Acoustic Absorption Performance Research Articles

Experimental study on durability and acoustic absorption performance of biomass geopolymer-based insulation materials

The biomass-based insulation materials have been identified as low-embodied energy materials that reduce energy use and CO2 emissions, particularly in construction. However, as satisfactory insulation material, it must also ensure durability under changing climate conditions. This work investigated the effect of freeze–thaw, wet-dry and cool-heat cycles on the appearance, compressive strength and thermal conductivity of biomass (wheat straw, rice husk and sawdust) geopolymer-based insulation materials. The results showed that, these samples had a good skeleton structure. By contrast, the wheat straw geopolymer-based (WG) samples showed better durability with compressive strength losses of 19.4 %, 1.3 % and 2.6 % in the three modes, respectively. In addition, the WG samples had high sound absorption performance (0.71 at 1028 Hz). This study provides a more systematic analysis of the durability and sound absorption of biomass geopolymer-based insulation materials, which are crucial for their practical application.

Sound Absorption Improvement in Porous Ferroelectret Polyethylene with Effective Piezoelectric Mechanism.

Airborne sound absorption in porous materials involves complex mechanisms of converting mechanical acoustic energy into heat. In this work, the effective piezoelectric properties of polyethylene ferroelectret foams on sound absorption were investigated by comparable samples with and without the piezoelectric response. Corona poling and thermal annealing treatments were applied to the samples in order to enable and remove the piezoelectric property, respectively, while the microstructure and the mechanical properties remained substantially unchanged. The effective piezoelectric properties and airborne sound absorption coefficients of the polyethylene foam samples before and after material treatments were measured and analyzed. Our experimental results and theoretical analysis showed that the open-cell ferroelectret polymer foam with an effective piezoelectric property provides an additional electromechanical energy conversion mechanism to enhance the airborne acoustic absorption performance.

Improving the sound absorption of a highly deformable nanocomposite foam based on ethylene-propylene-diene-monomer (EPDM) infused with multi-walled carbon nanotubes (MWCNTs) to absorb low-frequency waves

In this study, an elastomeric nanocomposite foam was fabricated on the basis of ethylene-propylene-diene-monomer (EPDM) rubber filled with multi-walled carbon nanotubes (MWCNTs) to absorb low-frequency acoustic waves. The effects of adding MWCNTs on the cellular structure, viscoelastic behavior, and sound absorption characteristics were studied. Samples containing slight loadings of MWCNTs with increased damping factor and decreased pore sizes showed high viscose motions with superior sound absorption characteristic. The results revealed that the nanocomposite foam sample with only 0.1 phr (parts per hundred rubber) MWCNTs offered the best sound absorption coefficient with a sharp peak (0.98) within the frequency range of 500–1000 Hz. This can be justified by the dissipation of acoustic energy through vibration and deformation of the flexible cell walls due to the highly elastic matrix, high porosity and airflow resistivity of the fine partially reticulated cellular structure. With further MWCNTs loadings beyond the optimum level, the cell size was reduced dramatically, leading to reduction in porosity and excessive increase in air flow resistivity. These factors resulted in poor sound absorption feature. Moreover, the sound insulation performance of the nanocomposite foams was characterized, and it revealed the improving effect of MWCNTs, especially in high loading, by yielding a sound reduction index equal to 27–33 dB. According to the obtained results, the potential role of MWCNTs in the improvement of the acoustic absorption and sound insulation performance of EPDM foam can be inferred. Thus, a lightweight sound absorber with high resistance to harsh environmental conditions and excellent low-frequency sound absorption can be achieved by the infusion of nanoparticles.

