Normal Incidence Sound Absorption Research Articles

Ventilated acoustic metamaterial window panels for simultaneous noise shielding and air circulation

The conflict between the acoustical performance and ventilation efficiency in conventional noise barriers limits their application potentials in several settings. To address this challenge, we design and experimentally demonstrate a ventilated tunable acoustic metamaterial for noise mitigation at targeted frequencies. Through the structure, a peak normal incidence sound absorption coefficient of more than 0.96 at 1000 Hz and the peak normal incidence sound transmission loss of 18 dB is achieved while maintaining the air circulation with a 45% open area of ventilation. Such a high absorption near the resonant frequency under the ventilation condition originates from high impedance matching between the metastructure and air. Next, we present a proof-of-concept for the fabrication of acoustic metapanels for large scale applications. A simple jigsaw puzzle-like assembly technique is used to interconnect the multiple unit cells, leading to the production of integrated metapanels. The experimental findings validate the potential applications of the proposed metapanels for low-to-mid frequency noise control in a small ventilated spaces.

Sound absorption coefficient measurement by extracting the first reflected wave in a short tube

This study presents a method for measuring the normal incident sound absorption coefficients of acoustic materials by extracting the first reflected wave in a short pulse tube. Instead of generating a short pulse in a tube with an open end, the inverse filter method is used when a sample is added at the end of the tube, which makes it possible to extract the first reflected wave directly from the superposed sound field in the time domain. The length of the tube used in this proposed pulse separation method is half as long as that of the traditional pulse separation method, which makes it can be used in a general commercial impedance tube. Two different materials are investigated. When the proposed method is compared to the traditional transfer function method and the traditional pulse separation method, good agreement can be observed among these three methods.

Measurement and analysis of sound absorption by a metallic composite foam

A composite foam consisting of polyurethane foam embedded in metallic foam is fabricated and evaluated for sound absorbing properties. The normal incidence sound absorption coefficient is measured in an impedance tube for three different types of foams including the composite foam, a rigid framed metallic foam, and an elastic framed polyurethane foam. A lumped element model is used to predict the energy dissipation mechanisms, which include viscous and thermal losses, structural losses, and coupling losses at the metal and polyurethane interface. The best performing composite foam increased sound absorption by a factor of 6 (from 0.1 to 0.6) in the low frequency test range (around 600 Hz) and by a factor of 2 (from 0.2 to 0.4) over the entire test frequency range (250–4500 Hz). The composite foam maintains the high static stiffness of the metal foam and exhibits acoustic compliance characteristic of the polyurethane foam. Composites such as those presented in this work are advantageous for engineering applications where combined high stiffness and energy absorption are required.

Numerical modelling of the sound absorption spectra for bottleneck dominated porous metallic structures

Numerical simulations are used to test the ability of several common equivalent fluid models to predict the sound absorption behaviour in porous metals with “bottleneck” type structures. Of these models, Wilson’s relaxation model was found to be an excellent and overall best fit for multiple sources of experimental acoustic absorption data. Simulations, incorporating Wilson’s model, were used to highlight the relative importance of key geometrical features of bottleneck structures on the normal incidence sound absorption spectrum. Simulations revealed significant improvements in absorption behaviour would be achieved, over a “benchmark” structure from the literature, by maximising the porosity (0.8) and targeting a permeability in the range of 4.0 × 10−10 m2. Such a modelling approach should provide a valuable tool in the optimisation of sound absorption performance and structural integrity, to meet application-specific requirements, for a genre of porous materials that offer a unique combination of acoustic absorption and load bearing capability.

Sound absorbers with a micro-perforated panel backed by an array of parallel-arranged sub-cavities at different depths

This paper presents a type of wide band micro-perforated panel (MPP) absorber with an array of parallel arranged sub-cavities at different depths based on the quadratic residue sequence (QRS). An analytical prediction model is proposed for evaluating the normal incidence sound absorption coefficients of this type of MPP absorber in its design stage. Next, simulations with a finite element procedure and experimental measurements were conducted to provide cross-validations of the proposed analytical model and to facilitate the discussions on the sound absorption performance of this type of MPP absorber. The results show that the analytical model provides simple yet accurate prediction on the sound absorption performance of this MPP absorber; the absorption bandwidth of this type of MPP absorber can be significantly enhanced through the fine design on the sub-cavity depth sequence. It is also shown that the normal incidence absorption coefficient of this type of MPP absorber can remain beyond 0.5 in the frequency range of 450–3500 Hz, with the maximum value of 0.98.

