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The acoustic absorption of granular aerogel layers with a granule sizes in the range of 2 to 40 μm is dominated by narrow-banded, high absorption regions in the low-frequency range and by reduced absorption values at higher frequencies. In this paper, we investigate the possibility of developing new, low-frequency noise reduction materials by layering granular aerogels with traditional porous sound absorbing materials such as glass fibers. The acoustic behavior of the layered configurations is predicted using the arbitrary coefficient method, wherein the granular aerogel layers are modeled as an equivalent poro-elastic material while the fibrous media and membrane are modeled as limp media. The analytical predictions are verified using experimental measurements conducted using the normal incidence, two-microphone impedance tube method. Our results show that layered configurations including granular aerogels, fibrous materials, and limp membranes provide enhanced sound absorption properties that can be tuned for specific noise control applications over a broad frequency range.

Object and purpose of research. The study of the acoustic pulse changes regularities during its propagation in con-fined media is one of the fundamental problems of acoustics, which allows to pose and solve the inverse problem of determining the dissipative and resonant properties of these media. The physical processes occurring during the propagation of a pulse in a cylindrical waveguide with rigid walls were investigated. Materials and methods. To analyze the mechanism of dissipation, experimental studies of pulse propagation in a hy-droacoustic tube were carried out, and the theoretical description of the obtained results was carried out using analytical methods. The simulation of the propagating pulse in the finite element waveguide model was used to confirm the theoretical assessments and the experiment. Main results. Experimental studies of physical processes during the propagation of an acoustic pulse in confined medium of cylindrical waveguide bounded by walls with characteristics close to absolutely rigid are carried out. The data showed that it is possible to control changes in the phase velocity, amplitude, and waveform, which made it possible to quantify the impedance of the internal walls of the waveguide and the dissipation of acoustic energy with a sufficient degree of accuracy. The numerical model calculation, taking into account the theoretically obtained quantitative assessments of the dissipation values and the impedance value of the waveguide inner surface, showed a good correspondence between the model and experimental characteristics of the change in the propagating pulse. Conclusion. In the studies devoted to the propagation description of acoustic waves in waveguides, the issues of energy dissipation are usually not considered, especially in cases where it has a weak effect on the measurement result. The theoretical value of the research is to quantify the wave energy dissipation by the parameters that can be determined with sufficient accuracy in the experiment: the phase velocity, the pulse form. Further accuracy improvement of the experimental data, especially in a wide frequency range, will improve the theoretical model of dissipation by taking into account the mechanism of inhomogeneous viscous and thermal waves near the inner surface of the waveguide. The practical significance of the research is to increase the reliability of experimental data and to develop physical and mathematical models of underwater sound absorption due to a forced variable flow with a highly transformed velocity of a viscous liquid in a thin surface layer near the elastic wall.

Pure polyurethane foam and nanocomposite foam are used to absorb sound. In this study, hollow silica nanospheres and rigid silica nanoparticles were added to the polyurethane matrix and their sound absorption properties were investigated by impedance tube and compared with pure polyurethane foam. Reinforcement phase influences on the morphology of the matrix were studied by scanning electron microscopy. Due to greater effects of the rigid silica nanoparticles on the morphology of the matrix, it was expected to increase the sound absorption coefficient of the rigid silica nanoparticles/polyurethane, more than hollow silica nanospheres/polyurethane, but the results show that the hollow silica nanospheres increased absorption coefficient of the composite more efficiently. The crust of hollow silica nanospheres increases the number of boundaries in a sound wave, and the air gap inside them cause the sound wave to damp. So the intrinsic property of the hollow silica nanospheres is more effective than the matrix morphology. Thus, by the same content of reinforcement in the matrix, hollow silica nanosphere/polyurethane sample with sound absorption coefficient of 0.87 for a thickness of 9 cm has the highest sound absorption coefficient compared to the rigid silica nanoparticles/polyurethane sample and pure polyurethane foam. In pure and nanocomposite samples, sound absorption coefficient increased by increasing the thickness of samples.

