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This study explores a novel approach to produce silica-cellulose aerogel from geothermal solid waste and newspaper trash for enhanced sound insulation applications. The silica-cellulose aerogel composite was synthesized by using cellulose as the matrix and impregnating it with silica particles through a sol–gel process involving sodium silicate and hydrochloric acid (HCl), followed by solidification using freeze drying. Through central composite experimental design (CCD) with 13 runs, the study optimized the formulation, examining the effects of HCl, silica, and cellulose content on the resulting cellulose aerogel and silica-cellulose aerogel. The developed empirical model signifies that it can be seen that the single component factors and their linear interaction significantly influence the sound absorption coefficient (SAC) positively, while higher-order interaction terms of quadratic and cubic interactions contribute negatively on SAC values. Response surface plots revealed that optimized HCl, silica, and cellulose content significantly influenced SAC. The embedded silica in the aerogel improved physical, chemical, mechanical, and thermal stability, enhancing its functionality for the acoustic insulation material. The optimized formulation was found by applying 34.35%w cellulose, 2 M HCl, and 5.26% silica, achieved the highest SAC of 0.9837 with a desirability score of one. Validation experiment confirmed a SAC of 0.9785, suggesting the model accurately predicts actual SAC values (error = 0.53%), affirming the potential of the silica-cellulose aerogel as an effective acoustic insulation material. [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]

A microperforated panel (MPP) often suffers from a limited absorption bandwidth and poor sound absorption in the low frequency range. The present study aimed to propose an MPP-panel-type resonator (PR) compound structure that can simultaneously widen the half-absorption bandwidth and improve the poor absorption at the low frequency range. A vibroacoustic model is developed and compared to finite element simulations to demonstrate its accuracy. The vibration characteristics of the compound structure and the surface impedance are analyzed to reveal the mechanism forming the absorption characteristics of an MPP-PR structure. It is found that backing an MPP with a panel-type resonator (PR) creates a low frequency absorption peak but followed by an absorption valley that decreases the absorption bandwidth. The analysis showed that the phase difference between velocity of air particles in perforation and the PR panel velocity dictates the absorption characteristics associated with the PR resonance. A multi-resonators panel-type resonator (MPR) is found that the absorption valley associated with the host panel resonance splits into multiple smaller amplitude absorption dips improving absorption performance. The proposed MPP-MPR compound structure demonstrated a relatively wider half-absorption bandwidth with improved low frequency absorption. [ABSTRACT FROM AUTHOR]

Multifunctional materials are highly desired for the design of compact engineering structures, such as aircraft where weight reduction, sound absorption, load carrying, and energy harvesting are key considerations. However, design challenge remains in the balance of multiple functionalities. Here, we combine the sandwich structure with the neck-embedded cavities to design a cellular metamaterial having sound-absorption, compression/impact resistance and energy harvesting functionalities. For sound absorption, an autoencoder-like neural network is constructed to generate an instant design, after which a probabilistic module is inserted to optimize it by searching solutions in a slightly expanded design space. This inverse design has been experimentally validated, showing broadband sound absorption from 400 to 650 Hz merely with nine ultra-thin resonators. Beyond serving as absorber, the resonant cavities, once installed with well-tuned piezoelectric membranes, can gather broadband acoustic energy at low frequencies. Additionally, the cellular metamaterial inherits the excellent mechanical properties of honeycomb cores, having a low density of 0.64 g/cm3 yet displaying a high yield strength (21.2 MPa) in out-of-plane compression test and a superior energy absorption capability (8.6 J/cm3) in low-velocity impact tests. This work presents an effective approach to design lightweight metamaterials of superior mechanical and acoustic functionalities highly sought-after in practical engineering.

One of the uses of Helmholtz resonators is as sound absorbers for room acoustic applications, especially for the low frequency range. Their efficiency is centered around their resonance frequency which mainly depends on elements of their geometry such as the resonator volume and neck dimensions. Incorporating additional necks on the body of a Helmholtz resonator (depending on whether they are open or closed) has been found to alter the resulting resonance frequency. For this study, tunable Helmholtz resonators to multiple resonance frequencies, are proposed and investigated utilizing additional necks. The resonance frequencies of various multi-neck Helmholtz resonators are first modeled with the use of the finite element method (FEM), then calculated with the use of an analytical approach and the results of the two approaches are finally compared. The results of this study show that Helmholtz resonators with multiple resonances at desired frequencies are achievable with the use of additional necks, while FEM and analytical methods can be used for the estimation of the resonance frequencies. Analytical and FEM approach results show a good agreement in cases of small number of additional necks, while the increasing differences in cases of higher neck additions, were attributed to the change in effective length of the necks as demonstrated by FEM. The proposed approach can be useful for tunable sound absorbers for room acoustics applications according to the needs of a space. Also, this approach can be applied in cases of additional tunable air resonances of acoustic instruments (e.g., string instruments). [ABSTRACT FROM AUTHOR]

