Non-acoustical Parameters Research Articles

Prediction of acoustic properties of polyurethane foams from the macroscopic numerical simulation of foaming process

This work aims to present a method to predict the acoustic properties of flexible polyurethane foams from the numerical results of a macroscopic simulation of the foaming process. The foaming process simulation is carried out using a meshless method and a set of models taking into account the main chemical reactions, the exothermic effect as well as the thermo-rheo-kinetic coupling. To predict the acoustic properties of the foam, a semi-phenomenological model is used to avoid the experimental characterization of the foam microstructure. The distribution of the non-acoustical parameters involved in the Johnson-Champoux-Allard model were determined using the proposed method for a square panel mold. These results are used to calculate the normal-incidence sound absorption coefficient, which is compared to experimental and numerical results presented in the literature. The predicted acoustic behavior of the polyurethane foam is in good agreement with results in previous works.

Acoustic media with pore size distribution and assignment of the pore characteristic lengths and permeabilities

The model by Champoux and Allard [J. Appl. Phys. 70(4), 1975-1979 (1991)] is used extensively for research and development of new porous media solutions and for predicting sound propagation in the presence of porous media. Four key non-acoustical parameters in this model which are rarely measured non-acoustically are: the viscous and thermal characteristic lengths, thermal permeability and Pride parameter Pride et al [Phys. Rev. B 47, 4964-4978 (1993)]. In this work we show how these parameters unambiguously relate to the pore size distribution which is a characteristic measured nonacoustically and more routinely. We compare the predictions by this model against the log-normal pore size distribution model by Horoshenkov et al. [J. Acoust. Soc. Am. 104, 1198-1209 (1998)] and show that these two models provide very close predictions when the four non-acoustic parameters are expressed through the mean pore size and its standard deviation. We also confirm these results through a 4-microphone impedance tube experiment in which we determine the dynamic density and complex bulk modulus of a range of porous media. We also study the values of the viscous and thermal characteristic lengths and viscous and thermal permeabilities which fit our acoustic data.

Wind Turbine Noise - A Pilot Study in India

In the context of achieving a national set target of 60 GW by 2022, a study was carried out in Kayathar, Tuticorin district, Tamil Nadu, India on noise propagation from wind turbines. Acoustic and non-acoustic parameters were measured according to IEC 61400-11 at wind speeds of 4-13 and 0.7-8.6 m/s. The onsite measurements were compared with ISO 9613-2; this showed a difference of ± 5 dB(A) for every increment in the distance. An equation was derived to obtain sound pressure level (dB(A)) at the base of the tower for wind turbines at varying distance of measurement.

Inverse acoustical characterization of natural jute sound absorbing material by the particle swarm optimization method

Abstract For modeling of jute as acoustic material, knowledge of its non-acoustical parameters like porosity, tortuosity, air flow resistivity, thermal and viscous characteristic lengths is a prime requisite. Measurement of these non-acoustical parameters is not straightforward and involves a dedicated measurement setup. So in order to overcome this issue, the inverse acoustical characterization can be used. In this paper, the particle swarm optimization method (PSO) is used as an optimization method. This method estimates the non-acoustical parameters of jute material in felt form by minimizing the error between experimental and theoretical sound absorption data. In this work, the impedance prediction models for fibrous materials like Johnson–Champoux–Allard model with rigid and limp frame and Garai–Pompoli model is used for sound absorption coefficient calculation by the transfer matrix method along with the PSO. The inverse estimated non-acoustical parameters for jute material are then compared with estimated and experimentally measured parameters for jute felts. Using these inversely predicted parameters, sound absorption of multilayer sound absorbers is also studied.

Effect of non-acoustic parameters on heterogeneous sonoporation mediated by single-pulse ultrasound and microbubbles