International audience; This chapter discusses the role of energy absorption in a reverberation chamber and some related applications. The composite Q-factor is the key indicator of the total amount of losses in a reverberation chamber since it quantifies the energy stored (per unit of time) per unit of transmitted/dissipated power. In a first part of this chapter, we settled the definition of this composite quality factor and described a method to estimate it in different ways with a set of two antennas. The scattering parameters at antenna ports enable to estimate the composite Q-factor in three different ways and enables to check for the enhanced backscattered coefficient. Obviously, an accurate enough estimation requires a high number of RC states. We suggest that averaging over small frequency bands is a solution to minimize intrinsic statistical variation of Q-factors estimated from a limited set of stirrer positions. The first application consists of evaluating the average absorption cross section of any piece of material within the RC. This absorption coefficient is retrieved from the contrast of quality factors measurement before and after putting the material in the chamber. For simple geometries of the inserted absorber, it is possible to estimate its efficiency. Moreover, from the knowledge of the intrinsic EM properties of homogeneous and thick (with regard to the skin depth) materials, we showed that the measured AACS computed from the modification of the Q-factor is consistent with the theoretical one. This last fact was at the origin of a specific calibration procedure for mm-wave RC dedicated to animal exposure. This is the second application of this chapter. In order to calibrate the EM dose during exposure, the power density in the RC must be controlled and proved to be consistent with the expected temperature rise. Using a rectangular homogeneous phantom, mimicking the properties of animal’s skin, we show that the temperature rise at its surface and measured with an infrared camera was indeed predictable. On a theoretical point of view, the measurement of the Q-factor, the knowledge of intrinsic parameters of the phantom and the properties of the diffuse field in the RC fully determine the heat deposition on the phantom surface. Solving the heat-equation gives then the correct temperature rise prediction with a reasonable accuracy. Chapter Contents: • 6.1 Losses and Q-factor • 6.1.1 Role of losses in a RC • 6.1.2 Definition of the Q-factor • 6.1.3 Origin of losses and their contribution to the Q-factor • 6.1.4 Q-Factor measurements • 6.1.4.1 Test setup for Q-factor measurements • 6.1.4.2 Q-Factor measurements using the full scattering matrix • 6.1.4.3 Q-Factor measurements using the reflection coefficient from each antenna • 6.1.4.4 QTx and QRx measurements • 6.1.4.5 Experimental results of Q-factor estimation • 6.2 The average absorption effective area of an object in a RC • 6.2.1 The Q-factor of an object in a RC and its average absorption cross section • 6.2.2 Theoretical absorbing cross section of a rectangular piece of absorbing material • 6.2.3 Measurement examples • 6.3 Dosimetry for animals • 6.3.1 Using RC for animal exposure • 6.3.2 Theory about heat transfer in a RC • 6.3.2.1 The phantom • 6.3.2.2 The heat equation • 6.3.2.3 The electrothermal coupling • 6.3.2.4 The 1-D approximation • 6.3.2.5 Solution of the 1-D heat equation • 6.3.3 Experimental validation • 6.4 Discussion • References. © The Institution of Engineering and Technology 2021.

In this study, new sound-absorbing composite materials in the form of nanofiber web-reinforced nonwovens and sandwich structures were produced by incorporating polyurethane (PU) nanofibers of different durations to recycled polyethylene terephthalate (PET) bottle waste nonwovens in order to execute environmentally friendly noise control in the lower frequency region. Effect of nanofiber spinning duration on the prepared structures was examined and the results were compared to commercial sound absorbers. Nanofiber spinning period optimization studies were done by producing nanofibers from 15 wt% PU solution for several durations and 5, 20, 60 and 120 min were chosen as the nanofiber spinning durations. The bead free PU nanofibers of 509.9 nm diameter were spun on the 250, 500 and 750 g/m2 needle punched nonwovens for the selected durations in order to produce the nanofiber web-reinforced nonwovens, afterward sandwich structures were prepared by laminating them with another nonwoven of the same. Noise reduction coefficient (NRC) values of all samples were calculated after the sound absorption coefficients (SACs) have been measured using a two-microphone impedance tube. It was seen that as nanofiber production time increased, resonance frequencies decreased and thicker nanofiber webs provided more effective sound absorption in the lower frequency range. There were statistically significant differences between noise reduction coefficients (NRCs) of the composites depending on the nanofiber spinning duration. In particular, 1560 (nanofiber web produced for 60 min) added nonwovens had the highest NRCs of all durations. However, in sandwich structures, better sound absorption coefficient (SAC) values were not obtained as the nanofiber spinning time increased. Amogst all samples, H750-1520-H750 had the highest NRC value of 0.574 with a thickness of 8.471 mm. H250-1560 which was formed by spinning nanofibers on H250 for 60 min was also regarded as a good sound absorber with a thickness of 1.548 mm and 0.435 NRC. The produced materials may be referred to as potential environmentally friendly sound absorbers with a value added utilization area for recycled polyethylene terephthalate fibers. © 2020 Elsevier Ltd