In this study, an experimental procedure is developed in order to elaborate an acoustic absorber based on date palm fibers. This kind of material has a double interest: The first is due to its great importance in environment protection by recycling wastes; the second is the fact that these materials are used in the treatment of a second environmental problem which is the noise pollution by reducing it. In this work, wastes from the date palm were used to manufacture an acoustic absorber. In fact, agriculture wastes are increasingly used in the acoustic field. In this work, wastes from date palm were used to manufacture acoustic absorbers. To achieve this goal, date palm waste is acoustically evaluated by an experimental characterization based on measuring the acoustic absorption coefficient of the studied material in the frequency band [0-4000 Hz] using the Kundt tube. The obtained results are encouraging with acoustic absorbent coefficient reaching 0.9 in some frequencies. [ABSTRACT FROM AUTHOR]

Today, the majority of the commercial electrified vehicles use Interior Permanent Magnet Motors (IPMSMs) for propulsion. An IPMSM can deliver high starting torque and high efficiency in the low- and medium-speed operation, which makes it attractive for propulsion applications. However, IPMSMs generally utilize rare-earth permanent magnets and there are growing concerns about the price volatility and supply chain of these materials. Switched Reluctance Motors (SRMs) can potentially replace IPMSMs in various applications including propulsion. An SRM has a simple and low-cost construction, and it can provide reliable operation at high-speed and high-temperature conditions. Compared to IPMSMs, SRMs radiate considerably higher acoustic noise, which have historically hindered their widespread acceptance. Various approaches to mitigate acoustic noise and vibration at the source level have already been explored in the literarture by improving the electromagnetic design and current control. This paper explores multiple noise and vibration mitigation methods that can be applied at the transmission stage. First, the noise comparison between the Internal Combustion Engine (ICE) and SRMs is discussed, and then the methods used for ICEs are presented for their applicability to SRMs.

In this study, an optimized dynamic vibration absorber (DVA) was applied to improve the sound absorption performance and simultaneously attenuate vibrations and structure-borne noise at multiple frequencies. As the size parameters and eigenfrequencies of a DVA demonstrate a nonlinear relationship, tuning the eigenfrequencies by heuristically modifying the geometric parameters of the DVA is difficult. To avoid this intricate process, geometric parameters were optimized using a gradient method solver to tune the eigenvalues of the DVA to the target frequencies. Post-processing was performed for the sake of manufacturing. To confirm the validity of the performance of the sound absorption and vibration/noise attenuation, impedance tube experiments and impact experiments were conducted using the manufactured DVA. The impedance tube experiment verified that the values of the sound absorption coefficient increased with the application of the DVA. From the impact experiments, it was verified that the values of the frequency response function and sound pressure level decreased with the application of the DVA. The present study validates the notion that optimized DVAs improve the sound absorption performance of structures and simultaneously reduce vibrations and sound pressures at multiple desired frequencies. [ABSTRACT FROM AUTHOR]

Natural fiber materials are often favored as a replacement to synthetically made sound absorbers due to its comparable sound absorption properties as well as in the effort to reduce the environmental issue. However, studies are weighted heavily on evaluating the performance of a single or composite fiber and only few studies focused on the multi-layer fiber. In this paper, the sound absorption performance of multi-layer coir and kapok fibers is presented. The kapok fiber is used as the assisting layer element for the coir fiber. A combination of coir fibers with different layer thicknesses and with additional 2 mm thick kapok layer shows significant improvement of the absorption bandwidth. It is also found that the kapok fiber layer placed in between the coir fiber layers show the best improvement of the absorption frequency bandwidth. [ABSTRACT FROM AUTHOR]

Natural fiber materials are often favored as a replacement to synthetically made sound absorbers due to its comparable sound absorption properties as well as in the effort to reduce the environmental issue. However, studies are weighted heavily on evaluating the performance of a single or composite fiber and only few studies focused on the multi-layer fiber. In this paper, the sound absorption performance of multi-layer coir and kapok fibers is presented. The kapok fiber is used as the assisting layer element for the coir fiber. A combination of coir fibers with different layer thicknesses and with additional 2 mm thick kapok layer shows significant improvement of the absorption bandwidth. It is also found that the kapok fiber layer placed in between the coir fiber layers show the best improvement of the absorption frequency bandwidth.

