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This paper presents sound absorbing structure constructed using multiple layers of fibrous papers. Sound absorbing materials converts sound wave into thermal energy due to the porosity. The proposed sound absorbing structure has helical-like shapes in layered format with cyclic winding of the mulberry papers and coconut fibers. The acoustic properties changes depending on the thickness of the air cavity between the thin layers and porosity of the layers themselves. Sound absorption coefficients were measured using two-microphone impedance tube in order to investigate the acoustic property variations with porosity. The measured results were compared to predictions utilizing wave propagations through a slit for verification of the sound absorbing mechanism. A simple method for improving the sound absorption with less material by composite structure is proposed and verified through the experiment. Dynamic stiffness and loss factor of the helical-shaped sound absorbers were measured and compared by analyzing supporting properties to the vibrating beam. Depending on the construction method, the frame dynamic properties showed variation even when the sound absorption was identical. This information is required for optimal design of absorbing layers for specific noise control applications. [ABSTRACT FROM AUTHOR]

The massless Dirac electron shows a linear energy dispersion in topological insulators (TIs) and these materials posses interesting properties such as high mobility, tuneable density and protection against backscattering over the time reversal symmetry and strong spin-orbit interaction. The flexibility is also an important parameter for development of wearable devices specially THz detector for imaging and quantum devices for optoelectronics applications. In this research work first time Bi 2 Te 3 TIs thin film is fabricated on inhouse develop metallic multi wall carbon nanotubes flexible paper (MWCNT-FP) and characterized by terahertz-time domain spectroscopy acquired at room temperature. This research work delivers exclusive opportunities to explore interesting phenomena in topological surface states and topological phase transition in Bi 2 Te 3 TIs thin film. Here, we measured terahertz absorption, optical density conductance and conductivity. An excellent broad band absorption is observed between 0.02 and 3.0 THz. The real conductance level is scientifically quite linear below 3.0 meV and thereafter certain increase. Similar trend is observed for the study of an imaginary conductance with respect to photon energy. The optical conductivity is observed in the range of 0.02–2.0 THz. • The Bi 2 Te 3 TIs thin film on multi wall carbon nanotubes flexible paper (MWCNT-FP) is developed and examine by using THz spectroscopy. • The sample is measured over a wide frequency band ranging from 0.02 to 3.0. • A very good broad band absorption is observed between 0.02 and 3.0 THz. • We measured quantize optical density, conductance and conductivity. • The real and imaginary conductance are measured with respect to photon energy. [ABSTRACT FROM AUTHOR]

• Demonstration of the efficacy of FT-IR in characterizing materials. • Usage of FT-IR considering of thickness and absorption coefficient effects. • Explanation of dipolar electric moment, overtone bands, and Fermi resonance. • Vibrational modes in 1D, 2D, and 3D systems and IR spectrum representation. • Criteria for the correct FT-IR band identification. This paper investigates the application of Fourier-transform infrared (FT-IR) spectroscopy by analyzing a commercially available plastic bag as an example. The study investigates the vibrational modes, mechanical properties, and structural changes of a bare sample during a tensile test. By measuring the absorption bands and their intensities, valuable information about the chemical composition and the concentration of certain components in the plastic bag is obtained. The behavior of vibrational modes in one, two, and three-dimensional systems is studied, with a focus on their representation in the IR spectrum. The concept of the dipolar electric moment and phenomena such as overtone bands and Fermi resonance are also explained. The X-ray patterns of the bag show talc and polyethylene crystalline phases. Scanning electron microscopy (SEM) determines the thickness of the sample. Tensile tests are performed to investigate the mechanical behavior of the plastic bags. This study demonstrates the informative and educational capabilities of FT-IR spectroscopy in studying the vibrational, mechanical, and structural properties of materials, with an emphasis on the correct application of this technique. The study highlights the importance of considering the effects of thickness and absorption coefficient and applying the Beer-Lambert law to understand the changes in vibrational modes during tensile testing. The results contribute to a deeper understanding of the structural behavior of plastic bags and demonstrate the effectiveness of FT-IR spectroscopy in characterizing such materials. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, we study the electromagnetically induced transparency (EIT) of a quantum ring by considering both interband and intraband transitions, under the influence of external electric and magnetic fields, as well as Rashba spin–orbit interaction (RSOI). Four band k ⋅ p model along with effective mass one band approximation were respectively employed to compute valence and conduction bands energy eigenvalues. We investigate the effects of applied electric and magnetic fields, as well as RSOI and the ring dimensions, on EIT. Our results reveal that the external magnetic field and the ring dimensions have a great influence on the EIT phenomenon. Ultimately, our numerical results will be used to develop devices with controllable nonlinear optical absorption and refraction. • Kohn-Luttinger Hamiltonian for holes is derived within the cylindrical coordinates. • Fields effect as well as RSOI on interband EIT in a QR are investigated. • Magnetic field has a great influence on EIT and the transparency window. • The ring size considerably changes the transparency window. [ABSTRACT FROM AUTHOR]

Herein, we propose that the commonly used methylammonium cation (CH 3 NH 3 +) in CH 3 NH 3 PbI 3 (MAPbI 3) be replaced with ethyl-phosphonium cation CH 3 CH 2 PH 3 + (EP+), allowing stronger electronic coupling between the PbI 6 octahedra and the organic cation and thereby increasing its stability. This paper will examine EP + based hybrid perovskite (CH 3 CH 2 PH 3 PbI 3 or EPPbI 3) as an alternative absorber material for photovoltaic cells of high efficiency and that too at affordable processing costs. By using FP-LAPW + lo methodology as used in density functional theory (DFT), we have examined physical features such as the bandgap energy, distribution of valence electron density, DOS, and optical and thermodynamical coefficients. We also simulated the performance of solar cells by employing EPPbI 3 as PVA material using SCAPS-1D. The findings of the current study viz. an absorption coefficient surpassing 10 4 cm−1, a direct forbidden energy gap of 1.388 eV and simulated PCE of 31.8%, is enough to support applicability EPPbI 3 perovskite as a PVA material. [ABSTRACT FROM AUTHOR]