The paper presents the design and the acoustic properties evaluation of complex shape metamaterials. Complex metamaterials are a better alternative to conventional acoustic materials to absorb acoustic energy at low frequencies when the available volume is limited. Moreover these metamaterials are well adapted for harsh environments (high temperature, high pressure). In addition, traditional methods of performance evaluation are generally developed and used for simple shape geometries with known acoustic properties. In the present study, the metamaterials were placed in the restricted available volume. Their design is based on periodic neck-cavity configurations. In order to broaden the sound absorption coefficient frequency range, different cells were connected in parallel and in series. Owing to the fact that the cavities are small and of complex shape, a numerical Thermo-Visco-Acoustic (TVA) model was used to simulate the acoustic behavior of each periodic cell. Finally, the TVA was combined with transfer matrix methods to deduce the acoustic absorption coefficient of the metamaterials. The numerically predicted results using the present approach are in good agreement with measurements on prototypes. It is shown that these relatively thin metamaterials allow for low frequency sound absorption with wide absorption coefficient peaks., 49th International Congress and Exposition on Noise Control Engineering, INTER-NOISE 2020, August 23-26, 2020, Seoul, Korea

Composite structures with various stacking sequences were formed by combining electrospun polyurethane (PU) nanofibers with needle-punched recycled nonwovens from polyester textile wastes and bottles as a new sound absorption material in order to contribute to the solution of noise control and waste generation. PU solutions of 12, 13, 14 and 15 wt % concentrations were prepared for nanofiber optimization studies. Scanning electron microscope (SEM) images showed that the bead free nanofibers were obtained with diameters 296.5 and 509.9 nm from 13 and 15 wt % PU concentrations, respectively. In order to evaluate the potential of nanofibers and nanofiber-recycled nonwoven composites as noise reduction materials; sound absorption coefficients (SACs) were measured and noise reduction coefficients (NRCs) were calculated. The most ideal sample was regarded as the 3.24 mm thick sandwich structure (recycled nonwoven-nanofiber-recycled nonwoven), coded with A250-1320-A250 with a 0.504 NRC value. When the two nanofiber diameters were compared, finer nanofibers (296.5 nm) and finer nanofiber enhanced nonwovens had better NRCs than the thickers (509.9 nm) of the same. The developed composite materials can be regarded as promising sound absorbers. © 2019, © 2019 The Textile Institute.

In this paper, the natural and resonant frequencies of porous media are studied based on Biot's equations. The governing equations of porous media are analytically solved by using the recent developed potential function method. Based on the obtained results, the natural and resonant frequencies of the porous medium can be investigated. In this research, several foams with different acoustical and non-acoustical properties are considered and the natural and resonant frequencies are studied. In addition, for a better understanding of the natural and variation of resonant frequencies of the considered foams, various damping gains are defined and by changing them gradually, the variations of absorption coefficient and field variables are studied. The results show that the trends of absorption coefficient versus frequency for porous media can be predicted by considering the arrangement of structural and fluid natural frequencies. Also, around the structural natural frequencies two types of variations in absorption coefficient occur (i.e., maximum-minimum or maximum variations). Additionally, after computing the corresponding results of rigid frame and Biot's models it can be seen that the rigid frame theory cannot correctly predict the sound absorption coefficient in the vicinity of structural natural frequencies.

Absorption of acoustic nanowave in specific frequency region is important for the design of acoustic filter. This paper puts forward a meta material model made up of fluid-conveying carbon nanotubes (CNT), which can absorb acoustic nanowave in a given frequency range by adjusting the lengths and fluid velocities of themselves. Absorption coefficients are calculated out through the combination of the finite element method with the theoretical model, which are 0.4∼0.55 relating to different fluid velocities for the crossing-distributed model. Comparisons are made between the crossed model and the aligned one, which prove that the CNT forest with crossed distribution is more effective in acoustic wave absorption.

Tapestries represent an important and valuable cultural heritage. In the past they played a significant role in enriching indoor environments, as well as contributing to change the indoor characteristics of the space. Considering typical mounting conditions, acoustics was likely to be strongly affected by hung tapestries. Even though their acoustic properties could be ideally inferred from their physical features, a proper characterization would be preferable to understand how they could modify sound propagation. However, given their dimensions and their delicacy, on-site measurements are often the only option. In the present paper, absorption coefficients of the Barberini tapestries were measured during a temporary exhibition held in the Cathedral of St. John the Divine in New York City. Measurements were carried out in a large chapel with and without the tapestries installed. The effect of acoustical coupling with the adjacent cathedral volume was investigated using Bayesian methods to exclude unwanted influence on the measurements. Results were obtained using either the classical reverberation time formula or the indirect approach based on the use of geometrical acoustic modeling. Finally, results proved to be in good agreement with predictions based on theoretical formulas, once the relevant non-acoustical parameters were estimated.