One of the uses of Helmholtz resonators is as sound absorbers for room acoustic applications, especially for the low frequency range. Their efficiency is centered around their resonance frequency which mainly depends on elements of their geometry such as the resonator volume and neck dimensions. Incorporating additional necks on the body of a Helmholtz resonator (depending on whether they are open or closed) has been found to alter the resulting resonance frequency. For this study, tunable Helmholtz resonators to multiple resonance frequencies, are proposed and investigated utilizing additional necks. The resonance frequencies of various multi-neck Helmholtz resonators are first modeled with the use of the finite element method (FEM), then calculated with the use of an analytical approach and the results of the two approaches are finally compared. The results of this study show that Helmholtz resonators with multiple resonances at desired frequencies are achievable with the use of additional necks, while FEM and analytical methods can be used for the estimation of the resonance frequencies. Analytical and FEM approach results show a good agreement in cases of small number of additional necks, while the increasing differences in cases of higher neck additions, were attributed to the change in effective length of the necks as demonstrated by FEM. The proposed approach can be useful for tunable sound absorbers for room acoustics applications according to the needs of a space. Also, this approach can be applied in cases of additional tunable air resonances of acoustic instruments (e.g., string instruments).

Taking as bioinspiration the remarkable acoustic absorption properties of moth wings, we develop a simple analytical model that describes the interaction between acoustic pressure fields, and thin elastic plates incorporating resonant sub-structures. The moth wing is an exemplar of a natural acoustic metamaterial; the wings are deeply subwavelength in thickness at the frequencies of interest, the absorption is broadband and the tiny scales resonate on the moth wing acting in concert. The simplified model incorporates only the essential physics and the scales are idealized to flat rigid rectangular plates coupled via a spring to an elastic plate that forms the wing; all the components are deep-subwavelength at desired frequencies. Based on Fourier analysis, complemented by phenomenological modelling, our theory shows excellent agreement with simulation mimicking the moth-wing structure. Moth wings operate as broadband sound absorbers employing a range of scale sizes. We demonstrate that a random distribution of scale sizes generates a broadband absorption spectrum. To further illustrate the potential of the model, we design a deeply sub-wavelength acoustic counterpart of electromagnetically induced reflectance. This article is part of the theme issue 'Wave generation and transmission in multi-scale complex media and structured metamaterials (part 2)'. [ABSTRACT FROM AUTHOR]

Low-frequency noise generated by ventilation systems propagates into indoor environment through ducts. In this study, a novel micro-perforated panel (MPP) muffler, with the absence of porous material in cavities, backed radially with parallel-arranged cavities of different depths (PCD) is proposed. Based on the two-load method, numerical simulations and experiments are conducted to evaluate sound transmission loss (TL) and sound wave radiating coefficients (SWRC) of the PCD-MPP muffler. Influences of muffler parameters on TL and SWRC are then discussed by numerical simulation. A wideband compound PCD-MPP muffler with high performance is finally proposed. Results show that the numerical model provides accurate prediction on acoustic properties of the PCD-MPP muffler by comparing with the results by experiments. Numerical results demonstrate that, the coupling effect of absorption resonance occurred by cavities of different depths contributes to the wideband silencing performance. TL, absorptance and reflectance improve when enhancing the length of the muffler, while TL and absorptance move to lower frequencies with the increase of cavity depth. In addition, TL of the compound PCD-MPP muffler is higher than 20 dB within the frequency range of 700–1600 Hz, with a maximum TL of 42.5 dB. The present work also highlights the higher TL and absorptance than other types of MPP mufflers, and silencing performance can be customized by adjusting the perforation parameters and cavity depths, which may provide a valuable reference in ventilation noise reduction with the absence of porous material. • Acoustic properties of a micro-perforated muffler with parallel-arranged cavities of different depths are investigated. • The coupling effect of absorption resonance occurred by cavities of different depths contributes to the wideband silencing performance of the muffler. • Sound transmission loss of a compound PCD-MPP muffler higher than 20 dB within the frequency range of 700–1600 Hz is found. [ABSTRACT FROM AUTHOR]