In this paper, we studied the effects of the annealing temperature of nickel oxide (NiO x) thin films on the structural, optical, and electrical transport properties. Thin NiO x was synthesized by the sol-gel method, followed by annealing at different temperatures varying from 200 to 450 °C. From the obtained AFM image data, an increase in the surface Ra value is observed with rising film annealing temperature. The decrease in film thickness with increasing annealing temperature is associated with significant densification of NiO x. During thermal annealing, a change in the redistribution of Ni/O elements is seen in the films. The increase in the intensity of the (1P)LO scattering peak in NiO x with rising annealing temperature is associated with a growth in the density of defects due to Ni vacancies. When the annealing temperature reaches 250 °C, an increase in Eg = 3.49 eV is detected; a further rise of the annealing temperature leads to a decrease in Eg = 2.92 eV. The increase in the index n of films with rising annealing temperature is associated with the formation of a denser and more ordered NiO x film. A rise in the annealing temperature of NiO x films leads to an increase in the value of the total resistance of the films, a decrease in the resistance of hole recombination at the NiO x /electrode interface, and the value of the conditional mobility of holes. • An increase in the RMS value of films with increasing annealing temperature is associated with an increase in grain size on the surface. • At an annealing temperature of 250 °C, an increase in Eg is observed; a further increase in the annealing temperature leads to a decrease in Eg. • An increase in the extinction coefficient and dispersion index is associated with the densification of the NiO x film with increasing annealing temperature. • An increase in film density with increasing annealing temperature leads to an increase in the NiO x resistance and a decrease in the value of the conditional hole mobility. [ABSTRACT FROM AUTHOR]

In this research paper, we studied the optical and nuclear shielding efficiency of newly developed BGO glasses with the following compositions (in wt%): 32Bi 2 O 3 –68GeO 2 , 42Bi 2 O 3 –58GeO 2 , 47Bi 2 O 3 –53GeO 2 , 52Bi 2 O–48GeO 2 , 62Bi 2 O 3 –38GeO 2. BGO glasses were prepared by traditional melt quenching method. To obtain the band gap values of fabricated BGO glasses, optical absorption spectra were used for evaluation of optical properties. The mass attenuation coefficients (μ/ρ) were achieved for prepared glasses at 0.015–15 MeV photon energies employing MCNPX Monte Carlo code and WinXcom program. Moreover, broad-range of nuclear shielding parameters for gamma ray, neutrons and charged particles such as mass attenuation coefficient, half value layer, effective atomic number, buildup factors, mass stopping powers, projected ranges, fast neutron removal cross sections and damage factors were calculated. The refractive index is calculated from E opt , As Bi 2 O 3 concentration is enhanced, E opt is also increased as well as the optical electronegativity and consequently the refractive index. In addition, the results showed that BIGE5 glass sample with highest Bi 2 O 3 contribution has excellent nuclear radiation shielding ability among the other fabricated glass samples. [ABSTRACT FROM AUTHOR]

• The underwater low-frequency sound absorption performance and mechanism of membrane-type acoustic metamaterials (MAM) are investigated by numerical simulation. • The effects of key parameters on absorption performance are explored by numerical simulation, and these are used to optimize the underwater sound absorption performance. • A parallel combination design of multiple MAM units is proposed to achieve broadband and efficient absorption of underwater low-frequency sound by the acoustical siphon effect, and the accuracy of numerical simulation is verified by the experiment. The underwater low-frequency sound absorption performance of membrane-type acoustic metamaterials (MAM) is investigated in this paper. The MAM unit is composed of a tensioned membrane, an upper cavity frame, a lower cavity frame, a steel backing and a cylindrical mass block located at the geometric center of the membrane. The sound absorption mechanism of MAM and the influence of critical structural parameters are analyzed by numerical simulation. The absorption performance of MAM is further optimized by the parallel combination design of multiple MAM units and the acoustical siphon effect. A design of MAM with 19 units is proposed to improve the absorption bandwidth. The calculation result demonstrates that the structure can effectively absorb sound in the low-frequency range of 250–750 Hz, and the average absorption coefficient is above 0.6. Lastly, the corresponding sample is fabricated, and the underwater sound absorption performance is tested in the standing wave tube. The accuracy of numerical simulation is validated by experiment. [ABSTRACT FROM AUTHOR]

The authors of recently published "Materials Science and Engineering B 281(2022)115754" try to estimate the optical conductivity (σ) using the refractive index (n) and the absorption coefficient (α) values through this relationship (σ = αn/4π). Unfortunately, this equation in this form is incorrect whether in Gaussian (G) or Standard International (SI) units due to the dimensions are not the same on the two sides of this relation. Such relationship must be corrected to the following form (σ = αnc/4π where c is the light velocity), which agree with that published in textbooks and research papers. According to the Gaussian system σ is measured by s - 1 while the authors use S c m - 1 units (see the Y-axis units of Figure 5) which corresponding standard international (SI) units. Such Scm−1 units should be corrected to s - 1 or multiplied by the term (4 π ε o where ε o = 8.85 × 10 - 12 F m - 1 ) which is the transformation factor form Gaussian to SI units. [ABSTRACT FROM AUTHOR]

• Creation of three transparency windows. • Absorption and refractive index depend on the length of quantum dot for z-polarization. • Absorption and refractive index depend on the radius of quantum dot for x-polarization. • Transparency window and FWHM do no depend on the size of quantum dot. • By increasing Rabi frequency, transparency window is increased. In this paper, electromagnetically induced transparency (EIT) is investigated in a four-level C-model of GaAs cylindrical quantum dot (QD) with parabolic potential. In these system three transparency windows is created. By solving the Schrӧdinger equation in effective mass, eigenfunctions and eigenvalues of QD are found. By obtained eigenfunctions, eigenvalues and with solving the density matrix equations in steady-state, susceptibility of cylindrical QD is found. Dependence of EIT, refractive index and absorption coefficient on control laser fields, radius and height of cylindrical QD, decay rates, control laser detuning and polarization of probe and control lasers are investigated. [ABSTRACT FROM AUTHOR]

Abstract Acoustic impedance is typically measured using an impedance tube, which requires a material sample physically fitted to the tube. However, the impedance can vary greatly between the material mounted in the tube and the material located in a real environment, where the mounting conditions are likely to be different. Also, oblique incidence cannot be measured in an impedance tube. In this paper, we investigate the use of a double-layer microphone array for in-situ measurement of surface impedance and absorption coefficient. With the array positioned near the material surface, a source emits broad-band sound towards the array and the material. A measurement is taken, and the sound pressure and the surface-normal particle velocity at the material surface are calculated using Statistically Optimized Near-field Acoustical Holography (SONAH). From the surface pressure and velocity, the impedance across a selected area is calculated, and finally the absorption coefficient is calculated from the impedance. A set of tests has been performed on porous material samples in an anechoic chamber as well as in a fitted room. Different sample sizes and different sound incidence angles have been considered. The results show consistency between the measurements in the anechoic room and the ordinary room as well as good agreement with Miki's model up to large oblique incidence angles. [ABSTRACT FROM AUTHOR]

