Acoustic Properties Research Articles

Poroelasticity and acoustic properties of bio-based materials : from characterization to the understanding of mechanical dissipation effects

The building sector is a key element in reducing the effects of climate change. To operate sustainably and efficiently, solutions based on building materials having a low environmental impact, such as bio-based materials, are needed. In fact, they present high-level multi-functional properties and they capture atmospheric carbon dioxide. However, even if a number of works has examined visco-thermal dissipation effects, the understanding of the mechanical dissipation effects on the acoustic performances of bio-based materials is still incomplete and little data are available in the literature. Therefore, it seems particularly relevant to investigate by experimental characterizations and modelling methods the effects of mechanical dissipation on the acoustic properties of a representative variety of bio-based materials such as vegetal wools, vegetal aggregate stacks and vegetal concretes. To do this, Young's modulus, Poisson's ratio and structural damping were measured on the various typology of materials by a quasi-static analysis method. The non-linear elastic behaviour in compression of bio-based samples is analyzed. Finally, the influence of Young's modulus and structural damping on the acoustic absorption and attenuation properties of bio-based materials is described using modelling approaches.

Development and analysis of Fe-doped ZnO nanoparticle-infused sisal fiber reinforced hybrid polymer composites for high-performance sound absorption and thermal insulation applications

In the current study, sisal fiber-reinforced hybrid composites have been developed by integrating a bidirectional mat with a blend of epoxy using Fe-doped ZnO (IDZO) nanoparticles at concentrations of 0.2, 0.4, 0.6, and 0.8 wt%. Hand-lay-up approach was used to create the composites. The composites were characterized through a comprehensive analysis of their density, water absorption, elasticity, thermogravimetric analysis, mechanical resistance, thermal conductivity, microhardness, and propagation of sound properties, including sound absorption coefficients and sound transmission class ratings. As filler loading increased, the bio-composite sample’s hardness increased under the density and void volume fraction values, with 0.8 wt% composite sample showing the superior micro-hardness. A positive correlation was observed between the ZnO nanoparticle weight percentage and thermal conductivity, with improve nanoparticle content leading to improved thermal performance. Composites containing 0.4 wt percent nanoparticles showed superior tensile strength (66.8–84.16 MPa), flexural strength (97.69–120.02 MPa) and flexural modulus (3.56–7.68 GPa). It has been observed that the weight percentage of IDZO filler greatly influences the sound absorption efficiency of the current composites. These findings highlight the promising potential of these novel biocomposites in advanced engineering applications, emphasizing their usefulness in demanding environments with biodegradability and high-performance thermal, mechanical, and acoustic properties. Based on the sound transmission class ratings, the sisal fiber biocomposites have been regarded as great adoptions for use as sound-absorbing materials such as wall partitions, doors, enclosures, and structural components, thermal insulation systems, and electrical insulation.

Virtual assessment of phone booths' acoustic performance in laboratory and office environments

The irrelevant speech noise is one of the significant issues affecting workers' productivity and comfort. Acoustic furniture is the most common solution addressing this problem: mobile phone booths and partially closed workstations are two examples. The acoustic performance of such devices is defined with laboratory tests according to ISO 23351-1:2020. The sound power level measured in reverberation rooms with and without the product determines the reduction of speech A-weighted sound power level (DS,A). However, predicting acoustic performance in real-world scenarios is still challenging. The present work explores the discrepancies between DS,A, and apparent speech reduction (D'S,A) through numerical models. The study assesses the performance difference between a calibrated virtual reverberation room and virtual open-plan offices maintaining the same acoustic properties, i.e., sound transmission and sound absorption, of a typical two-person phone booth. Moreover, the study investigates the fluctuation on D'S,A changing main room acoustic criteria, e.g., the reverberation time (T) and the total absorption area (A). Results help define ray-based simulations' reliability in predicting furniture ensembles' acoustic performance.