Both specific airflow resistance and air permeability can be used as a parameter to estimate the sound absorption of textiles. The measurement of specific airflow resistance is specified in ISO 9053 (Int. Standards Org., 1991), but it is known to be inaccurate for low specific airflow resistance. This paper compares the measured specific airflow resistance according to ISO 9053 and those calculated from air permeability according to ISO 9237 (Int. Standards Org., 1995). The sound absorption coefficients predicted by Pieren's model [R. Pieren, Textile Res. J. 82(9), 864–874 (2012)] are compared with measurements by the impedance tube method, which concludes that those predicted from the air permeability are more accurate than those from the measured specific airflow resistance for textiles.Both specific airflow resistance and air permeability can be used as a parameter to estimate the sound absorption of textiles. The measurement of specific airflow resistance is specified in ISO 9053 (Int. Standards Org., 1991), but it is known to be inaccurate for low specific airflow resistance. This paper compares the measured specific airflow resistance according to ISO 9053 and those calculated from air permeability according to ISO 9237 (Int. Standards Org., 1995). The sound absorption coefficients predicted by Pieren's model [R. Pieren, Textile Res. J. 82(9), 864–874 (2012)] are compared with measurements by the impedance tube method, which concludes that those predicted from the air permeability are more accurate than those from the measured specific airflow resistance for textiles.

We measured the acoustic absorption, on the 0.5-6 kHz frequency range, of polyurethane foams with mean pore diameters between 0.6 and 3.2 mm. Two types of foams were investigated: classical open-cells ones versus membrane foams, in which thin polyurethane membranes were preserved during solidification. Interestingly , the latter presented better absorption abilities, indicating that membranes could be an asset for sound absorption. Noise pollution has become a major problem in our modern life. Industrial and academic research has continuously tried to design more efficient soundproofing materials. Recently, exotic stuctures have been considered, using the concept of double-porosity 1,2 or introducing low frequency resonators to enhance the dissipation. 3–5 Traditionally , sound absorbers have been porous media, such as mineral wools or foams. A first general rule for their efficiency is that no obstacle should prevent the sound from propagating in the medium, otherwise the acoustic energy is reflected back instead of being absorbed. For foams, for example, it means that open-cell structures are preferred. On the other hand, the physical picture is that when sound penetrates such a medium, it loses a lot of energy because of the large surface area it can interact with. As a rule of thumb, one finds that good absorption is obtained when the typical pore size corresponds to the heat and viscous diffusive length in air, which is of the order of 50 µm at 2 kHz, for example. Hence, open-cell porous materials with pores sizes of tens of micrometers are good candidates for efficient sound absorption and are therefore used extensively for sound insulation. 6,7 In this letter we show that there are exceptions to these established rules: closed-cell foams with millimeter-sized pores can actually be good sound absorbers. The foams we studied were provided by the company Foampartner. They were made of polyurethane, with a porosity (air volume fraction) of 98%. Their most interesting feature, for us, was that most of the membranes which separate neighbouring pores (Fig. 1) were preserved during solidification. 8 As these membranes are not desired for most of the applications, the manufacturer employs a technique by which the membranes a) Electronic

Passive acoustic monitoring with widely-dispersed hydrophones has been suggested as a cost-effective method to monitor population densities of echolocating marine mammals. This requires an estimate of the area around each receiver over which vocalizations are detected - the "effective detection area" (EDA). In the absence of auxiliary measurements enabling estimation of the EDA, it can be modelled instead. Common simplifying model assumptions include approximating the spectrum of clicks by flat energy spectra, and neglecting the frequency-dependence of sound absorption within the click bandwidth (narrowband assumption), rendering the problem amenable to solution using the sonar equation. Here, it is investigated how these approximations affect the estimated EDA and their potential for biasing the estimated density. EDA was estimated using the passive sonar equation, and by applying detectors to simulated clicks injected into measurements of background noise. By comparing model predictions made using these two approaches for different spectral energy distributions of echolocation clicks, but identical click source energy level and detector settings, EDA differed by up to a factor of 2 for Blainville's beaked whales. Both methods predicted relative density bias due to narrowband assumptions ranged from 5% to more than 100%, depending on the species, detector settings, and noise conditions. © 2018 Acoustical Society of America.