Jute and hollow conjugated polyester fiber (HCP)-reinforced nonwoven composites were designed by Box and Behnken model approach. Its thermal properties such as thermal conductivity, thermal resistance, thermal transmittance, and thermal diffusivity were evaluated. From the test results of response surface method (RSM), the weight of composite material maintained as 3280 g/m2 with the proportion of jute/HCP fiber in the ratio of 50/50%, 60/40%, and 70/30% played a significant role in insulation and mechanical properties. Various needling density maintained on the composite material such as 300, 150, and 75 punches/cm2 contributes to the role of significant impacts on the research process. The optimum of 150 needling density with the proportion of 50/50 Jute and HCP fiber was selected to produce four kinds of composite structures. Its thermal, sound, electrical insulation values and limiting oxygen values were analyzed and suggested for building environments.

The paper presents the sound absorption coefficient of acoustic absorbers fabricated from natural durian husk fibers, which are currently still considered as agricultural wastes, especially in Malaysia. Samples were fabricated with different fiber densities and thicknesses and the sound absorption performance was measured using the impedance tube method. The results reveal that the durian husk fibers can have absorption coefficient of more than 0.5 above 1 kHz for a minimum thick sample of 20 mm and with minimum density of 160 kg/m3. The optimised macroscopic parameters for various densities were calculated using the inverse method employing the well-known Johnson-Champoux-Allard (JCA) model for porous material. [ABSTRACT FROM AUTHOR]

• Ventilated sound absorber is designed based on Helmholtz acoustic metamaterials. • Sound absorption is up to 98.6% at 687 Hz and maximum TL reaches 24.9 dB at 724 Hz. • Broadband absorption at 453–697 Hz is achieved by stacking four layers. In this work, we present a novel Helmholtz acoustic metamaterial (HAM) designed for ventilated low-frequency sound absorption. Based on COMSOL Multiphysics simulation results, a double-layer HAM achieves near-perfect sound absorption at approximately 687 Hz. Transmission loss within the range of 661 to 1074 Hz exceeds 10 dB. Both experimental measurements and theoretical calculations using the transfer matrix method validate the simulation results. A four-layer HAM enables effective broadband sound absorption across the range of 454 to 698 Hz. Consequently, our study presents a potential breakthrough in achieving effective low-frequency sound absorption across a broad spectrum in ventilated environments. [ABSTRACT FROM AUTHOR]

One of the dominant causes of noise pollution is the disruptive honking while traveling on a road and is particularly problematic as well as resulting in irritating sound. This results in many effects that need to be addressed taking into consideration stress, depression, mental and physical illnesses. The proposed system comprises of an embedded module that will be of great benefit to citizens of society adjacent to heavily populated roads, students studying in schools near crowded roads, patients admitted to roadside hospitals, etc. Noise Armor's alternative approach needs immense expenditure and personnel for both repair and surveillance as there is the need for expensive noise absorbers. By developing a real-time (smart) honking device that automatically allows vehicles on the road to reduce the volume of horn near conscious areas such as schools , hospitals, old-age homes, baby care centers and other organizations that are adversely damaged, our research aims to develop an effective solution for excessive honking.

• An analytical model is presented for predicting the absorption performance of the micro-perforated panel absorber (MPA) with parallel-arranged backing-cavities at different depths (PCD) in a diffuse field. • An enhanced parametric method is proposed to improve random incident absorption performance of the PCD-MPA based on thorough discussions on key parameters, such as cavity depth, MPP perforation diameter, panel thickness and perforation rate. • The PCD-MPAs can have an average random incidence absorption coefficient beyond 0.8 within 490 Hz-4000 Hz by precise parameter modulations. This study investigates sound absorption performance of a kind of wideband micro-perforated panel (MPP) absorbers with parallel-arranged backing cavities at different depths in a diffuse field. An analytical prediction model is presented for evaluating the random incidence absorption coefficient of this type of absorbers, and it exhibits excellent agreement with finite element model and experimental measurements. Then, numerical cases were established using this model to investigate the absorption characteristics of this type of absorbers under random incidence, considering the influence of cavity depths and MPP parameters. Results show that the cavity depth and MPP perforation diameter in this absorber are primary influencing parameters controlling the overall sound absorption performance. Cavities at different depths introduce multiple resonant absorption peaks, while the perforation diameter determines matching conditions between the MPP and air impedance, which simultaneously broaden the absorption bandwidth of this MPP absorber. Although individually adjusting the panel thickness and perforation rate has a smaller impact compared to the perforation diameter, the interaction between these two parameters has significant influences across the entire absorption frequency band. [ABSTRACT FROM AUTHOR]