Reconstructing the soundscape of the ancient Hippodrome of Olympia for an immersive sonic experience of heritage sites based on sound source description in texts

This study attempts to reconstruct and auralize the soundscape of the Hippodrome of Olympia in Greece. This UNESCO world-heritage-listed-site is considered the evidence of the Greek hippodromes. Hippodromes used to have several activities from which different combinations of sound sources created the soundscape. To collect sound sources, description in texts of site is sought by implementing methods of the archaeology of soundscape and the suggested criteria of contextual, internal, and comparative certainty for archaeoacoustics studies. The description of these activities facilitates reconstructing and auralizing the soundscape during different phases of events in the hippodrome. The results of the study showed that the information about the activities on the hippodrome, on similar hippodromes of different civilizations, and on similar hippodromes of later eras are rather sufficient to create a comprehensive soundscape of the site. Auralizing the reconstructed soundscape by using current auralization software and techniques promises of immersive experience of ancient sites that enhances understanding the sonic intangible cultural heritage element and the experience of the ancient soundscape. Furthermore, ancient users' perception of the soundscape can be approached by localizing the sound sources on the site and obtaining the corresponding acoustical properties, providing information of users' response to the sonic outcomes

Characterisation and simulation of a general ventilation filter based on a double porosity approach

The air filters installed in ventilation systems are designed for collecting dust and particles. These filters usually consist in folded paper or textile, arranged either as a flat slab or in a V-shape frame. Some of them may also contain activated carbon in order to remove molecular compounds. Experimental results show that they can significantly attenuate the upstream sound sources such as fans or other regenerated noise. Though, these acoustic properties are not strictly speaking designed and often result from the tailored filtering capacities. The present work proposes a three-stage methodology to design and optimize the acoustic performance of these filters. The first stage is the characterization of the intrinsic acoustic parameters of the paper or textile. The second stage consists in the validation of these parameters regarding sound attenuation as measured in impedance tube. This includes analytical and finite element computations to correctly account for the mounting conditions of the specimen in the impedance tube. The final stage consists in predicting the sound attenuation measured on the full-size filter, set in shape using the porous composite material theory. The obtained results provide a satisfactory correspondence allowing for an accurate design at an early stage of the project.

Experimental characterization of the effective acoustic properties of anisotropic porous materials via free-field measurements

Porous media are commonly described as effective isotropic fluid materials, such that their acoustic properties can be adequately determined from their bulk modulus and dynamic density. Most acoustic absorbents, however, possess a marked anisotropy, which influences their acoustical behavior and the parameters necessary to deduce it. Specifically, the influence of anisotropy translates into a full symmetric density tensor (in place of a scalar). This study presents a method for retrieving the bulk modulus and all six components of the density tensor of a layer of porous material from reflection coefficients measured in free field with an array of microphones. The procedure consists in expressing the sound field measured in the vicinity of the layer as a superposition of plane waves, from which the surface impedance and the reflection coefficient are reconstructed. The reflection properties, as well as the pressure at the rigid backing interface, are estimated for various source positions (i.e., various angles of incidence) and an inverse problem is formulated to infer the effective fluid parameters. The validity of the method is examined experimentally on a manufactured porous material, and the resulting fluid parameters compared with experimental results obtained from reflection and transmission coefficients measured in an impedance tube.

Acoustic properties of porous metamaterials featured by acoustic streaming, homogenization based modelling

We consider secondary effects induced by acoustic waves in viscous barotropic fluids saturating pores in elastic, or rigid scaffolds with elastically suspended particles. The asymptotic homogenization method is applied to derive models of the acoustic wave propagation and of the related acoustic streaming (AS) as the secondary effect. This phenomenon originating due to the nonlinearity (divergence of the Reynolds stress) in the Navier Stokes equation is retained using the perturbation analysis. This yields the first and the second order sub-problem enabling to linearize the model governing fluid dynamics. The local AS in the microstructure produces forces acting on the suspended particles, transforming the pore geometry, hence, modulating the acoustic properties. As the consequence, the permeability involved in both the subproblems is modified due to the slowly oscillating particles. The sensitivity analysis can be employed to introduce the homogenized coefficients of the model depending on the particle motion.