Effect of silica aerogel on thermal insulation and acoustic absorption of geopolymer foam composites: The role of aerogel particle size

Silica aerogel features super thermal insulating and excellent sound absorption. Although the application of silica aerogel in building materials for energy saving has been investigated recently, the role of aerogel particle size on geopolymer foam render can be crucial but is rarely studied. In this research, four kinds of silica aerogel with different particle sizes (2–40 μm, 100–700 μm, 100–1200 μm, 700–4000 μm) are applied in geopolymer foam aerogel render (GFAR) to improve the thermal insulation and acoustic absorption performance. Results show that larger silica aerogel particles are more beneficial for acoustic absorption and thermal insulation while smaller aerogel particles are less effective for insulation properties. The microstructure of GFAR is investigated in-depth with micro-CT, optical microscopy and ultrasonic pulse velocity test. The mechanism of the improved performance of GFAR with different silica aerogel is explained. The optimal geopolymer foam aerogel render (GFAR) reaches a thermal conductivity of 0.133 W/(m·K), an average value of acoustic absorption coefficient of 0.51 with a density of 715.2 kg/m3. The result of this study is essential to enlighten the use of larger silica aerogel particles in geopolymer foam render, focusing on a low dosage of silica aerogel while utilize its full benefit.

Sustainable Sound Absorber from Nonwoven Fabric of Natural Biduri Fibers (Calotropis gigantea) with Polyester Binder

ABSTRACT In recent years, the research on sound-absorbing material has been directed to sustainable materials such as kenaf fiber and coconut coir fiber. Herein, this study offers the acoustic absorption performance of a novel natural fiber that is biduri fiber (Calotropis gigantea). Biduri fiber is a cellulose fiber that has a low density and a hollow structure along the fiber; hence, it has potential for sound absorption. Biduri fibers were thermally bonded with polyester fibers into nonwoven fabric. The sound absorption coefficient was measured using an impedance tube system, according to International Organization for Standardization (ISO) 10534-2:1998. The nonwoven fabric effectively absorbed high-frequency sound (2000–5000 Hz). The absorption coefficient reached 0.95 at 1600 Hz for nonwoven fabric with 95 wt.% of biduri fibers, a density of 25.9 kg m−3, and a thickness of 15–21 mm, which was better than glass wool and several natural fibers. Nonwoven fabrics with a thickness of 15–21 mm and ≥80 wt.% of biduri fibers met Indonesian National Standard 8443:2017 for high- and low-frequency sound silencers. Using the approximate relationship of the absorption coefficient between normal and random incidence sound, it was found that those nonwoven fabrics were categorized as class B sound-absorbing material according to ISO 11654:1997.

Experimental and Theoretical Investigation of the Acoustic Absorbers Fabricated from Natural Kenaf Fibres and Rice Husk

ABSTRACT Fabrication of acoustic absorbers from a combination of natural waste materials is a novel approach in the field of noise control. The aim of this study was to investigate the acoustic absorption performance of several sample sound absorbers made from natural kenaf fibers and rice husk. A new method of fabricating these sample sound absorbers was also employed to study the usability as well as performance improvement via the approach fibro-granular sound absorbers (FGSA). The values of acoustic absorption performance, porosity, and airflow resistivity of the samples were used in the experimental design. The Dunn and Davern (D-D) theoretical model as well as the reversed D-D model with Nelder–Mead Simplex were applied to evaluate the sound absorption as well. According to the obtained values, it was found that at low- and mid-frequency range, the sound absorption value of FGSA is higher than other acoustic materials. The FGSA (with a density of 150 kg/m3) increased noise reduction coefficient value by 31.25% compared to samples fabricated to the rice husk and also by 10.53% compared to kenaf. Based on comparison of the theoretical and experimental results, the model optimized with the Nelder–Mead method had higher accuracy and fitting in predicting the sound absorption values than the D-D model. Finally, it was found that improvement in the sound absorption performance of the natural acoustic absorbers might be achieved through adopting the approach of combining natural materials and fabrication of FGSA.

Multifunctional cellulose wood with effective acoustic absorption

Wood is a sustainable material with big sound absorption potential based on its natural porous characteristics. However, the acoustic absorption performance of wood is greatly limited by the low permeability of wood structures. In this study, we prepared a new type of cellulose wood (CW) using a two-step delignification process on a natural basswood matrix followed by supercritical CO2 drying. The as-prepared CW exhibited a high air permeability of 33.78 Darcys (increase of 496.82%) and superior porous characteristics compared to the natural basswood, resulting in a significant improvement in the acoustic absorption performance with an increased average absorption of 106.25% and maximum absorption of 126.32%. The dissipation of sound energy in the structure of CW is well predicted by a designed porous media model. Moreover, a high specific stiffness of 2.14 × 105 m2 s−2 and a diffuse reflectance of 97.02% across the visible light were demonstrated for the CW as well. Such a fascinating multifunctional wood-derived material may provide new insights into the development of efficient and sustainable acoustic absorbers for various applications.