Passive acoustic monitoring with widely-dispersed hydrophones has been suggested as a cost-effective method to monitor population densities of echolocating marine mammals. This requires an estimate of the area around each receiver over which vocalizations are detected - the "effective detection area" (EDA). In the absence of auxiliary measurements enabling estimation of the EDA, it can be modelled instead. Common simplifying model assumptions include approximating the spectrum of clicks by flat energy spectra, and neglecting the frequency-dependence of sound absorption within the click bandwidth (narrowband assumption), rendering the problem amenable to solution using the sonar equation. Here, it is investigated how these approximations affect the estimated EDA and their potential for biasing the estimated density. EDA was estimated using the passive sonar equation, and by applying detectors to simulated clicks injected into measurements of background noise. By comparing model predictions made using these two approaches for different spectral energy distributions of echolocation clicks, but identical click source energy level and detector settings, EDA differed by up to a factor of 2 for Blainville's beaked whales. Both methods predicted relative density bias due to narrowband assumptions ranged from 5% to more than 100%, depending on the species, detector settings, and noise conditions. © 2018 Acoustical Society of America.

A novel porous metallic foam has been studied in this work. This composite material is a mixture of resin and hollow spheres. It is lightweight, highly resistive to contamination and heat, and is capable of providing similar or better sound absorption compared to the conventional porous absorbers, but with a robust and less degradable properties. Several configurations of the material have been tested inside an expansion chamber with spatially periodic area changes. Bragg scattering was observed in some configurations with certain lattice constants. The acoustic properties of this material have been characterized from the measurement of the two-port matrix across a cylindrical sample. The complex density and speed of sound can be extracted from the transfer matrix using an optimization technique. Several models were developed to validate the effect of this metallic foam using Finite Elements and the Two-port Theory. There was a good agreement between both models and the measurement results. QC 20160930

Materials with well-defined microlattice structures are superlight, stable, and thus bear great potential in sound absorption. An integrated transfer matrix method (TMM) is proposed to evaluate the sound absorbing efficiency of these lattice materials, in which a massive number of micropores are densely placed. A comparison between integrated TMM and conventional TMM reveals that the proposed approach offers better predictions on sound absorption of microlattice. This approach is then employed to optimize the microlattice material to determine the best pore and porosity that lead to maximum absorbing efficiency capability and minimum required thickness to attain a target sound absorption.

Recently, the accuracy of the parallel transfer matrix method (P-TMM) and the admittance sum method (ASM) in the prediction of the absorption properties of parallel assemblies of materials was investigated [Verdière, Panneton, Elkoun, Dupont, and Leclaire, J. Acoust. Soc. Am. 136, EL90-EL95 (2014)]. It was demonstrated that P-TMM is more versatile than ASM, as a larger variety of different backing configurations can be handled. Here it will be shown that the same universality is offered by the equivalent circuit method.

Based on one-dimensional lossy Bragg stacks, an asymmetric absorber is realized for low-frequency sound waves, that is, perfect absorption can be obtained when sound waves are normally incident from one side while a small absorption can be obtained from the opposite side. Moreover, the asymmetric absorption persists for a wide incident angle of sound waves in the range from 0° to 42° with the absorptive coefficient larger than 90% from one side while less than 20% from the other side. By changing the thickness of the top sublayer, a series of interesting absorption phenomena such as Fano-resonance type absorption are further investigated.

Effects of impregnation with silver nano-suspension as well as heat-treatment on sound absorption coefficients (AC) were studied in tangential direction of five different solid woods based on their importance. AC was measured at two frequencies of 250 and 500 Hz. A 400 ppm nanosuspension was used for the impregnation; silver nanoparticles had a size range of 30-80 nm. Based on the obtained results, the species reacted significantly different in absorbing sound at the two frequencies. Impregnation with nano-suspension substantially decreased AC at the lower frequency of 250 Hz; it did not show any particular trend when AC was measured at the frequency of 500 Hz. Heat treatment significantly increased AC at the frequency of 250 Hz. ACs of mulberry tended to be similar at the two frequencies; in the other four species though, ACs were significantly different. High significant correlations were found in the hardwoods between the ACs measured at the two frequencies.

Measurements of the sound absorption of several blocks of chairs with varied perimeter-to-area (P/A) ratios were used to examine how chair absorption coefficients are related to their physical characteristics. Because the P/A method combines the results of several samples of each type of chair, a more accurate understanding can be obtained of how the physical properties of the chairs are related to their sound absorption characteristics. Unoccupied chairs are shown to have a wide range of characteristics and it is necessary to test each type of chair to accurately predict their effect in an auditorium. However, the absorption characteristics of occupied chairs are strongly influenced by the absorption of the occupants and an approximate average sound absorption characteristic can be useful for predicting the effect of occupied chairs in an auditorium. Although the variations of sound absorption with row spacing are related to P/A, the variations are quite different than when simply changing the P/A for chair samples with constant row spacing. Both chair underpass height and chair back height affect the absorption characteristics of theater chairs, but the effects can be influenced by the presence of carpet under the chairs or by occupants in the chairs. Common sources of increased sound absorption in each octave band are identified and discussed. © 2013 Elsevier B.V.All rights reserved.