Abstract This paper presents results of investigations of Fe+, Ge+, Mn+ ions implanted silicon samples with the use of the modulated free carrier absorption (MFCA) technique. These ions are significant in a modern electronics. The nondestructive character of the MFCA method makes it very useful for characterization of implanted materials. Spatial distributions and profiles of the amplitudes of the modulated free carrier absorption signal of the implanted silicon samples were measured and analyzed. The amplitude and phase frequency characteristics of the MFCA signal for the ion implanted regions have been measured and interpreted for different wavelengths of the illuminating light. The dependence of the amplitude of the MFCA signal on the lifetime of carriers and the optical absorption coefficient of the implanted layer has been analyzed theoretically and discussed. The influence of the ion implantation process on the values of the optical absorption coefficient and the lifetime of carriers of the implanted layers has been determined. Highlights • The influence of Fe+, Ge+, Mn+ ion implantation into silicon has been presented. • Frequency domain nondestructive MFCA method has been applied for investigations. • Changes of the optical and recombination parameters have been explained. [ABSTRACT FROM AUTHOR]

In this paper, an extension of the result of Wang et al. (“Modeling pulsed laser ablation of aluminum with finite element analysis considering material moving front,” Int. J. Heat & Mass Transfer , 113, 1246–1253, 2017) concerning the problem of uncertainty quantification for pulsed laser ablation (PLA) of aluminum is considered, when the source of uncertainty is due to an inherent randomness of the temperature-dependent absorption coefficient. In particular, we use a generalized polynomial chaos (gPC) method to incorporate the parameter uncertainty for the temperature-dependent absorption coefficient within the representation of the laser heat conduction phenomena. Furthermore, numerical simulation studies for the PLA of aluminum, with nanosecond Nd:YAG 266 nm pulsed laser, that demonstrate the proposed gPC predictions are presented. Finally, a sensitivity study is performed to identify whether small changes in the lower and/or upper parameter values of the absorption coefficient provide the most variance in the thermal and ablation responses. [ABSTRACT FROM AUTHOR]

In this paper the structural and optical properties of pure Polyvinyl alcohol (PVA) and La 3+ -doped PVA films in the concentration range of 4%, 12% and 20% weight percent of Lanthanum were prepared by the conventional casting technique. X-ray diffraction pattern and atomic force microscopy studies of the investigated samples reveal their semi-crystalline nature. It is found that, absorption coefficient and cluster size of lanthanum:PVA composite increase with increasing salt concentration. However, the optical energy gap shows a slight decreasing trend. [ABSTRACT FROM AUTHOR]

In this paper we present the optical properties of heterocyclic azo dyes containing polymers thin films. From spectroscopic ellipsometry combined with transmittance measurements we estimated the absorption coefficients, refractive indices and optical energy band gaps of studied compounds. One can notice that the optical properties of azo dyes polymers strongly depend on the type of substitution in the azobenzene moiety. The different substitution pattern in azobenzene moiety leads to a change in the value of refractive index and red shifts in the absorption spectrum. We also found that different polymer systems (such as guest-host system or side-chain) result in changes of the optical properties (such as the values of refractive index, optical energy band gap) of azo dyes. [ABSTRACT FROM AUTHOR]

In this paper, we report a theoretical study of the optical properties of InAs/In 0.52 Al 0.48 As quantum wire subjected to an inclined transverse electric field. For this purpose, we have used the effective mass approximation. The energy levels and the wave function of the ground and the first excited states have been calculated for different values of the electric field. The intersubband transition energy, the oscillator strength and the optical absorption coefficient have been also investigated. Thus, the effect of the tilt angle and the intensity of the electric field on the optical properties of the quantum wire, along the width direction, have been examined. [ABSTRACT FROM AUTHOR]

This paper presents the results of creating a system for diagnosing the defective structure of materials that are opaque in the visible region, based on the combination of methods: thermal imaging detection of large defects, digital holography of defects, wideband radiometry in the area of continuum absorption to measure transmission, determination of the complex refractive index of the dispersion medium and constants of interpolation polynomials for dispersion. It is shown that the combined use of interpolation polynomial constants and cluster characteristics allows determining the refractive index of the dispersed phase to further interpret its composition. • Use of interpolation polynomial constants allows determining the refractive index of the dispersed phase to further interpret its composition. [ABSTRACT FROM AUTHOR]

This paper reports two optical models of temperature dependent reflectivity, R, and absorption coefficient, α, during femtosecond laser interaction with aluminum. After a comparison with existing experimental data, the critical point model (CPM) was incorporated into a two-temperature model (TTM) to simulate laser energy deposition and thermal response in an aluminum film irradiated by femtosecond laser pulses. The dynamic change of optical properties R and α, distribution of laser heat density, electron and lattice temperature of an aluminum film were investigated. Compared with constant reflectivities and absorption coefficients, dynamic reflectivities and absorption coefficients are recommended to be employed in modeling ultrafast laser heating. [ABSTRACT FROM AUTHOR]

In this paper, the problem of determining the shape of plasmonic nano particle for maximum absorption coefficient in the visible part of light spectrum is considered. In order to determine a general optimal shape, a new linearization solution method in spherical coordinates is presented. Indeed, by using this approach which is based on an embedding process, the solution is determined via an appropriate finite linear programming problem by an iterative method based on particle swarm optimization (PSO) algorithm. This approach in comparison to the other methods has some advantages: the optimization is global and does not need any initial shape and mesh designing. This approach can be useful also in optical applications such as plasmonic nano antenna, plasmonic solar cell and plasmonic sensor. The numerical simulation results also show the efficiency of the proposed method especially in comparison. [ABSTRACT FROM AUTHOR]

In this paper, the absorption coefficient of the PbSe quantum well with random potential and different diameters is investigated. The Schrodinger equation is solved numerically and the optical properties are obtained using the density matrix approach. Results show that the absorption coefficient increases when size of the quantum well increases. Also, by increasing the size of the quantum well the absorption energy shifted to the lower energies. Finally absorption spectrums for different diameters and for two randomness cases are calculated. [ABSTRACT FROM AUTHOR]

