Behavior of the First Absorption Peak of Porous Materials with Changing Thickness

Several theoretical studies of the acoustic properties of homogeneous isotropic porous material have been presented in the past, and the results have been shown to be in good agreement with experiments. However, no corresponding satisfactory theoretical analysis of the commonly used perforated porous tile seems to have been given. In this paper this problem is treated in some detail, and from the resulting formulas and equivalent circuits the acoustic properties of the tile can be calculated in terms of the geometry and the properties of the porous material. As an example, the acoustic impedance and absorption coefficient have been calculated as a function of perforation depth for a typical tile, and the results are shown to be in good agreement with measurements.

Noise has become one of the four major pollution types in the world. Constant exposure to noises can cause all kinds of health problems such as hearing loss, cardiovascular disease and sleep disorder. Following paper investigates the ability of porous and solid materials to absorb the generated sound for different geometries. Many natural and synthetic material have been developed and tested for acoustic applications. Impedance tube is used to find absorption coefficient of specimen is tested experimentally and the research shows the peak of absorption coefficient changes with porosity and peak value affected with various geometrical shapes in specimens. Finally, we get the peak absorption coefficient in porous material than the solid geometry of same material. In this research, we study the different sound absorbing materials with different geometry and comparative study between the materials. A thorough analysis of the research on the future scope of work, issue statements and goals connected to converting two-wheelers with IC engines into electric two-wheelers. In accordance with our practical analysis, our bike come in second position in overall championship, it is found that motor of electric vehicle generates high frequency sound. The intensity of research and the development in manufacturing processes, we anticipate that the range of new sound absorbing materials will expand to manufacture the casing of motor over the next few years.KeywordsSound absorptionCarbon fibreElectric vehiclePollutionNatural and synthetic materialPorous material

Experimental evaluation on acoustic impedance and sound absorption performances of porous foams with additives with Helmholtz number

Acoustic porous materials are extensively used in many engineering applications like building, automobile, aviation, and marine. The health risk factor and environmental claims, associated with traditional materials such as glass wool, mineral fibers, and polymer foams demand for the alternative porous acoustic absorbing materials. Advances in additive manufacturing (AM) allow to manufacture complex structures and give an alternative method to produce porous materials. This study investigates the acoustic properties of porous sound-absorbing material produced by using additive manufacturing (AM) technique and explores the feasibility of AM to manufacture acoustic absorptive materials. For study, three samples with different aperture ratios were fabricated by AM technique, and their sound absorption coefficients were measured experimentally by using the impedance tube. The theoretical formulation for predicting normal sound absorption coefficient of sample with and without air gap was developed and compared with experimental results. The predicted absorption coefficient agrees well with measured results. The measured results indicate that the absorption coefficient of the structures fabricated through AM can be altered by varying aperture ratio and air gap behind the sample. This study reinforces the capability of AM for producing complex acoustic structures with better acoustic properties.

This paper is investigating the performance of Australian hemp in hempcrete including unretted and retted hurd and fines for wall and render applications, respectively. The mechanical, thermal and acoustic characteristics of hempcrete are assessed including the effect of retting process. Although the retting process caused about 12% decrease in shiv bulk density, which was attributed to the degradation of hemp and a lower solid volume fraction, hempcrete bulk density, mechanical characteristics, thermal conductivity and acoustic performance are not significantly influenced by the retting process. The thermal conductivity appears to be proportional to the bulk density which is proportional to the hemp content of hempcrete. Acoustic performance of wall mix specimens was outstanding with a maximum sound absorption coefficient around 0.90 for a frequency around 700 Hz. However, the acoustic performance of render mix specimens was extremely poor compared to that of wall mix specimens with a sound absorption coefficient less or equal to 0.13. The combined effects of fine particle size and high binder content is responsible for this drastic drop in sound absorption coefficient. Acoustic performance was much more impacted than thermal conductivity by the hemp fine particle size and high binder content of the render mix.KeywordsHemp concreteBio-aggregatesThermal performanceAcoustic performanceMechanical characteristics

<div class="section abstract"><div class="htmlview paragraph">Noise reduction is generally accomplished by applying appropriate noise control treatments at strategic locations. Noise control treatments consisting of poroelastic materials in layers are extensively used in noise control products. Sound propagation through poroelastic materials is governed by macroscopic material and geometric properties. Thus, a knowledge of material properties is important to improve the acoustical performance of the resulting noise control products. Since the direct measurement of these properties is cumbersome, these have been usually estimated indirectly from easily measurable acoustic performance metrics such as normal incidence sound transmission and/or absorption coefficient, measured using readily available impedance tube. The existing inverse characterization approaches fulfilled the estimation by curve fitting measured and predicted acoustic models. In this paper, in addition to the use of diffuse field performance metrics, a data driven machine learning approach is utilized to derive reduced order models. The properties of porous material are efficiently estimated by combining genetic algorithm with reduced order models. This approach is especially attractive to material suppliers, who usually lack information about material properties to improve their products.</div></div>