• MPP backed by panel resonator with a Helmholtz neck and multi-frequency resonators. • Helmholtz and structural resonance of PRH creates two low frequency absorption peaks. • PRH Helmholtz resonance enhances viscous loss in PRH neck creating a higher peak. • Resonators on PRHM improve the absorption dip due to its structural resonances. • An increase in neck radius deteriorates the absorption peak of MPP perforation. The present study is aimed to develop a compact sound absorber structure that can simultaneously improve low frequency sound absorption performance and broaden the half-absorption bandwidth of a microperforated panel (MPP)-type structure. A novel structure is proposed by backing an MPP with a panel-type resonator incorporating a Helmholtz neck and multi-frequency resonators (PRHM) to form an MPP-PRHM compound sound absorbing structure. A fully coupled vibroacoustic model based on the modal coupling method is developed to predict the sound absorption characteristics of the proposed sound absorbing structure. The accuracy of the model is verified against the finite element model and experimental result. The mechanism forming the sound absorption characteristics of the compound structure is investigated, allowing the widening of bandwidth in the low frequency range. The experimental and numerical results have shown that the proposed PRHM structure is able to create two distinctive low frequency sound absorption peaks respectively attributed to Helmholtz resonance (due to the relatively in-phase relationship between air mass in PRHM neck and air particle in MPP perforation, which simultaneously enhanced the viscous losses in MPP perforation and PRHM neck) and structural resonance of PRHM (due to the relatively in-phase relationship with the velocity of air particle in MPP perforation). The experimental results demonstrated that a relatively wide bandwidth of 362 Hz was observed and the averaged absorption coefficient was improved by 650 % in the very low frequency range of 100 Hz to 150 Hz as compared to a single MPP structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel metamaterial combining sound absorption, energy harvesting and mechanical strength is proposed. • The inverse design problem of acoustic absorption was solved using neural network modeling. • The proposed structure achieves quasi-perfect sound absorption and energy harvesting in the 2/3 octave. • The proposed structure has a mechanical load-bearing capacity comparable to titanium alloy foams. Multifunctional materials are highly desired for the design of compact engineering structures, such as aircraft where weight reduction, sound absorption, load carrying, and energy harvesting are key considerations. However, design challenge remains in the balance of multiple functionalities. Here, we combine the sandwich structure with the neck-embedded cavities to design a cellular metamaterial having sound-absorption, compression/impact resistance and energy harvesting functionalities. For sound absorption, an autoencoder-like neural network is constructed to generate an instant design, after which a probabilistic module is inserted to optimize it by searching solutions in a slightly expanded design space. This inverse design has been experimentally validated, showing broadband sound absorption from 400 to 650 Hz merely with nine ultra-thin resonators. Beyond serving as absorber, the resonant cavities, once installed with well-tuned piezoelectric membranes, can gather broadband acoustic energy at low frequencies. Additionally, the cellular metamaterial inherits the excellent mechanical properties of honeycomb cores, having a low density of 0.64 g/cm3 yet displaying a high yield strength (21.2 MPa) in out-of-plane compression test and a superior energy absorption capability (8.6 J/cm3) in low-velocity impact tests. This work presents an effective approach to design lightweight metamaterials of superior mechanical and acoustic functionalities highly sought-after in practical engineering. [ABSTRACT FROM AUTHOR]

Jute and hollow conjugated polyester fiber (HCP)-reinforced nonwoven composites were designed by Box and Behnken model approach. Its thermal properties such as thermal conductivity, thermal resistance, thermal transmittance, and thermal diffusivity were evaluated. From the test results of response surface method (RSM), the weight of composite material maintained as 3280 g/m2 with the proportion of jute/HCP fiber in the ratio of 50/50%, 60/40%, and 70/30% played a significant role in insulation and mechanical properties. Various needling density maintained on the composite material such as 300, 150, and 75 punches/cm2 contributes to the role of significant impacts on the research process. The optimum of 150 needling density with the proportion of 50/50 Jute and HCP fiber was selected to produce four kinds of composite structures. Its thermal, sound, electrical insulation values and limiting oxygen values were analyzed and suggested for building environments. [ABSTRACT FROM AUTHOR]

The paper presents the sound absorption coefficient of acoustic absorbers fabricated from natural durian husk fibers, which are currently still considered as agricultural wastes, especially in Malaysia. Samples were fabricated with different fiber densities and thicknesses and the sound absorption performance was measured using the impedance tube method. The results reveal that the durian husk fibers can have absorption coefficient of more than 0.5 above 1 kHz for a minimum thick sample of 20 mm and with minimum density of 160 kg/m3. The optimised macroscopic parameters for various densities were calculated using the inverse method employing the well-known Johnson-Champoux-Allard (JCA) model for porous material.