Estimation of the temperature-dependent absorption coefficient in laser welding process is known as high non-linear inverse problem. Thus, it is necessary to propose a robust and efficiency method to estimate this coefficient. In this paper, a modified Newton–Raphson method combined with the concept of future time is presented to estimate the absorption coefficient in the laser welding process. The special characteristics of this method are that no preselected functional form for the unknown absorption coefficient is necessary and non-linear least-squares is not needed in the algorithm. Two examples have been fulfilled to demonstrate the proposed method. The results show that the proposed method is an accurate, stable, and efficient method to estimate the absorption coefficient in the laser welding process. In addition, two kinds of future time description are considered in this work. One is a linear type and the other is a constant type. From the estimated results, it is evidenced that the accuracy of the estimated results for the linear type of future time description is higher than that for the constant one of future time description. [ABSTRACT FROM AUTHOR]

• Prediction of sound transmission loss of hemispherical shell using statistical energy analysis. • Measurement of sound transmission loss of hemispherical shell using the sound intensity technique. • Measurement of dissipation loss factor of hemispherical shell using the decay rate technique. • Measurement of natural frequencies of the hemispherical shell using experimental modal analysis test. The hemispherical shells are broadly used in many industries and applications such as aerospace, submarines, tanks, and architectural structures. Sound transmission loss prediction of such structures is essentially required during the design and assessment phases from the acoustic point of view. In the present paper, sound transmission loss of the hemispherical shell is predicted analytically and validated experimentally. The Statistical energy analysis (SEA) technique is employed to formulate an analytical model for computing the sound transmission loss of a hemispherical shell across a wide frequency range. The sound intensity experimental method was used to verify the proposed SEA formulation. The analytical predictions and the measured transmission loss were found to be in good agreement. Based on the proposed SEA model, the influence of design parameters such as the absorption coefficient, thickness, radius, and different materials on the transmission loss was investigated. [ABSTRACT FROM AUTHOR]

In this paper, the thickness and doping concentrations of the double buffer cell-based conventional CIGSSe solar cells are optimized, and alignment of the energy band of the optimized solar cell (ZnMgO/CIGSSe) is carried out, thus obtaining an increase in efficiency from 23.35 % to 27.88 %. Further, a new design is proposed of low cost, low resistivity, minimum absorption coefficient, tunable energy bandgap (3.6–4.44 eV) with tin-doped manganese oxide Sn 1−x Mn x O 2 material (x = 0.5) as a window layer in conventional solar cell design. The thin-film solar cell efficiency has improved from 23.35 % to 29.05 % by aligning the energy bandgap with an optimized thickness and doping concentration. The proposed Sn 1−x Mn x O 2 window layer-based thin-film solar cell is more convenient in instantly absorbing the maximum blue light with reduced recombination loss, increased efficiency and external quantum efficiency. The simulated parameters of the conventional model are compared and validated with the existing conventional solar cells. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the optical absorption coefficient of hybrid structure consisting of metal nano particle (MNP) coupled to multi-layer spherical quantum dot (MSQD). Energy eigenvalues and eigenfunctions of Schrödinger equation in this structure are obtained by using numerical solution (by the fourth-order Runge–Kutta method). The effect of MNP in the vicinity of MSQD is calculated by considering local field theory. Then the variation of optical absorption coefficient hybrid structure is calculated. The results show that the presence of MNP near MSQD enhances the optical absorption coefficient. Also, by changing the distance between MNP and MSQD and radius of MNP, variation of optical absorption coefficient and refractive index changes are introduced. [ABSTRACT FROM AUTHOR]

In this paper, the influence of impurity parameters on the electron energy spectrum and absorption coefficients in a parabolic quantum dot and in the presence of Rashba spin–orbit interaction subjected to a perpendicular magnetic field is studied. The impurity potential is approximated by a Gaussian form. We have shown that in the both cases of a repulsive and attractive Gaussian impurity, the absorption coefficients are strongly affected by the decay length. These coefficients show blue (red) shift as the decay length of repulsive (attractive) impurity is increased. The dependence of the absorption coefficients on the impurity position is also examined for different polarizations. Our results show that the absorption coefficient has local maximum (minimum) for a given value of impurity position for Y-polarized (X-polarized) light. [ABSTRACT FROM AUTHOR]

This paper presents aerosol light absorption measurements using a seven channel Aethalometer (AE-42), recorded during September 2011–November 2012 over a tropical semi-arid site in Kadapa, India. The annual average black carbon (BC) mass concentration ([BC]) during the studied period was 2.20 ± 0.78 μg m − 3 which is in agreement when compared to other sites of similar environment. Strong seasonal variation was observed with high values during winter (2.87 ± 0.81 μg m − 3 ) and low in monsoon (1.30 ± 0.31 μg m − 3 ) season, which could be attributed to regional synoptic meteorology and long range transport. The two peaks in diurnal variations of [BC] were observed during morning between 06:00 and 08:00 h local time (LT) and in the evening around 19:00–21:00 h LT, with a minimum peak in the afternoon (~ 16:00 h LT) which is strongly correlated with the boundary layer dynamics, local and regional sources, and associated meteorology. The relationships between measured [BC] and the corresponding meteorological parameters for the studied region were also analysed. The estimated monthly mean spectral absorption coefficient values range from 0.56 to 1.15 with an average of 0.90, which suggests high BC/OC ratio and possible source of emission of aerosols is fossil fuel burning. Trajectory cluster analysis showed significant impact of long range transport of BC aerosols towards the observational site during the period of study. [ABSTRACT FROM AUTHOR]

This paper introduces a technique to make an ultra-black surface by employing nanoporous anodized aluminum oxide as a template and deposition of nickel–phosphorus nanowires by the electroless process. The optical properties were compared with two other processes; a conventional black Ni–P deposition and a nickel electro-coloring process, on aluminum substrate. Surface morphologies of the samples were examined by field emission scanning electron microscope and elemental analysis of the coatings was performed by the energy dispersive spectroscopy method. Optical properties of surfaces were determined by spectrophotometry and infrared spectroscopy techniques. In addition, optical characteristics of the coated surfaces were evaluated by calculation of absorption and emission coefficients of the surfaces. The results showed that ultra-black duplex coating possessed an absorption coefficient higher than 99%, while emission coefficient decreased about 6% compared with simple black electroless Ni–P. Calculation of ξ factor indicated that a value of 5.1 proved that optical properties in the duplex coated sample had a significant improvement. [ABSTRACT FROM AUTHOR]

