Experimental measurements of acoustical properties of snow and inverse characterization of its geometrical parameters

Snow is a sound absorbing porous sintered material composed of solid matrix of ice skeleton with air (+water vapour) saturated pores. Investigation of snow acoustic properties is useful to understand the interaction between snow structure and sound waves, which can be further used to devise non-destructive way for exploring physical (non-acoustic) properties of snow. The present paper discusses the experimental measurements of various acoustical properties of snow such as acoustic absorption coefficient, surface impedance and transmission losses across different snow samples, followed by inverse characterization of different geometrical parameters of snow. The snow samples were extracted from a natural snowpack and transported to a nearby controlled environmental facility at Patsio, located in the Great Himalayan range of India. An impedance tube system (ITS), working in the frequency range 63–6300 Hz, was used for acoustic measurements of these snow samples. The acoustic behaviour of snow was observed strongly dependent upon the incident acoustic frequency; for frequencies smaller than 1 kHz, the average acoustic absorption coefficient was found below than 0.4, however, for the frequencies more than 1 kHz it was found to be 0.85. The average acoustic transmission loss was observed from 1.45 dB cm⁻¹ to 3.77 dB cm⁻¹ for the entire frequency range. The real and imaginary components of normalized surface impedance of snow samples varied from 0.02 to 7.77 and −6.05 to 5.69, respectively. Further, the measured acoustic properties of snow were used for inverse characterization of non-acoustic geometrical parameters such as porosity, flow resistivity, tortuosity, viscous and thermal characteristic lengths using the equivalent fluid model proposed by Johnson, Champoux and Allard (JCA). Acoustically derived porosity and flow resistivity were also compared with experimentally measured values and good agreement was observed between them.     

Influence of cement flow and aggregate type on the mechanical and acoustic characteristics of porous concrete

The influence of cement flow and aggregate type on the mechanical and acoustic characteristics of porous concrete is systematically investigated in the present study. Three levels of cement flow (80%, 110%, and 140%) and five types of aggregates (normal aggregates of 8-13 mm and 13-19 mm and lightweight aggregates of 4-8 mm, 8-12 mm, and 12-19 mm) are used, and effects of the application of AE admixtures in paste were also studied. Single-layered and double-layered porous concrete specimens are fabricated to examine the effect of different layer configuration on the acoustic characteristics. For the purpose of comparison, the void ratio, compressive strength, and sound absorption coefficient of the specimens are used as evaluation parameters. Based on the findings of the study, a sound absorbing porous concrete with a maximum absorption coefficient of approximately 1.00 is developed, and the minimum absorption coefficient of the ‘double-layered porous concrete’ structure is shown to be more than 0.60 with a frequency of 400 Hz or above, considering the tolerant error.     

Study of radon transport through concrete modified with silica fume

The concentration of radon in soil usually varies between a few kBq/m³ and tens or hundreds of kBq/m³ depending upon the geographical region. This causes the transport of radon from the soil to indoor environments by diffusion and advection through the pore space of concrete. To reduce indoor radon levels, the use of concrete with low porosity and a low radon diffusion coefficient is recommended. A method of reducing the radon diffusion coefficient through concrete and hence the indoor radon concentration by using silica fume to replace an optimum level of cement was studied. The diffusion coefficient of the concrete was reduced from (1.63 ± 0.3) × 10⁻⁷ to (0.65 ± 0.01) × 10⁻⁸ m²/s using 30% substitution of cement with silica fume. The compressive strength of the concrete increased as the silica-fume content increased, while radon exhalation rate and porosity of the concrete decreased. This study suggests a cost-effective method of reducing indoor radon levels.     

Characterization of partially sintered ceramic powder compacts using fluorinated gas NMR imaging.