Rigid polyurethane (PU) foam has been widely used for a multitude of applications, primarily as thermal insulation materials. With the exhortation of "green chemistry" and sustainable development, bio-based polyols are being introduced to replace the conventional petrochemical-based polyols. Hence, the use of 9, 10-dihydroxystearic acid (DHSA), derived from palm oil-based oleic acid, as a feedstock for rigid PU foam production was explored. In this study, the effects of the weight ratio percentage of DHSA on the characteristics of the resulting PU foams were investigated. The DHSA was utilized as a co-polyol to synthesize rigid PU foams by reacting with the diisocyanate compound, blowing agent and additives. In general, the effects of DHSA in the formulations on the quality of the foams were relatively similar to PU formulation without the addition of DHSA, particularly in terms of density, compressive strength, thermal conductivity, cell structure and dimensional stability. The remarkable effect of DHSA addition in the PU formulations was observed through the open PU cell content using a pycnometer and scanning electron microscopy (SEM). The addition of DHSA has the ability to open the cell structure of the resulting rigid PU foams. The open cell content increased notably from 6.83% to 90.99% when the DHSA content was increased from 0% to 50%. The ability of DHSA to produce open-cell rigid PU foam widens up the potential applications of rigid PU foams as sound absorption materials. • Incorporation of DHSA as a co-polyol significantly affects the PU cell matrix. • DHSA has a remarkable effect on enhancing water blown rigid PU cell content. • Open cell content of rigid PU foam notably increased from 6.83% to 90.99%. • No substantial changes to other thermal and structural properties. • Expands DHSA's potential as a sound absorption material. [ABSTRACT FROM AUTHOR]

• The mMPR improves the low frequency sound absorption and bandwidth of MPP. • Absorption characteristics are tuned by varying the resonators' coupling strength. • The mass ratio of resonators can be optimized to balance the performance and mass. • A circular resonator arrangement reduces the number of resonators required. The present work is aimed to broaden the half-absorption bandwidth and improve low frequency sound absorption of a microperforated panel (MPP) by investigating the effects of tuned multi-frequency resonators attached to a flexible panel (i.e. a multi-frequency panel resonator – mMPR), as the backing of MPP so to create a compact sound absorber. The effects of different configurations of multi-frequency resonators were investigated through theoretical and experimental studies. The results demonstrated that the coupling strength among MPP, the panel and resonators at their resonant frequencies had a significant contribution to shaping the sound absorption characteristics, that were particularly influenced by the placement of resonators, quantity of resonators and the configuration of resonators, and these parameters could be tuned to extend the half-absorption bandwidth as well as the low frequency sound absorption coefficient. It was found that there was minimal mass of resonators for them to be effective in improving low frequency sound absorption characteristics and an optimal mass ratio demonstrated a better sound absorption performance in terms of bandwidth and absorption coefficient without an unnecessary increase of structural mass. A circular arrangement of multi-resonators was found to be beneficial in achieving relatively similar MPP-panel-resonator coupling of multi-resonators so to achieve a more uniform sound absorption coefficient within the particular bandwidth. It was experimentally demonstrated that a 200 mm × 200 mm MPP with the backing of a panel and circularly-arranged multi-frequency resonators of only 2 % mass ratio, could achieve a wide low-frequency half-bandwidth from 160 Hz to 550 Hz with the averaged absorption coefficient improved by 18 % to 0.72 over 150 Hz to 260 Hz, in the frequency range within which the absorption valley was previously located. [ABSTRACT FROM AUTHOR]

One of the dominant causes of noise pollution is the disruptive honking while traveling on a road and is particularly problematic as well as resulting in irritating sound. This results in many effects that need to be addressed taking into consideration stress, depression, mental and physical illnesses. The proposed system comprises of an embedded module that will be of great benefit to citizens of society adjacent to heavily populated roads, students studying in schools near crowded roads, patients admitted to roadside hospitals, etc. Noise Armor's alternative approach needs immense expenditure and personnel for both repair and surveillance as there is the need for expensive noise absorbers. By developing a real-time (smart) honking device that automatically allows vehicles on the road to reduce the volume of horn near conscious areas such as schools, hospitals, old-age homes, baby care centers and other organizations that are adversely damaged, our research aims to develop an effective solution for excessive honking. [ABSTRACT FROM AUTHOR]

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 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.