In-depth radiation absorption significantly affects the ignition time and burning rate of flammable translucent materials. To improve the accuracy of flammability predictions by pyrolysis models, the spectral dependency of radiation sources and the depth of the computational cell in the sample should be considered. This paper provides the absorption spectra of black Poly(methyl methacrylate) (PMMA) in a wide spectral range of 0.25 to 25 μ m by measuring the high-resolution spectral transmissivity of thin samples. Compared to clear PMMA, black PMMA has a higher spectral absorption coefficient in the UV–Vis–NIR region (< 2.3 μ m). We extracted the complex index of refraction of black PMMA from the spectral absorption coefficient by applying the Kramers–Kronig transform (KK-transform). Furthermore, to investigate the effect of PMMA temperature on its absorption, the absorbance of the sample was measured with FTIR-ATR for the temperature up to the polymer's melting temperature. The results showed that the radiation absorption of PMMA may increase for higher material temperature. Implementing the measured optical constants, simulated temperature profiles in a layer of PMMA subjected to the heat flux of 15 and 28 kW/m 2 are obtained, showing good agreement with the experimental data. Finally, a set of data for the effective absorption coefficient as a function of source temperature and depth is proposed for the black PMMA to improve the gray modeling of radiation for pyrolysis simulations. [ABSTRACT FROM AUTHOR]

In this paper the optical absorption of the GeSn PIN photodetector was investigated. The vertical GeSn PIN photodetectors were fabricated by molecular beam epitaxy (MBE) and dry etching. By means of current density–voltage ( J – V ) and capacity–voltage ( C – V ) measurements the photodetector device was characterized. The absorption coefficients of GeSn material were finally extracted from the optical response of PIN structure. With further direct bandgap analysis the influences of device structure was proved negligible. [ABSTRACT FROM AUTHOR]

Silica aerogel is an excellent thermal insulation material with a low thermal conductivity and a high porosity and has attracted great concern in applications. This paper was to experimentally investigate the optical properties of optically thick silica aerogel in the visible, near-infrared and infrared spectrum region. The fiber-loaded silica aerogel sample was prepared through sol–gel technique and supercritical drying process. Silica fibers were added into the aerogel during the preparation procedure to strength the skeleton of aerogel. As a comparison with the fiber-load silica aerogel, a silica fiber composite sample with the same chemical component and different physical structure was also prepared. A simplified two-flux model neglecting the boundary effect was used to describe the radiation propagation characteristics inside the samples. The spectral normal-hemispherical reflectances, transmittances, and normal emittances of silica aerogel and silica fiber samples were measured and compared in the wavelengths of 0.38–15 μm. Then the spectral optical constants of samples were determined using the experimental data. The spectral absorption and scattering coefficients of silica aerogel were within (0.01 cm −1 , 31.0 cm −1 ) and (1.4 cm −1 , 25.8 cm −1 ). The results showed that the spectrum region where the scattering coefficient is low usually corresponds to a high absorption coefficient. In addition, the total radiation properties of samples were predicted at high temperatures. The analysis of optical properties of silica aerogel is necessary to provide valuable data in applications. [ABSTRACT FROM AUTHOR]

This paper proposes an evaluation procedure using a scale model to assess the noise reduction effects of vegetated façades for sustainable urban building designs. The absorption coefficients of the scale-model materials were measured to fit the absorption characteristics of real vegetation. The ground impedance of asphalt was also measured to deduce the acoustical properties of ground surfaces and to select the ground material. To assess the reduction of road traffic noise, a line source for a 1:10 scale model was modelled using ribbon tweeters that generated high frequencies ranging from 1 kHz to 40 kHz. Accordingly, the effects of adding vegetated façades were evaluated in the scale model of a street canyon. The noise reduction due to the vegetated façades was less than 2 dB at pedestrian level in a two-lane street canyon. The scale model results were compared with geometric computer simulation results, and both evaluation methods showed similar results. The suggested modelling method can be useful for evaluating the noise reduction in street canyons by vegetation considering realistic features such as the absorption, scattering, and diffraction associated with the materials and sound sources according to the shape of the vegetation. [ABSTRACT FROM AUTHOR]

Micro-perforated panel (MPP) has been widely used in the noise control field. However, the performance at low frequencies is relatively poor due to the restricted space limitations in actual applications. Hence in this paper a flexible micro-perforated panel absorber (PVDF-MPPA) based on Polyvinylidene fluoride (PVDF) piezoelectric film is proposed, which is composed of a perforated aluminum-electrode PVDF piezoelectric film and a small air cavity. Its sound absorption at low frequency can be improved by employing panel vibration effect and shunting the electrodes of piezoelectric film with some electrical impedance. The corresponding structure of the PVDF-MPPA was designed and fabricated. Experiments were performed under different shunting cases such as open circuit, short circuit, and resonant circuit in an impedance tube with the testing frequency range from 100 Hz to 1600 Hz. The results demonstrated that the PVDF-MPPA with shunt circuits had a wider absorption bandwidth compared with a rigid MPP absorber with the same structural parameters. When compared with the open circuit and short circuit cases, the PVDF-MPPA with resonant circuit definitely has a better low frequency absorption performance if the shunting parameters have been carefully tuned. By using shunt-damping technology, the PVDF-MPPA has a potential to be widely used as an efficient sound absorber in practical situations because of its simple structure and good absorption performance at low frequencies. [ABSTRACT FROM AUTHOR]

Objectives To use the Kubelka–Munk theory to evaluate the scattering ( S ), absorption ( K ) and transmittance ( T ) of non-colored and colored dental zirconia systems and human (HD) and bovine (BD) dentins. Methods Two zirconia systems were used: ZC- ZirCAD (Ivoclar Vivadent) and LV-LAVA (3M ESPE). Specimens from each ceramic system were divided into 3 groups ( n = 5): ZC1 and LV1 (non-colored); ZC2 and LV2 colored to shade A1, and ZC3 and LV3 colored to shade A3. Five human and bovine anterior teeth were flattened and polished through 1200 grit SiC paper to expose the superficial buccal dentin. All samples were prepared to a final thickness of 0.5 mm. Diffuse reflectance was measured against white and black backgrounds, using a spectroradiometer in a viewing booth with D65 illuminant and d/0° geometry. S and K coefficients and T were calculated using Kubelka–Munk's equations. Data was statistically analyzed using Kruskal–Wallis, Mann–Whitney tests, and VAF coefficient. Results Spectral distributions of S , K and T were wavelength dependent. The spectral behavior of S and T was similar to HD (VAF ≥ 96.80), even though they were statistically different ( p ≤ 0.05). The spectral behavior of K was also similar to HD, except for LV1 (VAF = 38.62), yet all ceramics were statistically different from HD ( p ≤ 0.05). HD and BD showed similar values of S and T ( p > 0.05). Significance The dental professional should consider the optical behavior differences between the zirconia systems evaluated and the human dentin to achieve optimal esthetics in restorative dentistry. [ABSTRACT FROM AUTHOR]