A New Methodology Based on Cell-Wall Hole Analysis for the Structure-Acoustic Absorption Correlation on Polyurethane Foams.

Polyurethane foams with a hybrid structure between closed cell and open cell were fabricated and fully characterized. Sound absorption measurements were carried out in order to assess their acoustic performance at different frequency ranges. The cellular structure of these systems was studied in detail by defining some novel structural parameters that characterize the cell wall openings such as the average surface of holes (Sh), the number of holes (h), and the area percentage thereof (%HCW). Therefore, these parameters allow to analyze quantitatively the effect of different structural factors on the acoustic absorption performance. It has been found that the parameters under study have a remarkable influence on the normalized acoustic absorption coefficient at different frequency ranges. In particular, it has been demonstrated that increasing the surface of the holes and the percentage of holes in the cell walls allows increasing the acoustic absorption of these types of foams, a promising statement for developing highly efficient acoustic insulators. Additionally, we could determine that a suitable minimum value of hole surface to reach the highest sound dissipation for these samples exists.

Sound absorption and transmission loss properties of open-celled aluminum foams with stepwise relative density gradients

We investigate the acoustical properties of uncompressed and compressed open-celled aluminum metal foams fabricated using a directional solidification foaming process. We compressed the fabricated foams using a hydraulic press to different compression ratios and characterized the effect of compression on the cellular microstructure using microtomography and scanning electron microscopy. The static airflow resistances of the samples are measured and related to the observed microstructural changes. We measured the normal incidence acoustical properties using two- and four-microphone impedance tube methods and show that the compression substantially improves their sound absorption and transmission loss performance. We then stack individual disks with different compression ratios to create various stepwise relative density gradient configurations and show that stepwise gradients provide a significant improvement in properties as compared to the uncompressed sample. The effect of increasing and decreasing relative density gradients on the overall absorption and transmission loss behavior is characterized. Finally, we use an experimentally informed and validated transfer matrix method to predict the effect of various layer thicknesses and stacking sequences on the global acoustical properties. Our results show that open-celled metal foams with stepwise relative density gradients can be designed to provide tailored acoustic absorption performance while reducing the overall weight of the noise reduction package.

Acoustic and thermal performance of polypropylene nonwoven fabrics for insulation in buildings

Thermal insulation and acoustic absorption materials play a significant role in the characteristics of energy-saving and comfortable modern buildings. However, these two types of materials are generally developed independently. This study highlights the potential of lightweight polypropylene nonwoven fabrics for building applications and investigates how fabric structure affects thermal and acoustical properties. An industrial-scale melt spinning machine was used to produce polypropylene fibers at linear densities of 1.4 and 1.8 dtex. A semi-industrial needle punching line was used to fabricate nonwoven samples with three thicknesses (2, 3, and 4 cm), four porosities (0.83, 0.88, 0.93, and 0.96), and different in- and through-plane fiber orientation distributions. Totally, 31 samples were produced and their sound absorption and thermal insulation performance were assessed using impedance tube and guarded hot plate techniques. Accordingly, the thermal conductivity (Keff) and sound absorption average (SAA) of the samples could be engineered in the range of 0.0270–0.0404 W m-1K-1 and 0.270–0.675, respectively. It was found that fabrics made of finer fibers provide superior acoustic absorption and thermal insulation performance. An increase in through-plane fiber orientation adversely affected the thermal insulation properties, while its effect on acoustic properties depends on the porosity of samples and the sound frequency. The results of statistical analysis showed that in-plane fiber orientation does not have a significant effect on sound absorption behavior, except at frequencies between 4000 and 6300 Hz. It was also found that in-plane fiber orientation does not significantly affect the thermal insulation properties. The best acoustic and thermal performance were achieved by fibrous samples of layered structure made of 1.4 dtex fibers with a thickness of 3 cm (Keff =0.0278 W m-1K-1, SAA = 0.552) and 4 cm (Keff =0.0277 W m-1K-1, SAA = 0.675). The performance of these samples was compared with some commercial products and literature data. The results pointed to the superior acoustic and thermal properties of the designed samples.