We use nuclear magnetic resonance (NMR) imaging of C2F6 gas to characterize porosity, mean pore size, and permeability of partially sintered ceramic (Y-TZP Yttria-stabilized tetragonal-zirconia polycrystal) samples. Conventional measurements of these parameters gave porosity values from 0.18 to 0.4, mean pore sizes from 10 nm to 40 nm, and permeability from 4 nm(2) to 25 nm(2). The NMR methods are based on relaxation time measurements (T(1)) and the time dependent diffusion coefficient D(Delta). The relaxation time of C2F6 gas is longer in pores than in bulk gas and it increases as the pore sizes decrease. NMR yielded accurate porosity values after correcting for surface adsorption effects. A model for T(1) dependence on pore size that accounts for collisions between gas molecules and walls as well as surface adsorption effects is proposed. The model fits the experimental data well. Finally, the long time limit of D(Delta)/D(o), where D(o) is the bulk gas diffusion coefficient is useful for measuring tortuosity, while the short time limit was not achieved experimentally and could not be used for calculating surface-area to volume (S/V) ratios.     

Acoustic absorption modeling of porous concrete considering the gradation and shape of aggregates and void ratio

The results of acoustic absorption modeling of porous concrete considering the gradation and shape of aggregates and void ratio are presented. To model the void texture of porous concrete, the multi-layered micro-perforated rigid panel model considering air gaps [1] and [2] is adopted. The parameters used in this acoustic absorption modeling are determined by a geometrical and experimental approach considering the gradation and shape of aggregates and void ratio. The predicted acoustic absorption spectra are compared with experimental results to verify the proposed acoustic absorption modeling approach. Finally, a parametric study is conducted to investigate the influence of design factors on the acoustic absorption properties of porous concrete.     

Acoustic properties of cenosphere reinforced cement and asphalt concrete

A detailed experimental study has been conducted to determine the effect of adding hollow ceramic-micro balloons, also known as cenospheres, on the acoustic properties of cement matrix and asphalt concrete. The motivation for this study was to explore the feasibility of using cenospheres in developing lightweight sound absorbing structural materials. Cement and asphalt concrete specimens with different volume fractions of cenospheres and varying diameter and thickness were tested to determine their acoustic characteristics over the range of frequencies (0-4000 Hz). Experimental results showed that a 40% volume fraction addition of cenospheres to cement matrix increased the Noise Reduction Coefficient by 100%. In contrast to cenosphere rich cement, the sound absorption coefficient of asphalt concrete decreased with an increase of cenosphere, volume fraction.     

Theoretical and experimental study of the ultrasonic attenuation in bovine cancellous bone

In this study a theoretical approach for the estimation of ultrasonic attenuation is proposed. The approach combines two models which take into account both absorption and scattering. Attenuation due to absorption is studied by using the Biot’s analytical model whereas that due to scattering is described by means of a generalized weak scattering model which is formulated for binary mixtures. The scattering model takes account of the density fluctuation of the porous medium in addition to the propagation velocity fluctuation. For the calculation of the attenuation coefficient due to absorption, experimental values have been used to link size of pores to porosity. The theoretical results have been compared with experimental data obtained on bovine cancellous bone samples filled with water. Using an immersion acoustic transmission method, the ultrasonic attenuation has been measured at a frequency range between 0.1 and 1.0 MHz for 12 bovine cancellous bone samples with a porosity range between 40% and 70%. The prediction of attenuation with this model appears to correspond more closely to its experimentally observed behavior. This study indicates that scattering is the predominant mechanism which is responsible for attenuation in trabecular bone. Furthermore, it shows that the density fluctuations contribute significantly to the phenomenon of attenuation and cannot thus be neglected.     