Sonoporation—transient plasma membrane perforation elicited by the interaction of ultrasound waves with microbubbles—has shown great potential for drug delivery and gene therapy. However, the heterogeneity of sonoporation introduces complexities and challenges in the realization of controllable and predictable drug delivery. The aim of this investigation was to understand how non-acoustic parameters (bubble related and bubble-cell interaction parameters) affect sonoporation. Using a customized ultrasound-exposure and fluorescence-imaging platform, we observed sonoporation dynamics at the single-cell level and quantified exogenous molecular uptake levels to characterize the degree of sonoporation. Sonovue microbubbles were introduced to passively regulate microbubble-to-cell distance and number, and bubble size. 1MHz ultrasound with 10-cycle pulse duration and 0.6MPa peak negative pressure were applied to trigger the inertial collapse of microbubbles. Our data revealed the impact of non-acoustic parameters on the heterogeneity of sonoporation. (i) The localized collapse of relatively small bubbles (diameter, D<5.5μm) led to predictable sonoporation, the degree of which depended on the bubble-to-cell distance (d). No sonoporation was observed when d/D>1, whereas reversible sonoporation occurred when d/D<1. (ii) Large bubbles (D>5.5μm) exhibited translational movement over large distances, resulting in unpredictable sonoporation. Translation towards the cell surface led to variable reversible sonoporation or irreversible sonoporation, and translation away from the cell caused either no or reversible sonoporation. (iii) The number of bubbles correlated positively with the degree of sonoporation when D<5.5μm and d/D<1. Localized collapse of two to three bubbles mainly resulted in reversible sonoporation, whereas irreversible sonoporation was more likely following the collapse of four or more bubbles. These findings offer useful insight into the relationship between non-acoustic parameters and the degree of sonoporation.

A comparison of impedance models for the inverse estimation of the non-acoustical parameters of granular absorbers

An impedance model relates acoustic properties of porous materials with non-acoustical parameters. Although such parameters can be measured, specific equipment is required for each of them, so that numerical methods have been proposed for estimating their value from a more manageable measurement of the normal incidence absorption coefficient in an impedance tube. This inverse procedure requires both an impedance model and an inversion technique. This paper compares four impedance models, Miki, Hamet–Berengier, Johnson–Allard–Champoux and Champoux–Stinson, when Simulated Annealing is used for the inverse estimation of the non-acoustical parameters of three granular materials, consisting of packings of small spherical glass beads of different diameters. Some of these parameters have also been measured, so that they can be compared with these estimated by the proposed inverse method.

A simplistic unit cell model for sound absorption of cellular foams with fully/semi-open cells

Abstract We present a simplistic yet accurate unit cell model of sound absorption for highly porous foams having either fully-open or semi-open cells. Employing an idealized periodic cubic unit cell to mimic the foam topology, we establish an analytical model for predicting the sound absorption coefficient of the foam. Analytical links between key non-acoustic parameters (viscous permeability and flow tortuosity) and cellular topological characteristics are also established. For fully-open foams, the model requires only two morphological parameters, porosity and pore window size, significantly reducing the complexity of sound transport modeling. For semi-open foams, an additional parameter, open-pore rate representing the influence of thin membrane covering on pore window, is introduced. The analytical model predictions are compared with existing experimental data and numerical simulation results for polyurethane foams, with good agreement achieved.

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 63–6300Hz, 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 1kHz, the average acoustic absorption coefficient was found below than 0.4, however, for the frequencies more than 1kHz it was found to be 0.85. The average acoustic transmission loss was observed from 1.45dBcm−1 to 3.77dBcm−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.

Inverse estimation of the non-acoustical parameters of loose granular absorbers by Simulated Annealing

Absorbing materials are fundamental for the acoustic design of buildings. They are used for increasing the insulation from either outdoor or neighbour noise and for raising the acoustic comfort of dwellings. If environmental and health issues are not of concern, conventional sound absorbers, namely synthetic foams and fibrous materials, offer an excellent acoustical performance at a moderate cost. However, the current trend towards eco-efficient products encourages the enhanced use of natural fibre and loose granular recycled materials. Whatever the material type, a model is required to reliably predict its acoustical performance. The microstructural Champoux–Stinson model has demonstrated to provide consistent acoustical predictions once five non-acoustical parameters are given. Instead of measuring these non-acoustical parameters, which is a rather sophisticated and time consuming procedure, an inverse method, based on Simulated Annealing, is proposed in this paper to estimate them from a single and simple measurement of the absorption coefficient. Experimental results on two samples of loose aquarium gravel validate the proposed inverse method.