Vegetal Fiber Additives in Mortars: Experimental Characterization of Thermal and Acoustic Properties

This paper investigates the influence of adding vegetal fibers on thermal and acoustic performance based on natural hydraulic lime. Mortar samples with 10% weight of vegetal fibers were fabricated adding water to obtain easily workable mortars with good consistency; their performance was compared to mortar samples without vegetal fibers. The fibers were of different types (rice husk, spelt bran, and Khorasan (turanicum) wheat chaff) and size (as-found and ground form). Thermal performance was measured with the Small Hot Box experimental apparatus. Thermal conductivity was reduced in the 1–11% range (with Khorasan wheat chaff and rice husk); no significant reduction was found with spelled bran in the mixture. When ground, fibers were characterized by both good thermal and acoustic absorption performance; a reduction of 6–22% in thermal conductivity λ was achieved with spelled bran (λ = 0.64 W/mK) and rice husks (λ = 0.53 W/mK), whereas the Khorasan wheat chaff had the highest sound absorption average index (0.38). However, the addition of fibers reduced sound insulation properties due to their low weight densities. This reduction was limited for rice husks (transmission loss value was only 2 dB lower than the reference).

Acoustic Performance and Flame Retardancy of Ammonium Polyphosphate/Diethyl Ethylphosphonate Rigid Polyurethane Foams.

Flame-retardant water-blown rigid polyurethane foams (RPUFs) modified by ammonium polyphosphate (APP) and diethyl ethylphosphonate (DEEP) were synthesized by a one-pot free-rising method. We performed scanning electron microscopy (SEM), compression strength tests, acoustic absorption measurements and thermogravimetric analysis, as well as limited oxygen index, vertical burning and cone calorimeter tests to investigate the mechanical properties, acoustic performance and flame retardancy of the foams. SEM confirmed that the open-cell structures of the foams were successfully constructed with the introduction of a cell-opening agent. Upon using 20 php APP, the average acoustic absorption coefficient of the foam reached 0.535 in an acoustic frequency range of 1500–5000 Hz. The results of thermogravimetric analysis demonstrated that the incorporation of APP and DEEP can effectively restrain mass loss of RPUFs during pyrolysis. In particular, the compressive strength of a foam composite containing 5 php APP and 15 php DEEP increased to 188.77 kPa and the LOI value reached 24.9%. In a vertical burning test and a cone calorimeter test, the joint use of APP and DEEP endowed RPUFs with a V-0 rating and they attained a THR value of 23.43 MJ/m2. Moreover, the addition of APP improved the acoustic absorption performance of the foam, verified by acoustic absorption measurements. Considering potential applications, the formulation containing 15 php APP and 5 php DEEP could be used in the preparation of a new flame-retardant acoustic absorption rigid polyurethane foam.

Proof-of-Concept Design for MPP Acoustic Absorbers with Elements of Art

A micro-perforated plate or panel (MPP) is a device used to absorb sound. It consists of a thin flat plate made from several different materials with small holes and a back cavity. Several reported modifications and enhancements to the original design of the MPP acoustic absorber were modified by the holes or the back-cavity shape and sizes following the original idea. The present study attempts to artistically beautify the MPP acoustic absorbers by incorporating dotted arts into the design of MPP. The perforation for micro-perforated panels could be dotted arts with a perforation size smaller than 1 mm for enhanced acoustic absorption performance in the form of various artistic designs. Small LED lights could be placed inside the acoustic chamber to create the color lights emanating from the perforations instead of dots with different colors. Several MPP incorporated artistic designs of dotted patterns were presented and their acoustic absorption performance was analyzed using impedance tube in this paper.