Noise is an unpleasant and unwanted sound for every human and animal. Noise can also interfere with one's communication and concentration. Nowadays, many sound absorber panels have been produced to overcome arising noise issues. A sound absorber panel works to absorb sound either in low or high frequencies. Usually, in Malaysia, synthetic materials are used for producing sound absorber panels. Glass fibre and minerals are the most common examples of synthetic materials used to produce sound absorber panels. In particular, glass fibre has a good result in sound absorption. However, it also brings negative effects to humans' health, such as skin inflammation, redness, cough, breathlessness, and others. Therefore, for this study, natural materials from recycling have been used to produce sound absorber panels, which are egg trays, papers, and plastics. The panels were tested with the impedance tube test to obtain the sound absorption coefficient (SAC) and the noise reduction coefficient (NRC). The values of sound absorption of plastics and papers are good at the frequency of 2,000 Hz (i.e., 0.75 and 0.74, respectively). However, the value of SAC for papers increased to 0.86, compared to the value of SAC for plastics that decreased to 0.67. Furthermore, the value of NRC for papers is better than plastics. Next, the reverberation room test was also performed on the panels to obtain the echo time value (RT) in the current unit. The comparison of echo time for this test is between the values for a room with and without panels. The results from the test showed that the value of echo time for the room with the panels is better than the room without panels. [ABSTRACT FROM AUTHOR]

Natural fibres are attractive as the raw material for developing sound absorber, as they are green, eco-friendly, and health friendly. In this paper, pineapple leaf fibre/epoxy composite is considered in sound absorber development where several values of mechanical pressures were introduced during the fabrication of absorber composite. The results show that the composite can absorb incoming sound wave, where sound absorption coefficients αn > 0:5 are pronounced at mid and high frequencies. It is also found that 23.15 kN/m² mechanical pressure in composite fabrication is preferred, while higher pressure leads to solid panel rather than sound absorber so that the absorption capability reduces. To extend the absorption towards lower frequency, the composite absorber requires thickness higher than 3 cm, while a thinner absorber is only effective at 1 kHz and above. Additionally, it is confirmed that the Delany- Bazley formulation fails to predict associated absorption behavior of pineapple leaf fibre-based absorber. Meanwhile, a modified Delany-Bazley model discussed in this paper is more useful. It is expected that the model can assist further development of the pineapple leaf composite sound absorber. [ABSTRACT FROM AUTHOR]

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.

Used cigarette butts represent a major and problematic form of waste, due to their abundance, toxicity, and durability. Moreover, the few proposals for their recycling are clearly insufficient, and new ones are welcome. For a new proposal regarding the reuse of used cigarette butts as acoustical absorbers in building construction, previous conditioning of the used butts is performed. This conditioning includes the elimination of moisture and toxic products accumulated in the filter of the cigarettes. Thus, in this work, the moisture content effect in acoustical absorption was analyzed, and a proposal for elimination is made. Moreover, a chemical cleaning procedure is proposed, and its influence on the acoustical behavior of the samples was also analyzed.