Three absorbing layers are investigated using standard rectilinear finite-difference schemes. The perfectly matched layer (PML) is compared with basic lossy layers terminated by two types of absorbing boundary conditions, all simulated using equivalent memory consumption. Lossy layers present the advantage of being scalar schemes, whereas the PML relies on a staggered scheme where both velocity and pressure are split. Although the PML gives the lowest reflection magnitudes over all frequencies and incidence angles, the most efficient lossy layer gives reflection magnitudes of the same order as the PML from mid- to high-frequency and for restricted incidence angles.

Sound absorption characteristics of specially designed high-loft nonwovens with minimum thickness were reported in this study. Three different polypropylene and polyester fiber-based high-loft, air-laid, and thermally bonded nonwovens varying in basis weight were produced. Heavier high-loft nonwoven samples at various thicknesses were formed using a specially designed mold. The sound absorption coefficients of samples with mass per unit areas ranging from 350 to 1575 g/m(2) and with thicknesses ranging from 5 to 45 mm were measured. Acoustical absorptive behavior of the high-loft nonwovens was explained by analyzing the displacements of small air control volumes in a high-loft nonwoven and the air velocities in the impedance tube. Results indicate that the velocity and the total displacement of the small air volumes inside the fiber network have a major effect on sound absorption. High-loft nonwovens can be much more effective in terms of sound absorption if they are produced at the thickness at which average maximum velocity of the air is calculated highest. If there is a desire to absorb more acoustic energy, heavier nonwovens can be produced. It is suggested that relatively heavy nonwovens (from 700 to 1575g/m(2)) can be produced thinner (5-10 mm) than the calculated thickness value based on the average maximum air velocity to get maximum sound absorption at lower thickness. Nonwovens Institute at North Carolina State University, NC, USA

Macroporous polymeric monoliths were prepared using, as main raw material, mixtures of hydrolysable and condensed tannins and Kraft black liquor from pulp industry. The process consists of mechanical frothing of aqueous solutions constituted with the different polyphenols, a hardener, and in the presence of a surfactant that maintains the foam stable before curing. The aim of this work was to study the contribution in the formulations of Kraft black liquor on physical properties for the foams. This objective was justified by the significantly lower cost of black liquor as compared with that of tannins. The target application is the thermal and sound insulation in the building industry. The materials obtained were characterized by scanning electronic microscopy, mercury porosimetry and mechanical, thermal and acoustical measurements.Solid, self-standing foams with an average porosity of 90% and an average cell diameter between 170 and 329 μm were obtained. The measured values of compression modulus ranged from 1.8 up to 21.6 MPa and the thermal conductivity was around 40 mW·K−1 m−1. The materials showed also good sound absorption characteristics at medium frequencies with coefficients of 0.5 (500 Hz) and 0.8 (1000 Hz) and at high frequencies with coefficients of 0.7 (2000 Hz) and 0.8 (4000 Hz). Keywords: Kraft black liquor, Tannins, Biobased porous materials, Physical characterizations

Metallic foams or sponges are materials with a cell structure suitable for many industrial applications, such as reformers, heat catalytic converters, etc. The success of these materials is due to the combination of various characteristics such as mechanical strength, low density, high specific surface, good thermal exchange properties, low flow resistance and sound absorption. Different materials and manufacturing processes produce different type of structure and properties for various applications. In this work a genetic algorithm has been developed and applied to support the design of catalytic devices. In particular, two substrates were considered, namely the traditional honeycomb and an alternative open-cell foam type. CFD simulations of pressure losses and literature based correlations for the heat and mass transfer were used to support the genetic algorithm in finding the best compromise between flow resistance and pollutant abatement. The CFD analysis was conducted by means of numerical simulations carried out on a geometry sample obtained by the micro-tomography technique to investigate the flow regime type and to extract pressure drop information. The result of this analysis was used to set guideline for the design of foam type substrate and to provide a first estimation of cost effectiveness of new type of substrates. © 2016 SAE International.