Sustainable Acoustics for Powertrain Noise Reduction

<div class="section abstract"><div class="htmlview paragraph">A common way to reduce the noise of automotive powertrains, both with internal combustion engine (ICE) and electric driveline, is to cover their noise radiating areas. However, the parts used for this purpose, besides being effective from the NVH standpoint, also have to fulfil several non-NVH requirements. For ICE, they have to provide a certain level of thermal insulation while favoring the reduction of fuel consumption and CO2 emissions. For electric powertrains, lightweight, environmental footprint and sustainability become preponderant in the design of engine covers. Combining these multidimensional requirements, a new fiber-based mono-material product for engine-mounted applications is proposed. It is able to provide optimum noise protection in the passenger cabin and greater driving comfort while keeping high level of sustainability, good heat resistance and relatively low weight. This paper deals with the acoustic benchmarking of the newly developed product against various more traditional solutions available on the market. Using an experimental approach, the newly developed solution is positioned against the other bills of materials, in terms of acoustic absorption and insulation performance first at material level. Then, an acoustic performance comparison is carried out at part level on an electric motor mock-up.</div></div>

Quasi-static indentation and sound-absorbing properties of 3D printed sandwich core panels

The use of 3D printing to produce acoustic panels with good mechanical and acoustic properties was investigated in this paper. Various fiber layups of the fiberglass face sheet and core designs were fabricated and tested for their indentation resistance and acoustic absorption performance. It was found that the bidirectional face sheet layup exhibited the best indentation energy absorption recording 4.2 J, which is 37% more than the 45-degree layout and 66% more than the quasi-isotropic layup. The specific energy absorption of the hybrid honeycomb core is the best among the three core designs recording 404 J/kg, which is 56% higher than the corrugated triangle with horizontal beam core (359 J/kg) and 20% higher than double ellipse core (335 J/kg). Computed-Tomography (CT) scan was used to study the fracture behavior of the sandwich structures. It was found that the bidirectional layup exhibited a different failure mode as compared to the 45-degree and quasi-isotropic layup. In terms of the acoustic properties, the face sheets with various layup patterns have a low acoustic absorption coefficient with minimal differences from each other at low frequencies (500 Hz–3000 Hz) and have higher absorption coefficients with greater differences from each other at frequencies between 3000 Hz–6500 Hz. The absorption curve was significantly affected by the design of the core. The orientation of the core also comes into play if the core is asymmetrical. The hybrid honeycomb sandwich structure was the optimal structure among the three designs for balanced indentation resistance and acoustic insulation.

Directly freeze-drying porous graphene aerogel as acoustic-absorbing material

Noise pollution, which is no less harmful than air, water and soil pollution, is becoming an environmental shortcoming that cannot be ignored. Scientists are also seeking various solutions to work on reducing noise pollution. In this work, porous graphene aerogel (FD-GA) and porous graphene oxide aerogel (FD-GOA) prepared by foaming combined with freeze-drying are reported as acoustic-absorbing materials. The effects of thickness and degree of compression on the acoustic-absorbing performance of FD-GA and FD-GOA in the 1000-6000 Hz range, where human ear hearing is sensitive, were investigated, respectively. FD-GA was found to have significantly higher acoustic absorption performance with a peak absorption of 96.7%. The present work provides a simple and straightforward strategy for the synthesis of the acoustic-absorbing material. Key words: freeze-drying, graphene aerogel, acoustic-absorbing

High-Temperature and Low-Frequency Acoustic Energy Absorption by a Novel Porous Metamaterial Structure