In this paper simultaneous effects of pressure, temperature and laser radiation on the optical absorption coefficient and refractive index of a spherical quantum dot with parabolic confinement and dressed impurity are studied. By means of matrix diagonalization technique, energy eigenvalues and functions are evaluated and used to find the optical properties of the system via density operator method. It is shown that linear and nonlinear optical properties strongly depend on pressure, temperature and dressing laser intensity. The interesting point is that the influence of laser radiation depends on pressure and temperature. [ABSTRACT FROM AUTHOR]

The temperature dependence of solar cells is a fundamental problem determines the device performance at unstable operating and environmental conditions. In this paper considering the temperature dependence of practical parameters, we have used an analytical drift-diffusion model to calculate the characteristic parameters of solar cell. We applied the Modified Tauc-lorentz temperature dependent model to describe the dielectric function of active layer, in order to study the performance of an organic P3HT: PCBM solar cell at different temperatures. Solving the steady-state continuity equation for electrons and holes regarding the dissociation probability of Onsager Braun along with bimolecular recombination rate, we have calculated the carrier's concentration in P3HT: PCBM organic active layer. The absorption coefficient and its temperature dependence is compared with the results of similar experimental data. [ABSTRACT FROM AUTHOR]

In the paper, we investigate spectral dependence of transmittance of a thin layer of a magnetic fluid with Fe 2 O 3 nanoparticles and transformer oil ITO 100 as base fluid, as a function of its thickness. Thin layer samples of the magnetic fluid with various thicknesses were prepared using standard microscope glass slides and spacers with defined thicknesses ranging from 25 µm to 530 µm. The stable spatial localization of the thin layer was ensured due to the existence of a surface tension emerging at the interface between the magnetic fluid surface and surfaces of the microscope glass slides. The transmittance as a function of the layer thickness was investigated in visible (VIS) and near-infrared (NIR) spectral ranges and spectral dependence of the coefficient of absorption of the magnetic fluid was determined, too. At the same time, the change of color of the layers depending on their thicknesses was evidenced. The results show the transmittance dependence on the thin layer thickness can be used to design magnetic fluid-based passive optical attenuators and filters. [ABSTRACT FROM AUTHOR]

In a recent paper [Physica E 114(2019) 113562] Jafarzadeh et al. have studied the optical and dielectric properties of Ag nanoparticles decorated by herceptin. The reflectance spectra of the specimens have been calculated using an incorrect approach and obtained results are unreasonable. Also, the erroneous values of reflectance have been used to study the optical properties of the samples. Consequently, reported optical parameters that have been calculated based on reflectance such as refractive index, extinction coefficient, as well as real and imaginary parts of the optical dielectric constant are incorrect. • The reflectance spectra have been calculated using a wrong approach. • The reported values of the refractive index are incorrect. • The reported values of the extinction coefficient are erroneous. • The calculated values of the optical dielectric constant are wrong. [ABSTRACT FROM AUTHOR]

Two-dimensional Janus transition metal dichalcogenides structures have drawn increasing importance due to their remarkable properties for versatile applications in optoelectronic, photo-catalytic, thermo-electricity, piezoelectricity and spintronic. In this Paper, by employing the Ab-initio computations based on the density functional theory, the structural, electronic and optical properties of Janus MXO (M = Ti, Hf and Zr; X = S and Se) mono-layers are investigated utilizing full potential linearized augmented plane waves (FP-LAPW) method. The lattice parameters of the six Janus were computed, which are close to the previous theoretical results. The density of states and the electronic band structures were investigated for the first time using (GGA-PBE) approximation for the potential of the exchange and correlation. The optical parameters like complex dielectric function, refractive index, reflectivity, extinction and absorption coefficients of all Janus were performed. Our results reveal strong absorption coefficient and low reflectivity in the visible and ultraviolet regions, which make them candidates for opto-electronic and photovoltaic applications. Image 1 • Janus transition metal oxides adopt a similar structure as transition metal dichalcogenides with MX 2 formula. • TiSO mono-layer is a semiconductor with a direct band gap. • Low reflectivity is observed for all MXO nanosheets. • Refractive index, extinction coefficient and optical conductivity are calculated. • All MXO mono-layers exhibit strong absorption coefficient in the visible and ultra-violet regions. [ABSTRACT FROM AUTHOR]

• A semi-analytical model was proposed to retrieve Chl- a using satellite imagery. • The model was based on the inherent optical properties (IOPs). • Chl- a distribution was mapped in inland waters from oligo-to-hypereutrophic status. • The model error was higher for hypereutrophic waters. • The model can be used operationally for long-term monitoring of the trophic status. Previous studies have shown the challenges in using a single model to estimate chlorophyll- a concentration (Chl- a) in water bodies with widely differing characteristics. A single model based on remote sensing to map the Chl- a distribution across the entire Tietê River Cascade System (TRCS) serves as a cost and time-efficient alternative to the conventional monitoring by providing trophic status over space and time. The Tietê River contains one of the largest cascade reservoir systems in the world, which sustains important ecological and socio-economic activities in the São Paulo State, Brazil. Surplus nutrients in water draining its surrounding catchments have been the main cause of eutrophication in the reservoirs of the TRCS. To assess the trophic state of the reservoirs, Chl- a has been regularly monitored by sampling points. However, they are limited by operational costs and dependent on weather conditions. Moreover, the current sampling method only produces point-based measurements. In this paper, we calibrate remote sensing images based on the absorption coefficient to map the spatial distribution patterns of Chl- a levels in the reservoirs. Mapping is done by estimating the Chl- a concentration. The absorption coefficients were retrieved from OLI/Landsat images using the Quasi-Analytical Algorithm (QAA). The total absorption (a t) in 482 nm and 655 nm retrieved by QAA presented NRMSE of 17% and 18.5%, respectively. Both a t (482 and 655 nm) were used in the model calibration and presented a satisfactory result covering all data ranges, with R2 of 0.646 and NRMSE of 15.3%. The proposed model in this study to retrieve Chl- a maps with relatively high accuracy can be incorporated into the operational monitoring system of the TRCS at a low cost that can provide timely information for reservoir managers to carry out necessary actions. This may include mitigating environmental impacts caused by sudden algae blooms. [ABSTRACT FROM AUTHOR]