Numerical investigation on the strategy of separating burner-flame thermoacoustics

Thermoacoustic instability in combustion systems is contingent upon the thermoacoustic properties of the burner when coupled with a flame. In instances where the burner is acoustically transparent, a straightforward approach involves the separation of the burner from its corresponding flame. Acoustic velocity measurement at the upstream of the burner, in such cases, can be equated with the acoustic velocity at the downstream of the burner due to its transparency. However, when the burner possesses a complex geometry with inherent acoustic properties, this separation strategy becomes pivotal. Near-field acoustics immediately following the burner may influence the effectiveness of this separation. Additionally, burners are typically characterized in the absence of a flame (cold condition), with a common assumption that the acoustic properties of the burner remain unchanged in the presence of the flame (hot condition). This paper employs numerical simulations to scrutinize these assumptions and the separation strategy of the burner from the flame. Utilizing direct numerical simulation and an acoustic network approach, we compare the direct evaluation of the total transfer matrix of the burner with flame against the separate evaluation of the burner and flame, derived from linearized Rankine-Hugoniot relations.

An experimental setup to investigate relevant validation parameters for the auralization of commercial aircraft flyovers in complex urban contexts

Auralization has a key role in the design of immersive virtual reality tools for the study of human environments and their evolution. In soundscape and multisensory research, the validation of auralization frameworks traditionally relies on the impressions engendered by the virtual acoustic environment on the human subjects, and whether those impressions match real-world experiences. It uses subjective surveys related to the perceptual and affective spheres of individuals. This may lead to unclear validation. In this paper, we present an experimental setup to search for a set of relevant validation parameters for the auralization of complex urban environments involving commercial aircraft flyovers. Our investigation is tailored to the study of working and learning activities in offices or universities neighboring the flight paths. The experimental setup focuses on the quantitative evaluation of cognitive performance instead of the classic perceptual or affective personal survey-based approach. In the experiment, subjects undergo distinct acoustic environments, synthesized from real-world binaural recordings. Simultaneously, subjects perform memory, fluency, or puzzle-like tasks. Different cognitive performance parameters are measured throughout the experiment. The parameters most strongly correlated to the environments' acoustic events and properties are considered meaningful validation parameters.

Modeling aeroacoustic interaction of rotating propeller with porous treatments using LBM (Lattice Boltzmann method) simulations

Porous treatments are more and more employed in presence of flow as passive noise control means to mitigate the generated noise. They are generally designed according to their acoustic dissipation properties under no-flow conditions. Nevertheless, in-flow porous treatments can alter the aerodynamic properties, modify the main acoustic source itself or even generate secondary acoustic sources. While most of the modelling methods (analytical, FEM, ...) are able to model the acoustic dissipation, they do not allow to model the interaction with the main source or the possible generation of spurious acoustic sources. The Lattice Boltzmann Method (LBM) is a computational fluid dynamics method able to run Direct Numerical Simulations (DNS) as well as Large Eddy Simulations (LES). The LES method resolves the macroscopic scale of turbulence and uses a sub-grid model for the small scales of turbulence. The low numerical dissipation of the LBM enables to compute both fluid flow and acoustics quantities in a single run. In this work, the LBM implemented in ProLB software is used to model the airborne noise generated by an electric propeller installed on an aircraft wing and the effects of sound absorbing treatments. Various treatments are investigated numerically and compared to experimental data when available.

Characterization and estimation of the acoustical and elastic parameters in stone wool samples

Mineral wool is widely used in acoustic applications. The properties of glass wool have been broadly investigated, thus resulting in a good knowledge of its structure and parameters. This information is used to describe mineral wool in general. Nevertheless, other types, such as stone wool, involve a different production process and therefore have a different fibre structure. e.g. stone wool is not transversally isotropic as glass wool. These differences can have important effects depending on the use of the material; therefore, assuming the same parameter values for both materials may lead to inaccurate results in simulations and calculations. Currently there are few studies that characterize both the acoustic and elastic properties of stone wool. This contribution reports on the characterization of two stone wool samples with different densities (~70 kg/m3 and ~140 kg/m3). The characterization aimed at obtaining the parameters that are required for the description of the propagation of sound in the material through the Biot poroelastic model, i.e. the acoustic and elastic parameters. The obtained knowledge enables the adequate modelling of the investigated poroelastic materials in different configurations by numerical methods such as finite elements and the transfer matrix method.