Further investigations of empirical improvements to the Johnson–Champoux–Allard model

Non-acoustical physical parameters (tortuosity, viscous characteristic length, thermal characteristic length, porosity and flow resistivity) of glass fibre, melamine foam, compressed melamine foam and reticulated polyurethane foam materials have been measured. Also tortuosity and two characteristic lengths have been deduced from impedance tube measurements using commercial software (Foam-X). Between 200Hz and 5kHz sound velocities in the materials have been deduced from impedance tube measurements. Between 79kHz and 1MHz they have been deduced from ultrasonic measurements.Predictions of sound velocities based on the Johnson–Champoux–Allard (JCA) model using measured non-acoustical physical parameters and, at high frequency, using ultrasonically-measured non-acoustical physical parameters (tortuosity, viscous characteristic length and thermal characteristic length) are found to be in good agreement with the ultrasonically-measured sound velocities above 100kHz thereby confirming the accuracy of the JCA model at ultrasonic frequencies.Differences in values of the two characteristic lengths between the Foam-X outputs and the results of ultrasonic measurements, using the slope method, and the important role of two correction factors in Kino’s empirical modification of the JCA model are explored in detail. It is shown that the values of characteristic lengths required to fit data using the original JCA model in the audible frequency range differ significantly from the ultrasonically-measured values. However it is shown that the Foam-X and the Kino model methods yield similar values for the two characteristic lengths in the audible frequency range. Further modifications of the JCA model to make it valid over a wider frequency range are proposed.

Impact of the irregular microgeometry of polyurethane foam on the macroscopic acoustic behavior predicted by a unit-cell model.

This paper deals with the prediction of the macroscopic sound absorption behavior of highly porous polyurethane foams using two unit-cell microstructure-based models recently developed by Doutres, Atalla, and Dong [J. Appl. Phys. 110, 064901 (2011); J. Appl. Phys. 113, 054901 (2013)]. In these models, the porous material is idealized as a packing of a tetrakaidecahedra unit-cell representative of the disordered network that constitutes the porous frame. The non-acoustic parameters involved in the classical Johnson-Champoux-Allard model (i.e., porosity, airflow resistivity, tortuosity, etc.) are derived from characteristic properties of the unit-cell and semi-empirical relationships. A global sensitivity analysis is performed on these two models in order to investigate how the variability associated with the measured unit-cell characteristics affects the models outputs. This allows identification of the possible limitations of a unit-cell micro-macro approach due to microstructure irregularity. The sensitivity analysis mainly shows that for moderately and highly reticulated polyurethane foams, the strut length parameter is the key parameter since it greatly impacts three important non-acoustic parameters and causes large uncertainty on the sound absorption coefficient even if its measurement variability is moderate. For foams with a slight inhomogeneity and anisotropy, a micro-macro model associated to cell size measurements should be preferred.

Noise annoyance is related to the presence of urban public transport

BackgroundThe association between noise annoyance and public transport as a source of noise has not been studied previously. The aim was to study noise annoyance in an urban population due to the presence, the type and the number of public transport vehicles, in relation to other acoustical and non-acoustical parameters. MethodThe study sample comprised 5861 adults residing in 118 streets in the city center of Belgrade. The presence, the type and the number of public transport vehicles were assessed using official transport maps and matched with residential addresses. Noise annoyance was assessed by a questionnaire including a self-report five-graded scale. ‘High noise annoyance’ was defined by merging ‘very’ and ‘extremely’ annoyed answers. ResultsSignificant predictors of high noise annoyance were the presence of public transport at daytime (yes vs. no) (odds ratio=1.47, 95% confidence interval=1.28–1.70), and at night (yes vs. no) (OR=1.39, 95% CI=1.20–1.61). Residing in the streets with more than 79 public transport vehicles per hour (3rd tercile vs. 1st tercile) predicted high noise annoyance at daytime (OR=1.64, 95% CI=1.18–2.27). Residing in the streets with buses and trams at night (‘bus and tram’ vs. no public transport) increased the risk of high noise annoyance (OR=2.67, 95% CI=1.78–4.09). These associations were independent from noise sensitivity, orientation of bedroom windows, floor level, and equivalent noise levels. Living in the apartment with bedroom windows facing the street was the strongest confounder for the association between noise annoyance, noise levels and public transport. ConclusionThe study has identified the presence of public transport at daytime and at night as a significant and independent predictor of high noise annoyance. Future intervention measures should concern the presence, the type and the number of public transport vehicles in order to reduce noise annoyance reactions in urban areas.

