverse arrangement can be recovered.

This is a comprehensive study on a sustainable sound absorber sourced from nettle fibers. The fibers were thoroughly characterized for properties linked to its performance as an effective and efficient sound absorber, like density, surface roughness, surface morphology, physical strength, and thermal stability. Nettle fibers were converted into non-woven form and characterized for its acoustical characteristics. The obtained results of sound absorption coefficients were analyzed to determine the optimum thickness for optimum performance during practical noise control applications. Further, a comprehensive comparison of experimental results and theoretical results obtained from empirical models were compared. The genetic algorithm and Allard model was used to estimate the characteristics impedance and complex wavenumber. These characteristics were further used to visualize the effect of the air gap behind the sound absorber. The results obtained showed nettle fiber to be an effective sound absorber. The results of theoretical and experimental studies matched well. The average noise reduction coefficient up to 0.72 was achieved, which was further boosted to 0.80 on incorporating a 30 mm air gap. The performance of nettle fiber with optimum thickness was compared with commercial glasswool, and waste jute fibers. The comparison established nettle to have better sound absorption properties.

This paper presents the results of a study in which the performance of palm oil clinker (POC) sandas an alternative acoustic material for controlling noise problems was investigated. The specimens were prepared using the basic mortar mixture of cement-to-river sand ratio of 1:4. The five mixtures used in the study containedPOC sand at the amounts of 0%, 25%, 50%, 75% and 100% of the total weight of the sand. Hardened mortar properties of density and compressive strength as well asacousticperformance were measured. The results show that the specimens containing POC sand gave an average noise reduction coefficient of 0.30, which is larger than the corresponding value of 0.25 for the specimens withriver sand only. The combination of 50% POC and 50%river sand achieved the highest sound absorption coefficient of 0.5 at 315 Hz and 0.4 at 1000Hz. It was also found that although POC sand reduced the compressive strength of specimens, the values obtained was still within the strength limit of non-load bearing structures. The findings suggest that POC sand has the potential to be used in the construction of non-structural wall in reducing the noise pollution.

Sound absorbing materials used for noise control is desired by the practitioners to be environment friendly and of low cost. This has motivated the researchers to look for alternate materials from natural sources. One step in this direction is exploring the use of recycled materials for potential application as sound absorbers. This paper identifies two materials, denim shoddy and waste jute fibers. They are characterized by their acoustical and physical properties. Experiments have been conducted as per 2-level 2-factor full factorial design of experiments. A comprehensive comparison of the sound absorption coefficients of denim shoddy and waste jute fibers, measured in an impedance tube and predicted by using empirical relations are shown. The absorption spectrum of these materials are compared with commercial glasswool with similar process parameters, and the results obtained show denim shoddy to be a better sound absorber than that of glasswool. A mathematical equation for Noise Reduction Coefficient is formed using the experimental results obtained and validated with a fresh set of experiments. Finally, economic comparison of these materials is conducted. The results obtained from technical and commercial research clearly states that the application of these materials for noise control will fetch better results apart from being cost-effective and environment-friendly.