This research introduces a novel approach to investigate porous sound absorbing materials using stochastic modeling techniques. The study employs the homogenization method and machine learning to predict the acoustic properties of porous materials. The homogenization method simplifies the analysis of microstructural properties in porous materials, while machine learning is a data analysis technique that utilizes algorithms to learn from data and make predictions. By integrating these methods, this study quantitatively assesses the impact of microscopic structural randomness on sound absorption properties. The study assumes that microstructural parameters are random variables and calculates the sound absorption coefficient through homogenization method. This dataset serves as input for machine learning. A Gaussian process regression model is developed using the dataset to evaluate the distribution of the sound absorption coefficient via the Gauss-Hermite quadrature method. The study implements Bayesian optimization, utilizing the variance of the resulting probability distribution as the acquisition function, to adaptively update the dataset. By iteratively performing these processes, the probability distribution of the sound absorption coefficient can be predicted efficiently and with high accuracy.

The porous materials, such as glass wool or foam, are generally used to attenuate noise. The most fundamental acoustic property of these porous materials is their sound absorption coefficient. The purpose of this paper is sintered fiber and porous materials sound absorption properties investigated. Sound absorption properties of sintered Al fiber has over 0.7 of sound absorption coefficient with 800-2000Hz frequency for 0.6 relative density and 10mm thickness. NRC (noise reduction coefficient) is 0.73. Metal foam have good sound absorption rate at 2000 ~ 4000Hz.

Nonwoven fabric coated with core-shell and hollow nanofiber membranes for efficient sound absorption in buildings

Experimental characterization of 3D-printed sound absorber

This study designs an underwater acoustic absorption metamaterial based on a multi-cavity diaphragm structure. The acoustic performance is carefully modeled and examined through simulations in COMSOL Multiphysics finite element software (v.6.1). First, a multilayer periodic unit model consisting of a main cavity and sub-cavities is constructed. A corresponding acoustic-structure coupled finite element model is established by incorporating diaphragm thickness and pre-tension parameters. The frequency domain analysis method is then employed to simulate sound wave transmission and resonance absorption within the structure, calculating the relationship between the acoustic absorption coefficient and frequency. Based on parametric sensitivity analysis, the study examines the influence of key parameters, including main cavity depth, slit width, sub-cavity depth, diaphragm thickness, and pre-tension, on acoustic absorption performance. The mechanisms by which these parameters regulate the absorption peak and bandwidth are revealed. The simulation results show that this metamaterial provides effective broadband acoustic absorption from 200 Hz up to 3000 Hz. The effective bandwidth with an absorption coefficient (α > 0.5) reaches 770 Hz, with a maximum absorption peak of 0.96 and an average absorption coefficient of 0.74, indicating excellent low-frequency underwater acoustic absorption capability. This study provides theoretical foundations and design guidelines for underwater noise control and related engineering applications.

Sound absorption is one of the important properties of porous materials such as foams and lattices. Many mathematical models in the literature are capable of modeling the acoustic properties of lattices. However, appropriate models need to be chosen for specific lattice structures on a case-by-case basis and require significant experience in acoustic modeling. This work aims to provide simplified insights into different mathematical models for the simple cubic lattice. The strut lengths and radii of the unit cells were varied, and the sound absorption properties were measured using an impedance tube. The sound absorption coefficients of the lattices generally increased and exhibited more resonant-like behavior as the strut radius increased. The Delany-Bazley (DB) model and the multi-layered micropore-cavity (MMC) model were used to simulate the acoustic properties of the lattices. The correction factors in the MMC were calculated based on empirical relations fitted using experimental data of the design geometry parameters. Results show that the DB model was able to model the sound absorption coefficients for lattice samples with porosities as low as 0.7, while the MMC with resonator theory is a more appropriate acoustics approach for lattices with porosities lower than 0.7. This work will be highly useful for materials researchers who are studying the acoustic properties of novel porous materials, as well as manufacturers of acoustic materials interested in the additive manufacturing of lattice structures for sound absorption and insulation applications.