Investigations on the sensitivity of the relationships between sound absorption characteristics and microstructure related parameters for polyurethane foams

Straightforward semi-phenomenological models have been developed for highly porous polyurethane foams to predict the macroscopic non-acoustic parameters involved in the classical Johnson-Champoux-Allard model (i.e., porosity, airflow resistivity…) from microstructure properties (i.e., strut length, strut thickness, and reticulation rate). These microstructure properties are measured using sophisticated optical methods (i.e., optical microscope, SEM) and a large variability can be observed due to great complexity of the 3D microstructure; variability also depends on the precision of the measurement device. This work investigates how the variability associated with the model inputs affects the model outputs (i.e., non-acoustic parameters, surface impedance, and sound absorption coefficient). The sensitivity analysis is based on the Fourier Amplitude Sensitivity Test (FAST). It helps quantify the correlation between the input parameters and identify the parameters contributing the most to output variability, thus requiring precise measurement. This study illustrates the preponderant impact of the reticulation rate (i.e., open pore content) on acoustic performances and guides the user on the required optical measurement device.

A semi-phenomenological model to predict the acoustic behavior of fully and partially reticulated polyurethane foams

This paper proposes simple semi-phenomenological models to predict the sound absorption efficiency of highly porous polyurethane foams from microstructure characterization. In a previous paper [J. Appl. Phys. 110, 064901 (2011)], the authors presented a 3-parameter semi-phenomenological model linking the microstructure properties of fully and partially reticulated isotropic polyurethane foams (i.e., strut length l, strut thickness t, and reticulation rate Rw) to the macroscopic non-acoustic parameters involved in the classical Johnson-Champoux-Allard model (i.e., porosity ϕ, airflow resistivity σ, tortuosity α∝, viscous Λ, and thermal Λ′ characteristic lengths). The model was based on existing scaling laws, validated for fully reticulated polyurethane foams, and improved using both geometrical and empirical approaches to account for the presence of membrane closing the pores. This 3-parameter model is applied to six polyurethane foams in this paper and is found highly sensitive to the microstructure characterization; particularly to strut's dimensions. A simplified micro-/macro model is then presented. It is based on the cell size Cs and reticulation rate Rw only, assuming that the geometric ratio between strut length l and strut thickness t is known. This simplified model, called the 2-parameter model, considerably simplifies the microstructure characterization procedure. A comparison of the two proposed semi-phenomenological models is presented using six polyurethane foams being either fully or partially reticulated, isotropic or anisotropic. It is shown that the 2-parameter model is less sensitive to measurement uncertainties compared to the original model and allows a better estimation of polyurethane foams sound absorption behavior.

Effect of the microstructure closed pore content on the acoustic behavior of polyurethane foams

The present paper proposes to investigate the links between the microstructure of polyurethane foams and their sound absorbing efficiency, and more specifically the effect of membranes closing the cells. This study is based on the complete characterization of 15 polyurethane foam with various cell sizes and reticulation rates (i.e., open pore content): (i) characterization of the microstructure properties (cell size Cs, strut thickness t, reticulation rate Rw…) from SEM pictures, (ii) characterization of nonacoustic parameters (porosity Φ, airflow resistivity σ, tortuosity α∞…) from direct and indirect methods. Existing analytical links between microstructure properties and nonacoustic parameters are first applied to fully reticulated materials. Then, they are improved empirically to account for the presence of the closed pore content. The proposed expressions associated to the Johnson-Champoux-Allard porous model allow a good estimation of the sound absorbing behavior of all polyurethane foams, fully reticulated or not. This paper also demonstrates the important effect of the presence of cell membranes: increase of the airflow resistivity, tortuosity, and the ratio between the thermal and viscous characteristic lengths while decreasing these two characteristic lengths. Thus, the sound absorption efficiency at low frequencies is improved but can be worsened in some higher frequency bands.