Heterogeneity and hierarchy of clinoptilolite porosity

The porous structure of the raw and the modified clinoptilolite has been investigated by nitrogen adsorption method, polarized light, scanning electron and atomic force microscopy as well as by thermogravimetric analysis. The relative moisture, specific gravity, bulk density and inter-particle porosity have been tested using thermostatic, pycnometric and hydrostatic weighing methods. The geometric method has been used to calculate the external surface area of the clinoptilolite crystallites in the clinoptilolite rock and to evaluate the volume of the clinoptilolite framework channels. Generally two types of porosities such as primary porosity and secondary one are observed. The primary porosity may be defined as microporosity presented by nanotube system of the clinoptilolite 3-dimensional aluminosilicate framework. The secondary porosity is formed by meso- and macropores, where the first ones are presented by slot pores determined mainly by cleavability of the zeolite crystallites. Total volume of the zeolite channels equals 0.332 cm³/g which has been obtained using a geometric method. Micropores volumes of the raw and the acid-treated samples are 0.0031 and 0.0342 cm³/g by nitrogen adsorption, respectively. According to thermogravimetric analysis the structural water lost at 150-750 °C placed the pore volume 0.087 cm³/g with the surface area 360 m²/g.     

Ultrathin porous pavement made with high viscosity asphalt rubber binder: A better acoustic absorption?

This work presents a non-destructive technique applied to ultrathin porous asphalt pavements that allows us to characterize their internal structure. Comparison between acoustic absorption data from experimental tests with an impedance tube and analytical simulation leads to an optimization of intrinsic parameters, such as tortuosity, airflow resistivity and porosity. The studied ultrathin specimens are of a high air void content porous mixture made with conventional polymer modified bituminous binder and high viscosity rubber binder. It has been found that one of the studied mixtures presents less air void content than specified by the manufacturer, and thus absorbs less noise as foreseen. Finally, in this case it appears that using a high viscosity asphalt rubber binder has no influence on maximum acoustic absorption but on the frequency of appearance of this maximum, tortuosity and airflow resistivity.     

Experimental Investigation on Pressure Drop and Heat Transfer Characteristics of Copper Metal Foam Heat Sink

Abstract

The effect of the cooling performance of a copper metal foam heat sink under buoyancy-induced convection is investigated in this work. Experiments are conducted on copper metal foam of 61.3% porosity with 20 pores per inch. The pressure drop experiment is carried out to find the permeability and foam coefficient of the porous media. It is found that the property of porous media changes by changing the angle of inclination of the porous media from a horizontal to a vertical position while keeping the orientation and porosity the same. The Hazen-Dupuit Darcy model is used to curve-fit the longitudinal global pressure drop versus the average fluid speed data from an isothermal steady-flow experiment across the test section of the porous medium. The study concludes that the permeability and foam coefficient for copper foam is found to be 1.11 × 10^?7 m² and 79.9 m^?1, respectively. The heat transfer study shows that the thermal performance of copper metal foam is 35–40% higher than the conventional aluminum metal heat sink under an actual conventional mode.     

The sound speed and attenuation in loose and consolidated granular formulations of high alumina cements

Clinkers of high alumina cements are separated into three granular formulations with particle sizes in the range 0.6-0.71 mm, 0.71-1.18 mm and greater than 1.18 mm. These are used to manufacture consolidated samples of porous concrete in an autoclave. The acoustic and microscopic properties of loose and consolidated porous samples of concrete are investigated using both experimental methods and mathematical modelling. Values of porosity, flow resistivity, tortuosity and parameters of the pore size distribution are determined and used to predict closely the sound speed, acoustic attenuation and normal incidence absorption coefficient of these materials. It is shown that high alumina cements do not require additional binders for consolidation and that the structural bonds in these cements are developed quickly between individual clinkers in the presence of water. The hydration product build-up during the consolidation process is insignificant which ensures good acoustic performance of the consolidated samples resulting from a sufficient proportion of the open pores. The value of porosity in the consolidated samples was found to be around 40%, which is close to that measured in some commercial acoustic absorbers. This work provides a foundation for the development of acoustically efficient and structurally robust materials, which can be integrated in environmentally sustainable concrete and masonry structures.     