Investigation of Peanut Shell as Alternative Sound Absorbing Material

Use of natural material for development of sound absorbers received considerable significance since in past few years. Natural materials are preferred due to their ease of availability, low environmental impact, distinctive internal structure and reusability. One such natural material Peanut shell is made up of natural cellulose fibers and has good internal pores which can be utilized for sound absorption application. In the current study samples of Peanut shell specimens were made of 100mm diameter having different material to binder weight ratios. The test specimens were made with thickness 10mm, 15mm, 20mm, 25mm, 30mm, 35mm and 40mm respectively. Normal incidence sound absorption coefficient for the specimens is measured using ASTM E1050-98 (Standard Test Method for Impedance and Absorption of Acoustical Materials Using a Tube, Two Microphones, and a Digital Frequency Analysis System) and values of the normal incidence sound absorption are compared with values obtained using Delany and Bazley Model. It was observed that Peanut shell test specimen with material to binder weight ratio 70:30 gives optimum average sound absorption for frequency range between 250Hz to 4500Hz. The current paper concludes that Peanut shell can prove to be good alternative natural material over existing conventional sound absorbing materials for application in field of sound absorption.

A Multi-Tone Sound Absorber Based on an Array of Shunted Loudspeakers

It has been demonstrated that a single shunted loudspeaker can be used as an effective low frequency sound absorber in a duct, but many shunted loudspeakers have to be used in practice for noise reduction or reverberation control in rooms, thus it is necessary to understand the performance of an array of shunted loudspeakers. In this paper, a model for the parallel shunted loudspeaker array for multi-tone sound absorption is proposed based on a modal solution, and then the acoustic properties of a shunted loudspeaker array under normal incidence are investigated using both the modal solution and the finite element method. It was found that each shunted loudspeaker can work almost independently where each unit resonates. Based on the interaction analysis, multi-tone absorbers in low frequency can be achieved by designing multiple shunted loudspeakers with different shunt circuits respectively. The simulation and experimental results show that the normal incidence sound absorption coefficient of the designed absorber has four absorption peaks with values of 0.42, 0.58, 0.80, and 0.84 around 100 Hz, 200 Hz, 300 Hz, and 400 Hz respectively.

Development of a narrow impedance tube to measure normal absorption coefficient and transmission loss at high frequencies around 10 kHz

To measure the normal incident sound absorption coefficient and transmission loss for frequencies around 10 kHz, a small impedance tube with an inner diameter of 15 mm was developed. It was found that the influences of sound attenuation, caused by the viscosity of air near the inner tube wall and the surface roughness on the wall of the tube at microphone, were non-negligible. To address these issues, we propose a correction method for the sound attenuation and an improved microphone holder to smoothen the inner wall surface. It was then possible to accurately measure the sound absorption coefficient and transmission loss with acceptable level of accuracy.

Airflow resistance of acoustical fibrous materials: Measurements, calculations and applications

The acoustic performance of fibrous materials is mainly determined by its airflow resistance, and it is a parameter of the resistance that the airflow meets through the materials. This paper has summarized the recent advances on the measurements, calculations and applications of airflow resistance. Firstly, different methods for airflow resistance measurements are presented, mainly including the direct airflow method, alternating airflow method and acoustical method. We have summarized the development history, current status and industrial applications of these methods. Secondly, this paper has summarized the models of calculating airflow resistance. Most of these empirical models are based on the characteristic parameters of fibrous materials, for instance bulk density, fiber diameter, porosity and thickness. Thirdly, this review has gathered the applications of airflow resistance in sound absorption and noise control. It is a crucial parameter in the prediction of both normal incidence sound absorption and reverberation chamber sound absorption. In conclusion, this review has concluded with some perspectives for the measurements, calculations and applications of airflow resistance.