Nonwovens have been increasingly used in car interiors for noise reduction. Most of these nonwovens are subjected to thermal treatments to give the nonwovens their final three-dimensional forms. Therefore, it became crucial to investigate the effects of thermal treatment on sound absorption characteristics of nonwovens. In this study, the effects of the material and treatment parameters on airflow resistivity and normal-incidence sound absorption coefficient of thermally treated three-layered nonwoven composites have been studied. The material parameters included fiber size and porosity. The treatment factors included the temperature and duration. The thermally treated three-layered nonwoven composites are classified into three types based on the material content and fiber blend. Sandwich structures consisting of polylactide/hemp/polylactide and polypropylene/glassfiber/polypropylene layers were called LHL and PGP, respectively. The sample which consisted of three layers of an intimate blend of polypropylene-glassfiber was named as PGI. Both temperature and duration of thermal treatment have been found to affect air flow resistivity and sound absorption. An increase in air flow resistivity and a decrease in sound absorption have been detected with heat treatment. A similarity has been observed between the thermal behaviors of PGP and PGI, which included the same thermoplastic polymer fiber. Variation in air flow resistivity of sandwich structure nonwoven composites increased with the increase in temperature, which was not observed in the intimate blend ones. The air flow resistivity of heat-treated nonwovens followed a steeper trend compared to unheated nonwovens per change in material parameters. In terms of treatment parameters, the difference between the thermal treatment and the melting point of the thermoplastics constituent of the nonwoven composite was found to be a significant factor on sound absorption. This effect of treatment temperature on sound absorption changed with treatment duration. The sound absorptive characteristic of the nonwoven composites in terms of sound frequency underwent a change with thermal treatment due to the structural changes with exposure to high temperature. © The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nave.

The accuracy of a phased beam tracing method in predicting transfer functions is investigated with a special focus on the positions of the source and receiver. Simulated transfer functions for various source-receiver pairs using the phased beam tracing method were compared with analytical Green’s functions and boundary element solutions up to the Schroeder frequency in simple rectangular rooms with different aspect ratios and absorptions. Only specular reflections were assumed and diffraction was neglected. Three types of error definitions were used: average error level over a narrow band spectrum, average error level over a 1/3 octave band spectrum, and dissimilarity measure. The narrow band error and dissimilarity increased with the source-to-receiver distance but converged to a certain value as the reverberant field became dominant. The 1/3 octave band error was found to be less dependent on the source-receiver distance. The errors are increased as the aspect ratio becomes more disproportionate. By changing the wall absorption from 0.2 to 0.8 for a rectangular room, the average narrow and 1/3 octave band error are deviated by around 1.5 dB. A realistic nonuniform distribution of the absorption increases the error, which might be ascribed to wave phenomena evoked by the impedance-discontinuous boundary.

In December 2005 construction work was started to replace a harbor wall in Kerteminde harbor, Denmark. A total of 175 wooden piles were piled into the ground at the waters edge over a period of 3 months. During the same period three harbor porpoises were housed in a marine mammal facility on the opposite side of the harbor. All animals showed strong avoidance reactions after the start of the piling activities. As a measure to reduce the sound exposure for the animals an air bubble curtain was constructed and operated in a direct path between the piling site and the opening of the animals' semi-natural pool. The sound attenuation effect achieved with this system was determined by quantitative comparison of pile driving impulses simultaneously measured in front of and behind the active air bubble curtain. Mean levels of sound attenuation over a sequence of 95 consecutive pile strikes were 14 dB (standard deviation (s.d.) 3.4 dB) for peak to peak values and 13 dB (s.d. 2.5 dB) for SEL values. As soon as the air bubble curtain was installed and operated, no further avoidance reactions of the animals to the piling activities were apparent.

The effects of material and treatment parameters on airflow resistivity and normal-incidence sound absorption coefficient (NAC) of compressed three-layer nonwoven composites have been studied. Material parameters included fiber size and porosity, and treatment factors included applied pressure and duration of compression. Fibers used included poly(lactic acid) (PLA), polypropylene (PP), glassfiber, and hemp. Three-layered nonwoven composites were classified based on material content and fiber blend. LHL and PGP were sandwiched structures consisting of PLA/Hemp/PLA and PP/glassfiber/PP layers, respectively. PGI consisted of three layers of an intimate blend of PP and glassfiber. Statistical models were developed to predict air flow resistivity from material parameters and the change in air flow resistivity from compression parameters. Independent variables in the first model were porosity and fiber size and, in the latter model, were compressibility, pressure, and initial material parameters. An increase in air flow resistivity was found with increased compression. No significant effect of compression duration was detected. Two additional statistical models were developed for the prediction of sound absorption coefficient based on material and treatment parameters. The independent variables of the first model were air flow resistivity, thickness, and frequency, and those of the second model were compressibility, initial thickness, and initial density of the composite, diameter and density of the fiber, compression pressure, and frequency. A decrease in sound absorption coefficient was detected with increasing compression, while no effect of duration was detected. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Recent development in noise control using T-shaped acoustic resonators calls for the development of more reliable and accurate models to predict their acoustic characteristics, which is unfortunately lacking in the literature. This paper attempts to establish such a model based on three-dimensional theory for T-shaped acoustic resonators containing sound absorption materials. The model is validated by experiments using various configurations. Predictions on fundamental and high-order resonance frequencies are compared with those obtained from the one-dimensional model and finite element analyses, and the effects of the physical and geometric parameters of the absorption materials on the resonance frequencies and Q-factor are also investigated numerically and experimentally. Limitations and applicability of existing one-dimensional models are assessed. The proposed general three-dimensional model proved to be able to provide an accurate and reliable prediction on the resonance frequencies for T-shaped acoustic resonators with or without absorption materials. This can eventually meet the requirement for resonator array design in terms of accuracy.