• Periodic signals lead to near 100 times faster processing for computing sound absorption. • Absorption coefficient determination using random inputs requires a high number of averages. • Periodic signals present a higher signal to noise ratio than random signals. This work evaluates the effect of the excitation signal used when measuring the absorption coefficient on an impedance tube. This paper aims to offer some guidance on the selection of the excitation signal to perform sound absorption measurements using the impedance tube. Four possible excitation signals defined in ISO 10534-2 Standard were studied: two of them, random noise and two of them sine sweep signals. Some hypotheses tests were executed to assess the homogeneity of each measurement. The signal-to-noise ratio (SNR) was also computed to verify the measurement quality. The results show that the best performing approach was accomplished using a logarithmic sweep, giving more precise sound absorption curves with an SNR of 34.15 dB. Random signals reported similar SNR (greater than 30 dB) after executing an average with 100 repetitions. [ABSTRACT FROM AUTHOR]

• A 3D model of the perforated plate wedge was created in COMSOL Multiphysics. • The experiment in a real impedance tube based on the ASTM C384 verified the model. • The effects of perforated plate properties on wedge absorption were studied. • Influences of geometrical parameters on wedge cutoff frequency were investigated. • 5- Influences of geometrical parameters on absorption coefficient were studied. The design of sound-absorbing wedges as the main elements used in anechoic rooms is of great importance. There are general standards for the design of an anechoic room and sound-absorbing wedges. The absorption coefficients of the wedges can be obtained in two ways, a numerical technique such as the finite element method and an experimental test either in the impedance tube or in the reverberation chamber. In this paper, the absorption coefficient of a wedge covered by a perforated plate is obtained by simulating the 3D wedge in an impedance tube using COMSOL Multiphysics. To verify the finite element model, the perforated plate wedge is fabricated and tested in a real impedance tube based on ASTM C384. A comprehensive study is carried out on the effect of geometrical parameters on the sound absorption coefficient. These parameters include geometrical properties of the perforated plate such as hole diameter, hole percentage, plate thickness, and geometrical properties of the wedge, such as hub thickness, air gap thickness, wedge cross-section, and wedge width. Moreover, side wedges (semi wedges), the gap at sides of the wedges and lack of absorptive material in the wedge tip under the perforated panel are investigated. Finally, the effect of the arrangement of the wedges on the sound absorption coefficient is also studied. [ABSTRACT FROM AUTHOR]

• Kaolin clay does not create a perfect hydraulic contact. • High absorptive samples assembled with kaolin show an interface resistance (R IF) in the order of 9 000 m/s and 25 000 m/s. • For low absorptive samples, the interface resistance is negligible. • The distance from contact interface to water plain influences the impact of the R IF in a free water uptake experiment. In literature, kaolin clay is often put forward to create a perfect hydraulic contact across the interface of building materials. This paper studies, based on imbibition experiments, if kaolin clay truly nullifies a hydraulic interface resistance between materials. Thereto, a comparison is made between the water uptake by monolithic samples and by samples composed using kaolin clay. An interface resistance is observed and estimated to be in the order of 6 000 m/s to 12 000 m/s for a high absorptive brick and around 25 000 m/s for calcium silicate. For low absorptive materials or for an interface far from the water plain, a hydraulic interface resistance is probably also present. Though, in this case, the interface resistance is too low to have a noticeable impact on the cumulative inflow. [ABSTRACT FROM AUTHOR]

This paper aims at consolidating research dealing with vibroacoustics of periodic media. The main goal of this work is to develop and to validate tools for the design of global vibroacoustic treatments based on periodic patterns, allowing passive control of acoustic paths in layered concepts. To this aim, some enhancements are introduced to the state of the art on the study and the design of Biot-modeled foams, in order to obtain desired acoustics performances through embedded periodic inclusions. At first, the properties of the studied acoustic package, constituted by a poro-elastic 3D unit cell, is introduced. Successively, through the use of the acoustic-structure coupling that comes from the implementation of Biot model, a non-rigid inclusion test campaign is carried out by considering some solid (but still non-perfectly-rigid) inclusions in a 3D-modeled unit cell; in particular, six setups are discussed herein. Then, some design guidelines are provided in order to predict at which frequency the first performance peak appears, together with its amplitude, as functions of unit cell dimensions, airflow resistivity, tortuosity, viscous and thermal characteristic lengths, frame density, Young modulus and loss factor; conversely, it is shown also the link between the unit cell dimensions and the first performance peak amplitude as functions of the design frequency. [ABSTRACT FROM AUTHOR]

In a recent paper [J. Alloys Compd. 817 (2020) 152711] Alasvand and Kafashan have studied the physical properties of the electrodeposited CdSe thin films. As shown in this comment, the authors have used two wrong equations to calculate the absorption coefficient, α, and refractive index, n, of the samples. Consequently, the rest of the calculated parameters based on the α and n such as optical band gap, extinction coefficient, complex dielectric constant, dissipation factor and evaluated dispersion parameters using the Wemple-DiDomenico single oscillator model and Walton-Moss model, are erroneous. • The absorption coefficient and refrective index have been studied using incorrect relations. • The reported values of energy band gap, extinction coefficient, dielectric constant and dissipation factors are wrong. [ABSTRACT FROM AUTHOR]

Owing to high power conversion efficiency and low-cost, methyl-ammonium lead-based halide (viz. CH 3 NH 3 PbI 3) Perovskites have been emerging as the innovative candidate in the development of optoelectronic devices. However, the toxic lead in these materials is a major hurdle in its commercialization. Thus, there is an urgent need to replace lead with an appropriate element. Ethyl-ammonium based lead-free hybrid halide perovskites may be an alternative photovoltaic (PV) absorber material with appropriate band gap, high stability and non-toxic properties. Herein, we have investigated structural, electronic, optical, thermoelectric and thermodynamic properties of ethyl-ammonium germanium iodide (CH 3 CH 2 NH 3 GeI 3 or EAGeI 3) by full-potential augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code within the density functional theory (DFT). In this paper, we have found that EAGeI 3 has direct band gap of 1.3 eV and high absorption coefficient greater than 104 cm−1 indicating its suitability as PV absorber material. We have also calculated thermoelectric coefficients as a function of carrier concentration, chemical potential and temperature. The thermodynamic calculations have been done within the quasi-harmonic approximation. As EAGeI 3 has been studied first time for PV applications, the present study may open a new vista for more exhaustive experimental and theoretical investigations in search of non-toxic and eco-friendly PV materials. • Physical properties of ethylammonium germanium iodide (CH 3 CH 2 NH 3 GeI 3). • EAGeI 3 is a direct band gap semiconductor. • Thermoelectric coefficients as a function of carrier concentration. • Thermodynamic properties have been done within the quasi-harmonic approximation. [ABSTRACT FROM AUTHOR]