Extension of the frequency range of normal-incidence sound absorption coefficient measurement in impedance tube using four or eight microphones

Extension of the frequency range of normal-incidence sound absorption coefficient measurement in impedance tube using four or eight microphones

Experimental and computational analysis of sound absorption behavior in needled nonwovens

In this paper application of X-ray micro-computed tomography (μCT) together with fluid simulation techniques to predict sound absorption characteristics of needled nonwovens is discussed. Melt-spun polypropylene fibers of different fineness were made on an industrial scale compact melt spinning line. A conventional batt forming-needling line was used to prepare the needled samples. The normal incidence sound absorption coefficients were measured using impedance tube method. Realistic 3D images of samples at micron-level spatial resolution were obtained using μCT. Morphology of fabrics was characterized in terms of porosity, fiber diameter distribution, fiber curliness and pore size distribution from high-resolution realistic 3D images using GeoDict software. In order to calculate permeability and flow resistivity of media, fluid flow was simulated by numerically solving incompressible laminar Newtonian flow through the 3D pore space of realistic structures. Based on the flow resistivity, the frequency-dependent acoustic absorption coefficient of the needled nonwovens was predicted using the empirical model of Delany and Bazley (1970) and its associated modified models. The results were compared and validated with the corresponding experimental results. Based on morphological analysis, it was concluded that for a given weight per unit area, finer fibers yield to presence of higher number of fibers in the samples. This results in formation of smaller and more tortuous pores, which in turn leads to increase in flow resistivity of media. It was established that, among the empirical models, Mechel modification to Delany and Bazley model had superior predictive ability when compared to that of the original Delany and Bazley model at frequency range of 100–5000 Hz and is well suited to polypropylene needled nonwovens.

Sound absorption of extracted pineapple-leaf fibres

This paper reports the utilisation of fibres from the pineapple leaf (PALF) to be an alternative natural acoustic material. We fabricated samples from raw pineapple leaf fibres with different densities and thicknesses to observe their effects on the sound absorption characteristic. Measurement was conducted for the normal incidence sound absorption coefficient in an impedance tube based on ISO 10534-2. It reveals that the pineapple leaf fibres can achieve sound absorption coefficient of 0.9 on average above 1 kHz by controlling the densities of the fibres and/or by introducing the air gap behind the samples. It is also demonstrated that the sound absorption performance is similar to that of the commercial rock wool fibres and synthetic polyurethane foam.

An equivalent fluid model based finite-difference time-domain algorithm for sound propagation in porous material with rigid frame.

An equivalent fluid model based finite-difference time-domain algorithm is proposed to simulate the frequency characteristic of porous material with rigid frame. The rigid-frame porous material is replaced by an equivalent fluid characterized by the effective density and the effective bulk modulus, which reflect the viscous effects and thermal effects in the porous material, respectively. The effective density and the effective bulk modulus are frequency-dependent complex values, which are designed in the form of infinite impulse response filters in the frequency domain. The Z transform theory is used to discretize the frequency-domain wave equations. The accuracy of the proposed algorithm is preliminarily validated by good agreement between the calculated and measured normal-incidence sound absorption coefficients of two samples composed of different multiple-layered materials in a wide frequency range.

Characterization of Sheep Wool as a Sustainable Material for Acoustic Applications.

In recent years, natural materials are becoming a valid alternative to traditional sound absorbers due to reduced production costs and environmental protection. This paper reports the acoustical characterization of sheep wool. Measurements on normal incidence and diffuse-incidence sound absorption coefficients of different samples are reported. The airflow resistance has also been measured. The results prove that sheep wool has a comparable sound absorption performance to that of mineral wool or recycled polyurethane foam. An empirical model is used to calculate the sound absorption of sheep wool samples. A reasonable agreement on the acoustic absorption of all sheep wool samples is obtained.

Sound absorption performance of natural kenaf fibres

In recent years, kenaf fibre has been highlighted for its superiority as the filler for composite materials. However, study of the fibres as an acoustic absorber is still lacking. In this paper, the sound absorption of kenaf fibre specimens are studied both under normal and random sound incidence. The normal-incidence sound absorption coefficient measurement was conducted using the impedance tube method. The effects of thickness involving full fibre and air-fibre specimen and the effect of bulk density were discussed. For the random-incidence sound absorption coefficient, the test was conducted in a reverberation chamber. From both methods, the results in general reveal that for bulk density of 140–150kg/m3 and thickness of 25–30mm, the absorption coefficient is above 0.5 starting from 500Hz and can reach 0.85 on average above 1.5kHz. These frequency bandwidth of absorption and the level of absorption coefficient improve significantly when bulk density and thickness are increased. Additional air gap also improve the absorption toward lower frequency.