Measurement of the resistivity of porous materials with an alternating air-flow method

Air-flow resistivity is a main parameter governing the acoustic behavior of porous materials for sound absorption. The international standard ISO 9053 specifies two different methods to measure the air-flow resistivity, namely a steady-state air-flow method and an alternating air-flow method. The latter is realized by the measurement of the sound pressure at 2 Hz in a small rigid volume closed partially by the test sample. This cavity is excited with a known volume-velocity sound source implemented often with a motor-driven piston oscillating with prescribed area and displacement magnitude. Measurements at 2 Hz require special instrumentation and care. The authors suggest an alternating air-flow method based on the ratio of sound pressures measured at frequencies higher than 2 Hz inside two cavities coupled through a conventional loudspeaker. The basic method showed that the imaginary part of the sound pressure ratio is useful for the evaluation of the air-flow resistance. Criteria are discussed about the choice of a frequency range suitable to perform simplified calculations with respect to the basic method. These criteria depend on the sample thickness, its nonacoustic parameters, and the measurement apparatus as well. The proposed measurement method was tested successfully with various types of acoustic materials.

Acoustic-Wave-Induced Analyte Separation in Narrow Fluidic Confinements in the Presence of Interfacial Interactions

In the present work, we attempt to analyze the influences of acoustic forces, in conjunction with the intrinsic electrokinetic effects as well as the near-wall attractive and repulsive forces, on the transport and size-based separation of charged analytes in a background-pressure-driven flow through a narrow fluidic confinement. By executing a regular perturbation analysis, we establish that the speed of traverse and the extent of spreading (dispersion) of the analyte bands is effectively determined by the ratio of particle to channel heights, channel height relative to the Debye length, and all other significant acoustic and nonacoustic parameters. These factors in tandem may dictate the analyte separation characteristics (quantified by the resolution of separation), in tune with the particular harmonic of the acoustic wave, the strengths of the induced electrical double layer fields, and the van der Waals interaction mechanisms. We quantitatively pinpoint the relationship between the harmonics to be employed and particle sizes to be separated. Our study reveals that there is a critical channel height beyond which the acoustic effects may effectively mask the near-wall interactions and below which the transverse migrative influences induced by the walls may influence the separation characteristics in a rather profound manner. The results implicate an interesting high-efficiency separation regime that can be obtained with a judicious combination of the background flow, energy intensity of the acoustic effects, and induced electrical double layer interactions.

Investigation of non-acoustical parameters of compressed melamine foam materials

A series of careful non-acoustical parameters measurements using 5 ‘Illtec’ melamine foam and 10 ‘Basotect TG’ melamine foam samples have been made. Flow resistivity, tortuosity, porosity, viscous characteristic length and thermal characteristic length of two types of compressed melamine foam materials with different foam magnifications have been investigated. It has been found that a relationship between the flow resistivity, fibre equivalent diameter and bulk density exists for each of the compressed melamine foam materials. This paper also discusses relationships between the non-acoustical parameters and compression rates in the compressed melamine foam media.

Sensitivity analysis and non linear parameters estimation of porous materials from normal sound coefficient absorption measurements

In literature, various models have been proposed in order to predict characteristic impedance Zc and wave number kc of porous materials. To use these models their involved nonacoustical parameters (i. e. flow resistivity, tortuosity porosityœ) are usually measured. Unfortunately measurements require specialised equipments and are often difficult on loose porous materials leading most often to erroneous values. Moreover, the inverse estimation of theses parameters from experimental data is not an easy task as believed. This is due to the non linearity of models and the presence of some parameters with weak and/or linearly dependent influences. In this work an approach based on the tradeoff between magnitude and linear independence of the sensitivity matrix components is presented in order to achieve an efficient ranking of parameters according to their inherent ease of estimation. Next this approach is associated to Genetic Algorithms to minimize the least squares norm between measured and modelled normal sound absorption coefficient. For five widely used models in literatures, the application of the proposed inverse estimation procedure on loose granular cork and vegetable fibres, shows that, unlike classical gradient approaches methods, better estimation has been obtained.

Evaluation of acoustical and non-acoustical properties of sound absorbing materials made of polyester fibres of various cross-sectional shapes

Four types of fibrous material samples made of polyester fibres of various cross-sections, i.e. circle, hollow, flat and triangle, have been prepared. The surface impedance and the non-acoustical parameters of them have been measured. The effect of the variation in the cross-sections on the acoustical properties and the non-acoustical parameters of the polyester fibre samples have been examined. The effects of the ‘circle’ and the ‘hollow’ samples are almost the same. However, for the ‘flat’ and the ‘triangle’ samples, the effects different from the ‘circle’ sample are found in the measured flow resistivity and the two characteristic lengths. This paper also discusses the equivalent diameters of the materials made of non-circular cross-sections.