In this paper, we propose a novel porous metamaterial structure with an improved acoustic energy absorption performance at high-temperature and in the low-frequency range. In the proposed novel porous metamaterial structure, a porous material matrix containing periodically perforated cylindrical holes arranged in a triangular lattice pattern is applied, and additional interlayers of another porous material are introduced around these perforations. The theoretical model is established by adopting the double porosity theory for the interlayer and the cylindrical hole which form an equivalent inclusion and then applying the homogenization method to the porous metamaterial structure formed by the equivalent inclusion and the porous matrix. The temperature-dependent air and material parameters are considered in the extended theoretical model, which is validated by the finite element results obtained by COMSOL Multiphysics. The acoustic or sound energy absorption performance can be improved remarkably at very low frequencies and high temperature. Furthermore, the underlying acoustic energy absorption mechanism inside the unit-cell is investigated by analyzing the distribution of the time-averaged acoustic power dissipation density and the energy dissipation ratio of each constituent porous material. The results reveal that regardless of the temperature, the acoustic energy is mostly dissipated in the porous material with a lower airflow resistivity, while the acoustic energy dissipated in the porous material with a higher airflow resistivity also becomes considerable in the high-frequency range. The novel porous metamaterial structure proposed in this paper can be efficiently utilized to improve the acoustic energy absorption performance at high temperature.

Acoustic absorption performance of cavity-backed magnetoactive elastomer film

This work aims to investigate the acoustic absorption performance of magnetoactive elastomer (MAE) film, which consisting of carbonyl iron particles (CIPs) and polydimethylsiloxane matrix. MAE film is a kind of smart material. The magnetized CIPs can be forced to move when suffering from a non-uniform magnetic field, and finally, cause the morphology change of MAE film. The magnetic-dependent deformation of MAE film was investigated using the digital image correlation method. The result shows that the deformation is center-symmetrically distributed and increases with the growth of current. Increasing the CIPs content and the film thickness can enlarge the magnetic-dependent deformation of the MAE film. The maximum deformation of sample 4 (75 wt. % CIPs) under the current of 4 A is nearly 0.89 mm. Then, the acoustic absorption performance of cavity-backed MAE film was fully studied. For CIPs embedded cavity-backed MAE film, increasing the current can raise its maximum acoustic absorption frequency (MAAF). The MAAF of sample 4 changes from 2.33 to 2.88 kHz when the current rise from 0 to 4 A. Meanwhile, increasing the CIPs content and film thickness can effectively reduce the MAAF of cavity-backed MAE film. Moreover, reducing the film thickness can increase the maximum acoustic absorption coefficient of cavity-backed MAE film. With exceptional acoustic absorption performance, the MAE film can find extensive applications in intelligent control and acoustic absorption devices.

Acoustical Properties of Fiberglass Blankets Impregnated with Silica Aerogel

It is known that aerogel impregnated fibrous blankets offer high acoustic absorption and thermal insulation performance. These materials are becoming very popular in various industrial and building applications. Although the reasons for the high thermal insulation performance of these materials are well understood, it is still largely unclear what controls their acoustic performance. Additionally, only a small number of publications to date report on the acoustical properties of fibrous blankets impregnated with powder aerogels. There is a lack of studies that attempt to explain the measured absorption properties with a valid mathematical model. This paper contributes to this knowledge gap through a simulation that predicts the measured complex acoustic reflection coefficient of aerogel blankets with different filling ratios. It is shown that the acoustic performance of a fibrous blanket impregnated with aerogel is generally controlled by the effective pore size and porosity of the composite structure. It is shown that there is a need for refinement of a classical Biot-type model to take into account the sorption and pressure diffusion effects, which become important with the increased filling ratio.

Mechanical and acoustic absorption characteristics of UHMWPE weft-knitted structures of flexible porous laminated composites

The study investigated the mechanical and acoustic absorption characteristics of ultrahigh molecular weight polyethylene (UHMWPE) weft-knitted structures of flexible porous laminated composites. Weft-knitted structure reinforcements were fabricated using UHMWPE filaments, with various yarn fineness, sinking depths, and laminating layers. The acoustic absorption performance of UHMWPE weft-knitted structures of flexible porous laminated composites was evaluated using a standing wave tube system. The results revealed that the acoustic absorption capacity of composites can be enhanced by increasing the thickness of the composites. The acoustic absorption coefficient of RL 400 D three-layered composites was 50% higher than that of the double-layered composites. The response surface methodology analyses results revealed that optimum acoustic absorption at low frequency was attained by the composites at a yarn fineness of 400 D, a sinking depth of 10.5 mm, and double-layered structures. However, response surface analysis at a frequency of 2000 Hz was not significant.