Modelling and optimization of the sound absorption of wood-wool cement boards

The present article aims to characterize and improve the sound absorption of wood-wool cement boards (WWCB) with varying strand widths, densities, thicknesses and applied with varying air cavity thicknesses by using impedance models. Different rigid-frame impedance models were analysed to predict the acoustic impedance of this material, and their suitability for the WWCB was evaluated. The Johnson-Champoux-Allard (JCA) model with its parameters, porosity, flow resistivity, tortuosity and characteristic lengths, was found to be the most appropriate to model the normal incidence sound absorption. The relations between the bulk density of the board and the impedance model parameters are established for material characterisation and optimization. Optimum density values were found per strand in terms of the sound absorption in the frequency range 200–2500Hz. Moreover, the use of a density variation in the boards leads to improvement of the sound absorption. Regarding the application of the board, the use of an air cavity with a thickness of 100mm leads to an optimized sound absorption for every strand width.

The influence of the synthesis parameters of nanoporous composite on the sound absorption coefficient

Metal materials with nanopore structure have multifunctional properties, they retain some properties of metal and also possess the characteristics of porous structures such as good sound absorption. This paper investigates the sound absorption characteristics of porous metal composites obtained by the mechanical mixing of polycrystalline powders. The mixture consists of (100—x) % Cu i x % Zn, (1≤ x ≤5). During the sample preparation process, several parameters were varied: activation time in the planetary mill in the non-oxide environment, pressing duration of activated powders and different sintering times of samples on the specific sintering temperature. Depending on the activation time, nanopowders with different sizes of nanocrystals are obtained. The activated powders were pressed into disk-shaped samples of different thickness, with a diameter of 30 mm. The normal incidence sound absorption coefficient of sintered samples was measured in an impedance tube. The obtained results showed that this mater...

Normal incidence sound absorption of parallel absorber at high sound pressure

A finite element model is developed to simulate the acoustic response of the parallel absorber array at high sound intensity. The device consists of an MPP and a rectangular backing cavity which is divided into two sub-cavities with different cavity depths. At high acoustic excitation, a nonlinear impedance model instead of the traditional linear model is adopted such that the effects of jets and vortex rings formed at the exit of orifices on the acoustic properties are taken into account. The normal incident pressure is considered as a main variable of the nonlinear acoustic impedance model. The performance of the MPP absorber array with different designed geometric parameters are studied. Based on the parallel absorption mechanism, the preliminary results show that the MPP absorber array provides good absorption performance with a wider frequency range by comparing with the single MPP absorber. Also, compared with the results obtained in linear regime, a better absorption performance is achieved by the MPP absorber array at high sound intensity due to the higher acoustic resistance. The acoustic behaviors of the MPP absorber array are also studied experimentally under normal incidence. The predicted and measured results show good agreement.

Experimental validation of Bayesian design for broadband multilayered microperforated panel absorbers

Single-layer microperforated panel (MPP) absorbers often exhibit limited absorbing bandwidth. A Bayesian inference framework (encompassing both model selection and parameter estimation) has been utilized to design broadband MPP absorbers. In this work, the broadband multilayered MPP absorbers are experimentally validated. In demonstrating ability to meet practical requirements on both high absorption and wide bandwidth, the current investigation uses a sample design scheme in the frequency range from 300 Hz to 2.4 kHz. A minimum requirement for three MPP layers and the relevant three-layer MPP parameters are derived by the two levels of Bayesian inference to meet the design scheme. MPP samples, based on the predicted design scheme, are fabricated and used to conduct normal-incidence sound absorption measurements in an impedance tube. In order to quantify the fabrication tolerance, the measured acoustic data are used to estimate the MPP parameters of the constructed absorber, using an inverse Bayesian inference method. This paper will discuss the initial MPP design, experimental validations of the designed absorption performance, as well as fabrication tolerance estimations of the MPP parameters.

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.