In acoustic engineering, noise control is of concern in various fields including industrial machines, vehicles, architectures and home appliances. Sound absorption is one of the major solutions to control noise and the efficiency of sound absorption can be evaluated by the acoustic absorption coefficient and the absorption frequency bandwidth. The development of novel acoustic absorbers is crucial in enhancing the efficiency of sound absorption and fulfilling various environmental requirements of acoustic absorption in different frequency ranges. The enhancement of functional properties of absorbers should also be considered by innovative designs. Additive manufacturing (AM) has been introduced to reduce the prototyping costs and lead time of sound absorbers, while the potential of AM for fabricating sound absorbers with complex structures has not been explored. This Ph.D. study designs and develops different types of novel acoustic absorbers with complex structures to enhance the efficiency of sound absorption in various frequency ranges by achieving wideband absorption. The development of the absorbers involves two different polymer AM processes owing to additive manufacturability of the complex designs as well as prototype cost. Three novel absorber prototypes with improved performances have been successfully developed. Firstly, for sound absorption enhancement in the range from low frequency to middle frequency, innovative multi-layer micro-perforated panels (MPPs) with adjustable geometries for tunable wideband absorption are designed and fabricated by selective laser sintering. The finite element method is applied to provide numerical predictions of the performances of the designed structures. The effects of structural parameters of multi-layer MPPs on absorption coefficients and frequency bandwidths are analyzed by theoretical predictions, numerical simulations and experiments. The results reveal that the absorption frequency bandwidths of MPPs are broadened by the multi-layer designs, while the absorption coefficients remain comparable or even higher. The frequency ranges can be tuned by varying the air gap distances and the inter-layer distances. An optimization method, in which the area under the sound absorption curve as the criterion to evaluate the sound absorption ability is maximized, is introduced to design acoustic absorbers with the most effective sound absorption. Secondly, for sound absorption enhancement in the range of high frequencies, triply periodic minimal surface (TPMS) structures that have shown potential absorption abilities but have not been experimentally validated are designed for characterization of the acoustic performances of innovative acoustic absorbers with multifunctionality. Three typical types of TPMS structures: Primitive (P), Gyroid (G) and Diamond (D) are designed and fabricated by stereolithography apparatus using polymeric resins. D structures exhibit superior absorption abilities in terms of high absorption coefficients and wide bandwidths. A series of D structures are designed to study the effects of the unit cell size, volume fraction and height on acoustic absorption. The analysis of the results provides an optimized design of geometric parameters with a small unit cell size and a large volume fraction for optimal acoustic absorption. Finally, for further enhancement of sound absorption of the TPMS absorbers, innovative micro-perforated TPMS absorbers are designed and investigated by combining MPPs and TPMS structures. Significant improvements on acoustic absorption are achieved by the micro-perforated TPMS structures, especially for P and G structures with lower absorption coefficients compared to those of D structures. In addition, the effects of the perforation ratio of P and G structures are studied. The increase of the perforation ratio in the observed range exhibits positive effects on improving the acoustic absorption of the structures. The fabrication materials are proved to have no major effects on the acoustic absorption performances of the TPMS structures and therefore can be selected according to the application environments and requirements. The study of innovative acoustic devices is significant in acoustic engineering and is facilitated by AM that efficiently manufactures the absorber prototypes with complex structures. AM is promising to be applied as an enabling technology that accelerates the development of acoustic engineering by fabricating innovative and customized designs.

Shallow coastal waters are characterized by high levels of suspended sediment particles relative to the open ocean. This kind of seawater is also called turbid seawater. A cylindrical resonator technique has been developed to measure the coefficient of sound absorption in turbid seawater by the reverberation technique. The resonator is a seamless aluminum barrel with inner diameter 100 cm, inner height 55 cm, and wall thickness 4.3 mm. The aluminum barrel has been anodized, and its inner surface has been covered by lacquer. The aluminum barrel is placed coaxially in a cylindrical steel chamber and supported at four points of its lower rim by means of hard-wood chocks, so as to reduce the energy loss through sound radiation. The measurement error of the system has been estimated by 15% with the calibrated liquids. Then, the coefficient of sound absorption in turbid seawater with the concentration from 40 mg/L to 320 mg/L, in steps of 40 mg/L approximately, has been measured. The results demonstrate that if the concentration of suspended sediment particles is beyond 140 mg/L, the coefficient of sound absorption in turbid seawater is as twice as that in clear seawater. The particle size distribution of suspended sediment particles has been measured by a laser diffraction analyzer. If the coefficient of viscous sound absorption can be calculated by the median diameter of suspended sediment particles, the calculated sound absorption coefficient agrees well with that measured. It is indicated that if the density, concentration, median diameter of suspended sediment particles in turbid seawater are known, the coefficient of sound absorption in turbid seawater can be calculated roughly by the combination of the clear seawater's sound absorption empirical formula and viscous sound absorption formula.

Experimental and frequency-domain study of acoustic damping of single-layer perforated plates