Highlights • Acoustic reactance of shunted diaphragm can be tuned for broadband absorption. • Shunt circuit introduces negative acoustic mass and stiffness. • Positive acoustic mass and stiffness of the diaphragm can be canceled in broadband. • A design procedure is proposed for uniform sound absorption over a broadband. • Experimental results agree with theoretical predictions. Abstract Recent study shows that a suspended diaphragm may function as an efficient sound absorbing device if a properly designed shunt electrical circuit is connected. The sound absorption performance mostly depends on the acoustic impedance of the suspended diaphragm, which is strongly coupled to the electrical impedance of the shunt circuit via the electromechanical coupling induced by a moving-coil in a DC magnetic field. This study concerns tuning the acoustic impedance of the diaphragm with shunt electrical circuit, or a 'shunted-electromechanical-diaphragm' (SEMD). A lumped parameter model is used to analyze the effect of each circuit element on the acoustic impedance of the SEMD. Results show that sound absorption coefficients of the SEMD can be adjusted at low, middle and high frequencies by tuning the capacitance, resistance and inductance of the circuit, respectively. Further analyses show that the shunt circuit introduces negative acoustic mass and reactance at middle frequencies, which can be used to cancel the acoustic mass and stiffness of the diaphragm, resulting in a vanishing reactance of the diaphragm in a broad frequency band. By tuning the resistance of the circuit to modify the acoustic resistance of the SEMD, the sound absorption coefficient of the SEMD can be adjusted to any specific level. A prototype SEMD is tested with different sets of circuit parameters using an acoustic impedance tube. Agreement is observed between the experimental results and theoretical predictions. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a type of wide band micro-perforated panel (MPP) absorber with an array of parallel arranged sub-cavities at different depths based on the quadratic residue sequence (QRS). An analytical prediction model is proposed for evaluating the normal incidence sound absorption coefficients of this type of MPP absorber in its design stage. Next, simulations with a finite element procedure and experimental measurements were conducted to provide cross-validations of the proposed analytical model and to facilitate the discussions on the sound absorption performance of this type of MPP absorber. The results show that the analytical model provides simple yet accurate prediction on the sound absorption performance of this MPP absorber; the absorption bandwidth of this type of MPP absorber can be significantly enhanced through the fine design on the sub-cavity depth sequence. It is also shown that the normal incidence absorption coefficient of this type of MPP absorber can remain beyond 0.5 in the frequency range of 450–3500 Hz, with the maximum value of 0.98. [ABSTRACT FROM AUTHOR]

Abstract: Environmentally friendly insulation materials based on renewable resources come more and more to the fore, replacing less sustainable materials, such as mineral wool or polystyrene foams. In this study, we present a straight-forward avenue to a highly porous cellulose II powder for application as sound absorber. The material was prepared from a lyocell-derived cellulose II gel by a freeze-drying approach. The powder has a porosity of 98% and a specific surface area of 177 m2/g. It featured a sound absorption maximum in the frequency range of one-third-octave bands of 400-800 Hz (65 mm thickness) and was significantly more efficient as acoustic insulation material than commercial alternatives. The high performance makes the cellulose II sound absorber especially attractive for a use in loudspeakers for small electronics.Graphical Abstract: [ABSTRACT FROM AUTHOR]

Cleanable and chemically resistant Sound Absorbers In modern energy‐efficient heating systems with low waste gas temperature, the water vapor in the exhaust flow generated during the combustion process condenses and forms, in connection with the waste gas, corrosive condensate. Thus, the absorber material for exhaust silencers must combine chemical resistance with acoustic properties. In cooperation with a manufacturer of exhaust silencers as well as a manufacturer of PTFE products, Fraunhofer‐IBP developed a sound absorber comprising of the integral properties of condensate resistance, sound absorption in the range of medium and high frequencies together with cleanability and reusability features. This paper presents the results of the research project carried out for this purpose [1, 2]. The approach for a sustainable solution to this acoustic, ecological and finally economic problem is presented that starts from the selection of materials. On a laboratory scale, patches of absorbers were created and their chemical and acoustic properties determined. The input data for the model of an absorber are derived using a material model. These calculations are compared with measurements and are discussed. Possibilities of cleanability and recycling of the sound absorber are illustrated with an example of a silencer for heating systems. The presented project approach for heating systems with cleanable and chemically resistant sound absorbers has a considerable potential for transfer in other sectors such as the medical and food industry. [ABSTRACT FROM AUTHOR]

For building applications, woven fabrics have been widely used as finishing elements of room interior but not in particular aimed for sound absorbers. Considering the micro perforation of the woven fabrics, they should have potential to be used as micro-perforated panel (MPP) absorbers; some measurement results indicated such absorption ability. Hence, it is of importance to have a sound absorption model of the woven fabrics to enable us predicting their sound absorption characteristic that is beneficial in engineering design phase. Treating the woven fabric as a rigid frame, a fluid equivalent model is employed based on the formulation of Johnson-Champoux-Allard (JCA). The model obtained is then validated by measurement results where three kinds of commercially available woven fabrics are evaluated by considering their perforation properties. It is found that the model can reasonably predict their sound absorption coefficients. However, the presence of perturbations in pores give rise to inaccuracy of resistive component of the predicted surface impedance. The use of measured static flow resistive and corrected viscous length in the calculations are useful to cope with such a situation. Otherwise, the use of an optimized simple model as a function of flow resistivity is also applicable for this case. [ABSTRACT FROM AUTHOR]