A recent paper [D. T. Bradley and L. M. Wang, J. Acoust. Soc. Am. 127, 223-232 (2010)] has reported inconsistencies between the results of two different approaches for characterizing non-exponential decays in coupled-volume systems. This letter aims to expose the origin of these inconsistencies, which are due to a limitation in the methodology utilized for the analysis presented in the paper referenced above.

The trade-off between the low-frequency sound absorption and ventilation efficiency in conventional acoustic absorbers limits their application potentials in many scenarios. Here, we design and experimentally implement a broadband compact acoustic absorber for absorbing low-frequency airborne sound, and the absorption coefficient above 0.5 ranges from 850 to 1000 Hz. We analytically analyze our proposed mechanism that uses double-layered perforated metastructure on each side to simultaneously lower the working frequency and ensures a high ventilation efficiency (with 70% cross-section open), which are verified both numerically and experimentally. Our work opens up possibilities for the design of acoustic absorbers working at a low frequency and may find practical applications such as noise control in small ventilating spaces.

The impedance of a simple artificial ear occluded with an earplug and bypassed with narrow air leaks was measured along with the attenuation of sound through the air leaks. A lumped element model is suggested for the simple occluded artificial ear with an air leak. The suggested model was adapted to the impedance measurements and the attenuation was predicted from the model. The attenuation predictions were compared to the attenuation measurements and were found to be within [-3.5,+3] dB of the measured attenuation over the frequency range of 50-1000 Hz and an attenuation range of -2-38 dB. The average difference between the measured and predicted attenuation for four different leaks in the frequency range of 50-1000 Hz was -0.7 dB, indicating a very slight underestimation of the attenuation.

Violin acoustics and concert hall acoustics are significantly different. Violins evolved three centuries ago, and since then every violin builder and owner has demanded an instrument that duplicates the physical properties and sound of that early period. Concert halls have evolved differently; architects have desired to make each a unique visual statement - certainly not a copy. Faced with this situation, acousticians have attempted to find a set of measurable characteristics that, if duplicated in halls of different appearances, will result in sounds that musicians and owners will find excellent, or at least satisfactory. This article recollects my efforts to isolate these characteristics - from the sequence of events that led to an understanding of how audiences in concert halls absorb sound, to finding acoustical measures needed for design and augmentation of concert hall acoustics.

When sound waves propagate in a duct in the presence of turbulent flow, turbulent mixing can cause attenuation of the sound waves extra to that caused by the viscothermal effects. Experiments show that compared to the viscothermal effects, this turbulent absorption becomes the dominant contribution to the sound attenuation at sufficiently low frequencies. The mechanism of this turbulent absorption is attributed to the turbulent stress and the turbulent heat transfer acting on the coherent perturbations (including the sound waves) near the duct wall, i.e. sound-turbulence interaction. The purpose of the current investigation is to understand the mechanism of the sound-turbulence interaction in low-Mach-number internal flows by theoretical modeling and numerical simulations. The turbulence absorption can be modeled through perturbation turbulent Reynolds stresses and perturbation turbulent heat flux in the linearized perturbation equations. In this thesis, the linearized perturbation equations are reviewed, and different models for the turbulent absorption of the sound waves are investigated. A new non–equilibrium model for the perturbation turbulent Reynolds stress is also proposed. The proposed model is validated by comparing with experimental data from the literature, and with the data from Direct Numerical Simulations (DNS) of pulsating turbulent channel flow. Good agreement is observed. QC 20150526

For many years geophysicists have attempted to exploit attenuation measurements in exploration seismology, because attenuation is perhaps the seismic property most closely related to the saturating fluid. The routine application of such ideas has proven elusive, however, largely because of the difficulty experienced when we attempt to measure attenuation in reflection data. Recent developments in the application of spectral decomposition methods to seismic data have opened the possibility of making further progress in this direction.