• A derived relation of the photoacoustic signal from multi-structures is presented. • Unlike predecessor models, light reflections at layers' interfaces are accounted. • Determination of the absorption coefficient spectral behavior of an unknown layer. • Experimental applications include an example of a three-layered sample. This paper reports on a theoretical relation for the normalized photoacoustic amplitude signal for use in determining the optical absorption coefficient spectrum of any layer of multilayered semiconductors' samples. The derived relation is for use in gas-coupled photoacoustic cells. The model, unlike its predecessors so far reported in the literature that are usually for specific cases in dependence of the optical properties of the sample, takes into considerations the geometrical sample, the photoacoustic cell parameters, the thermal reflections as well as the light reflections at different interfaces. We present the results analysis on the effect of increasing the number of layers on the normalized photoacoustic response by considering particular applications in optoelectronic devices' configurations. A computer program has been developed to extract the absorption coefficient spectral distribution of any particular layer. Three specific experimental applications are provided in evaluating the performance of the present model. Unlike conventional spectroscopic techniques, the photoacoustic method is known for high detectability in the region of weak absorption. Hence, the applications we present here include the cases of a single transparent sample, a thin solid single crystal and embedded layer in a three-layered sample. [ABSTRACT FROM AUTHOR]

Micro-perforated panel (MPP) absorber is increasingly gaining popularity as an alternative sound absorber in buildings compared to the well-known synthetic porous materials. A single MPP has a typical feature of a Helmholtz resonator with a high amplitude of absorption but a narrow absorption frequency bandwidth. To improve the bandwidth, a single MPP can be cascaded with another single MPP to form a double-layer MPP. This paper proposes the introduction of inhomogeneous perforation in the double-layer MPP system (DL-iMPP) to enhance the absorption bandwidth of a double-layer MPP. Mathematical models are proposed using the equivalent electrical circuit model and are validated with experiments with good agreement. It is revealed that the DL-iMPP produces a wider half-absorption bandwidth compared to the conventional homogeneous double-layer MPP. The absorption bandwidth to higher frequencies can be effectively controlled by reducing the air cavity between the iMPPs and to the lower frequencies by increasing the back cavity depth at the second layer. [ABSTRACT FROM AUTHOR]

Tumor genesis is accompanied by glycosylation of related proteins. Glycoprotein is usually regarded as a tumor marker since glycoproteins are consumed remarkably more by the cancer cells than the normal ones. In this paper, the terahertz time-domain attenuated total reflection (ATR) technique is applied to inspect the glycoprotein solution from a concentration gradient of 0.2 mg/ml to 50 mg/ml. A significant nonlinear relationship between the absorption coefficient and the concentrations has been discovered. The influence of the dynamical hydration shell around glycoprotein molecules on the absorption coefficient is discussed and the phenomenon is explained by the concepts of THz excess and THz defect. In order to identify glycoproteins, features are obtained by composite multiscale entropy (CMSE) method and clustered by the K-means algorithm. The results indicate that features extracted by the CMSE method are better than the Principal Component Analysis (PCA) method in both specificity and sensitivity of recognition. Meanwhile, the absorption coefficient and dielectric loss angle tangent are more suitable for qualitative identification. Research shows that the CMSE method has important directive significance for analyzing glycoprotein terahertz spectroscopy. And it has the potential for glycoprotein related tumor markers identification using terahertz technology in medical applications. Unlabelled Image • Terahertz technique is applied to inspect the glycoprotein solution. • The composite multiscale entropy method is introduced to identify glycoprotein sialylation. • The proposed method has a better result in specificity and sensitivity of recognition • It has potential for glycoprotein related tumor markers identification in medical. [ABSTRACT FROM AUTHOR]

In this paper we theoretically investigate the role of hydrostatic pressure by analyzing its influence on potential barrier's height in GaAs/Al x Ga 1 − x As core/shell spherical quantum dots. The values of hydrostatic pressure considered here are always below the Γ − X crossover. In addition, we take into account the barrier shell's size effects and the barrier's aluminum concentration, looking for a description of the features of the intraband optical absorption coefficient in the system. The electronic structure is calculated within the effective mass approximation. From the numerical point of view the hybrid matrix method was implemented to avoid numerical instability issues that appears in the conventional transfer matrix method. The main intersubband optical transition is considered to take place between the 1 s and 1 p computed electronic states. The results show that the absorption coefficient undergoes first a red-shift and later a more pronounced blue-shift, depending on the Al x Ga 1 − x As barrier width (w b 1). The absorption coefficient experiences a blue-shift as the barrier's aluminum concentration increases, and it is non monotonically red-shifted as the hydrostatic pressure augments, due to the barrier's height pressure dependency. For the chosen system parameters, the absorption coefficient resonant peak lies within the range of 20 to 30 meV, that corresponds to the THz frequency region. Accordingly, this system can be proposed as a building block for photodetectors in the THz electromagnetic spectrum region. [ABSTRACT FROM AUTHOR]

In room acoustic simulations the surface materials are commonly represented with energy parameters, such as the absorption and scattering coefficients, which do not carry phase information. This paper presents a method to transform statistical absorption coefficients into complex surface impedances which are needed for phased or time-domain calculation methods. Two 5-parameter impedance models based on fractional calculus are suggested to achieve a general model for common acoustic materials, thereby ensuring that the impedance found has a physical meaning. The five parameters for the general models are determined by solving an inverse problem with an optimization method. Due to the non-uniqueness of retrieving complex-valued impedances from real-valued absorption coefficients, prior information about the absorber of interest can be used as constraints, which is shown to help determine the impedance more correctly. Known material models, such as Miki's and Maa's models, are taken as references to assess the validity of the suggested model. Further stability and sensitivity investigations indicate that the method presented constitutes an efficient solution to convert sound absorption coefficients back to their original complex surface impedances. [ABSTRACT FROM AUTHOR]