Numerical modeling of acoustic scattering characteristic with wavenumber domain admittance

A conversion from the wavenumber domain acoustic reflection coefficient to the wavenumber domain acoustic admittance is formulated to deal with the acoustic reflection properties of arbitrary materials in numerical simulation. This method, which can represent arbitrary reflection directional properties, is an extended reactive boundary condition modelling method based on a spatial Fourier transform over the boundary surface. The advantages are that it is computationally inexpensive because it does not need to consider the interior of the material in the analysis, and that it can handle reflection characteristics obtained from actual measurements, etc. From the results of applying the wavenumber domain acoustic admittance to the numerical analysis, it has been confirmed that the proposed method can handle not only the specular reflection component, but also when arbitrary scattering components are included, or when the main reflection direction is changed from the specular reflection direction.

Low frequency, broadband and thin acoustic metamaterials for acoustic insulation

Elastic and locally resonant metamaterials can be used to improve the acoustic insulation properties of panels, particularly at low frequencies. Among the possible architectures, we are interested in the case of a very soft panel consisting of a porous material in which a periodic distribution of cylindrical inclusions is inserted. It has recently been established (JSV 571 (2024) 118005) that this configuration gives rise to a large and low-frequency transmission dip, explained by the coexistence of an elastic band gap and the destructive interference resulting from the interaction between an elastic resonance mode of the skeleton and a rigid body mode of the inclusions. The paper aims to describe the essential physical mechanisms involved in using a minimal lumped element model, the characteristics of which are extracted from the cell modes. The relevance of the minimal model is validated utilizing a comparison with reference results obtained using the finite element method. This approach focuses on the mechanisms involved and is not constrained by the choice of a particular geometric configuration (exact shape and size of the inclusion), allowing it to evolve.

Acoustical dissipation in biobased granular packings

Biobased building materials have many advantages, including local availability and functional performance in terms of mechanical, acoustic and hygrothermal properties. Besides, the biobased solutions coming from plants that uptake CO2 during their growth, are good opportunities to reach carbon-neutral building construction. However, mastering their acoustical performance still raises a number of questions directly linked to their microstructure, which is characterised by a high degree of porosity spread over several scales. The work presented here aims to highlight the effect of the porosity distribution in biobased granular packings on their acoustical performances. Several plant aggregates are considered, such as hemp shiv, sunflower pith or reed as a function of their density and water content. Characterizations are performed in normal incidence and compared to fluid equivalent modellings based on various assumptions, to analyze the effective contribution of the various types of pores at the aggregate level, and between the aggregates.

The "effects of wind turbine sound on working memory" (a pre-registration plan)

Although research on the effects of wind turbine sound (WTS) over mental health factors is increasing, very little attention has been given to the potential effects of WTS over subtle cognitive processes, such as concentration or memory. The "brainwave entrainment hypothesis" is rooted in neuropsychological research demonstrating that brain oscillations are modulated by external rhythms. In addition, perceived annoyance to wind turbines seems to be a phenomenon in which both visual and psychological variables interact with acoustic stimulation. However, annoyance is a concept rarely operationalised in the literature. Our study aims to experimentally assess the effects of WTS on working memory. Working memory is recognised as a basic process, playing a crucial role in unconscious everyday processes (e.g., perceiving an object's trajectory despite input interruptions when blinking), as well as deliberate processes (e.g., carrying out arithmetic operations). By exploring variations in performance during a memory task, while being exposed to WTS samples, we could highlight acoustic characteristics of the WTS stimuli that interfere-or not-with cognitive processes. These data, combined with within-block ratings of annoyance, can provide us with index of annoyance directly related to the acoustic properties present in WTS recordings.

Acoustic characteristics of the operating noise of vacuum cleaners that evoke the feeling of suction

Vacuum cleaner noise is mostly assessed by the sound when the cleaner is not picking up dust. However, users usually hear the operating noise when the cleaner is picking up dust, and this noise is used to recognize that the cleaner is working properly. Therefore, the vacuum cleaner operating noise during dust collection should be evaluated. Psychoacoustical experiments were conducted on three vacuum cleaner models to evaluate the sound quality of the operating noise when beads or pieces of paper were vacuumed. The results showed a strong influence of loudness. We therefore standardized the loudness of the operating noise at around 20 sone and carried out similar experiments. A relationship was found between the suction feeling and the impression of luxury. Most of the stimuli that evoked both the impression of luxury and the suction feeling were the noises made when there was no suction object. When comparing the acoustic properties of stimuli that evoked strong and weak suction feelings, the operating noise of the appliance itself, especially those with components in the low frequency range up to 500 Hz and in the high frequency range around 10 kHz, was rated as strong suction feeling.