Sound absorption properties of a sunflower composite made from crushed stem particles and from chitosan bio-binder

A recent study investigated the mechanical, thermal and acoustical properties of a bio-based composite made from crushed particles of sunflower stalks binded together by chitosan, a bio-based binder. The acoustical performance in absorption was found to be poor as the material was highly compacted and with low porosity. The present study focuses on the acoustical properties of a higher porosity composite, with lower density while the mechanical rigidity remains fairly high. A higher absorption coefficient is obtained. The experimental results on the absorption coefficient are compared to the prediction of a model involving 5 physical parameters (porosity, tortuosity, airflow resistivity, thermal and viscous characteristic lengths). The characterization methods to determine these parameters are described. The comparison between experimental and theoretical results shows that this material exhibits peculiar microstructural features. It is found that the sound absorption properties can involve dead-end pores or clusters and multiple porosity scales in the material.     

多重全内反射红外光谱原位研究混凝土渗透性

将多重全内反射傅里叶变换红外光谱(FTIR-MIR)技术应用于混凝土渗透性研究.考察了水灰比和养护龄期与混凝土微孔结构之间的关系及其对混凝土渗透性的影响.简要分析了水和硫酸根离子在混凝土中传输规律.原位FTIR-MIR研究表明,水在混凝土中的传输主要依靠混凝土内部孔隙两端的毛细势能梯度.SO2-4离子的传输过程则要考虑对流和浓度差引起的扩散两种方式以及SO2-4与混凝土中水化产物的反应.混凝土抗渗性能与其微观孔结构有着密切的关系,随着水灰比降低或养护龄期延长,混凝土孔隙率减小,孔的连通性下降,混凝土抗渗性能提高.     

Preparation of activated carbons from cherry stones by activation with potassium hydroxide

Using cherry stones, the preparation of activated carbon has been undertaken in the present study by chemical activation with potassium hydroxide. A series of KOH-activated products was prepared by varying the carbonisation temperature in the 400-900 °C range. Such products were characterised texturally by gas adsorption (N₂, −196 °C), mercury porosimetry, and helium and mercury density measurements. FT-IR spectroscopy was also applied. The carbons prepared as a rule are microporous and macroporous solids. The degree of development of surface area and porosity increases with increasing carbonisation temperature. For the carbon heated at 900 °C the specific surface area (BET) is 1624 m² g⁻¹, the micropore volume is 0.67 cm³ g⁻¹, the mesopore volume is 0.28 cm³ g⁻¹, and the macropore volume is 1.84 cm³ g⁻¹.     

Analysis of the alterations in porosity features of some natural stones due to thermal effect

Three types of commercially available carbonate rocks were used in the study to determine the effect of thermal treatment in the range from 100 °C to 500 °C on porosity features in terms of two different approaches such as pore shape factor and quality index values. The ratio of the ultrasonic velocity measurements before and after water saturation was used to differentiate porosity of pores from porosity of cracks under varying temperatures. It was found that, pores in Burdur Beige and Usak White are in the form of cracks, which are situated through inner structure. On the other hand, pores in Patara Limestone are in the form of porosity with lower pore shape factor values. Quality index calculation is another approach based on the comparison of the measured and theoretical ultrasonic velocity values. When the rocks were subjected to higher temperatures, internal stress was developed, crack lengths and numbers were increased and finally the higher pore shape factor and lower quality index values were obtained. This situation was proven by the higher water absorption values for all the stone types with the higher pore shape factor and lower quality index values depend on the noticeable increase in effective porosity values.     

Optical properties of microphytobenthic biofilms (MPBOM): Biomass retrieval implication

Microphytobenthos Optical Model (MPBOM) provides the optical properties, absorption coefficient and refractive index, of a laboratory simulated microphytobenthic biofilm using the reflectance measurements derived from HySpex laboratory images, with the final aim of estimating photosynthetically active biomass. The high correlation between this biomass, expressed in chlorophyll a (mg Chl a m⁻²) and the absorption coefficient at the corresponding absorption wavelength of Chl a (673 nm) made possible the estimation of biomass for any absorption coefficient calculated from reflectance measurements of any other data set. The latter was validated for an independent data set which performed an acceptable estimation of biomass in comparison with the biomass measured by HPLC (R²=0.93). Finally, this model is designed to be applied to hyperspectral images, like airborne or satellite, in order to map biomass in the field.     

Ultrasonic characterization of shrinkage microporosity in aluminum castings

Shrinkage microporosity in cast aluminum was characterized utilizing the frequency dependence of ultrasonic attenuation caused by scattering from the pores. Measurements were made with the plate specimen immersed in water, and, by using a focused transducer, spatial resolution of about 2 mm was obtained. An accurate measure of attenuation was obtained by comparing the specimen’s ultrasonic signal with that from a pore-free reference specimen. Although the attenuation could be fitted using a single spherical pore size, better fits were obtained by assuming a lognormal distribution of spheres. Pore volume fraction inferred from the lognormal fits overestimates the actual volume fraction, determined from density measurements, by the same factor for all volume fractions. The actual volume fraction is overestimated by more than 100%, due to the complicated, nonspherical pore shapes, and must be taken into account to obtain accurate values of porosity. The strong correlation (r²=0.97) between ultrasonic and density-derived volume fractions permits reliable, nondestructive laboratory measurements of porosity.     

Variation of microstructure with carbonation in lime and blended pastes

Carbonation, as a reaction of the curing process of both, cement and lime binders, modifies the microstructure. Several microstructure properties, namely porosity, pore size distribution, surface fractal dimension, and specific surface area have been investigated in this study to describe the effect of carbonation on microstructure. Both carbonated and non-carbonated pastes of lime and blended pastes of lime and cement having varying water/binder (W/B) ratios are studied. Results show that carbonation decreases the porosity, but not with the same intensity in all pore size ranges. The highest modification is between 0.03 μm and 0.01 μm in lime pastes and between 0.2 μm and 0.02 μm in 50% lime pastes, while in 80% lime pastes the modification is very small. It is also observed that carbonation is a function of the binder composition but not of the W/B ratio. Moreover, surface fractal dimension decreases during the carbonation process, while the specific surface area varies depending of the binder composition.     

Investigation and optimisation of a multipass resonant photoacoustic cell at high absorption levels

A theoretical and experimental investigation of photoacoustic (PA) signals in a resonant multipass PA cell with high background absorption (up to 29 m^-1) is presented. An analogous electric transmission line model including discontinuity inductances at cross section changes was used to model the first longitudinal acoustic mode of the multipass PA cell equipped with two buffer volumes. This model was validated with experimentally obtained results and used to predict the behaviour of the PA cell for different multipass arrangements and different buffer volume diameters. The highest PA signal is obtained for high pass number and large buffer radius. Increasing the absorption coefficient of the medium enhances the PA signal until a maximum is reached, leading to a minimum for the PA signal sensitivity. For a given background absorption, the number of passes required to maximise the sensitivity depends on the absorption coefficient. The model allows the determination of the best-suited number of passes for a given absorption coefficient and cell geometry. PACS 82.80.Kd; 42.62.Fi; 82.80.Gk     

Low frequency in situ metrology of absorption and dispersion of sound absorbing porous materials based on high power ultrasonic non-linearly demodulated waves

The present work is related to acoustic in situ free-field measurements of sound absorption in porous materials, such as cellular plastic foams, glass-wool or recycled felt materials. The emphasis is given towards fine metrology of absorption in view of potential industrial applications. A powerful ultrasonic array working at 40 kHz is used. It enables to measure absorption acoustical data down to 100 Hz due to the exploitation of the non-linear ultrasonic demodulation phenomenon in air. Fine measurements of acoustic absorption are compared to numerical predictions based on the “equivalent-fluid” model (when the squeleton frame is motionless), and to some measurements performed on a Brüel and Kjaer impedance tube. Dispersion curves are also measured and compared to the numerical predictions for some automotive felt materials which are compressed at various ratios. Data obtained with a dedicated portable instrument are also discussed for the same type of materials and configurations.