Sound absorption metamaterials designed by machine learning and their applications

Acoustic metamaterials are artificial materials composed of acoustic functional elements along the spatial order, which have extraordinary acoustic properties that are not possessed by homogeneous materials. Since the conception of phononic crystal in the 1990s, acoustic metamaterials have gradually matured, which prospects industrial application. In our report, we present an acoustic metamaterial design method based on deep learning for sound absorption. The design method is firstly set up with the array of Fabry-Perot resonators, then adapted for the array of Helmholtz resonators. The method uses a tandem inverse design model based on the convolution neural network(CNN), which is capable of on-demand design. It can generate acoustic metamaterial design of high absorption in targeted frequency range. Three designs of acoustic metamaterials for different purposes are presented. Firstly, sound absorption metamaterials with selective bandwidth absorption, are used to control tonal noise from electric transformers. Secondly, metamaterials with broadband absorption are used in vehicle cabin noise control. Lastly, full-band near-perfect absorption materials are created to construct acoustic anechoic chambers.

Comparative analysis of acoustic testing methods of a multi-layered material: uncovering the membrane effect

This paper presents a comprehensive investigation into the differences observed when evaluating the acoustic properties of a multi-layered material using different standardized methods. The primary focus is on the membrane effect of the upper aluminum layer, which has been shown to contribute to the overall acoustic absorption. This phenomenon, crucial for applications requiring enhanced acoustic performance at lower frequencies, is observed in tests conducted with large sample sizes but remains undetected in impedance tube measurements. Through a series of experiments, including reverberant chamber, impedance tube, and in-situ absorption measurements, this study investigates the complexities of accurately estimating sound absorption characteristics in multi-layered materials. Additionally, 2D colormaps to visualize the spatial distribution of sound absorption across different samples are examined, offering deeper insights into the uneven absorption patterns that standard testing methods may overlook. The experimental findings highlight the importance of selecting an appropriate evaluation technique for multi-layered materials, particularly in applications where acoustic performance is critical at lower frequencies.

Modal identification as a tool to increase the accuracy of acoustic absorption measurements in reverberation rooms

Measuring the acoustic properties of materials may be very challenging. At low frequencies, the sample may interfere with the room's modal behavior. When it occurs, the longest modal decays are influenced. Differences in reverberation time - empty Vs with-specimen conditions - are often emphasized by these modal effects, and, as a matter of fact, the absorption coefficient may be overestimated. The present work aims to increase the accuracy through data augmentation at low frequencies. Modal identification was performed on measured impulse responses of porous specimens, allowing the determination of modal-decay times within the third-octave bands of 80-160 Hz. A statistical analysis of natural modes highlights the modes whose decay exceeds the density function as outliers. This preliminary study ultimately provides some statistical considerations on the modal distribution of decay times and their relationship with the reverberation time measured through the Schroeder integral.

Estimation and experiment on acoustic properties of rice straw (estimation of sound absorption coefficient based on CT image)

The sound absorption characteristics of rice straw were estimated using micro-CT scan images and theoretical analysis. Since it is difficult to express the structure and dimensions of rice straw in a regular manner, the voids in the straw were approximated as clearance between two parallel planes based on the micro-CT scan images. Using the surface area of the straw and the volume of the voids, a theoretical estimation of the sound absorption coefficient of the straw was made. Propagation constants, characteristic impedance, and normal incident sound absorption coefficient were obtained for the derived clearance. Since the rice straw is tubular, the sound absorption coefficient was also obtained assuming that an arbitrary CT scan image is continuous in the thickness direction. In addition, the sound absorption coefficient was estimated from ordinary photographs taken of the sample incident surface. In the experiments, the sound absorption coefficient was measured using a two-microphone impedance tube. The theoretical values were close to the experimental value.