Normal Incidence Absorption Coefficient Research Articles

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.

Determining the Sound Absorption Coefficient of Bamboo Composites: Theoretical- and Laboratory-based Approaches

Abstract Aim: The use of natural fiber-based composites in various applications is increasing due to their cost-effectiveness, low density, nontoxicity, and environmental friendliness. Therefore, in this study, the acoustic performance of natural bamboo fiber composites was investigated using experimental and theoretical methods. The properties of these composites, which include affordability, low density, nontoxicity, and eco-friendliness, contribute to their growing popularity. Methods: In this study, three types of composites with a density of 200 kg/m3 and thicknesses of 50 mm were fabricated. The normal incidence absorption coefficient was directly measured using the impedance tube method based on the ISO 10534-2 standard. In addition, for predicting the sound absorption coefficient of the composites, the software COMSOL and the Miki and Attenborough models were utilized. Results: The results indicate that for samples F (fine fiber) and F-C (fine coarse fiber), the average sound absorption above 1000 Hz is 0.8. The sound absorption values in sample C (coarse fiber) were lower than those in samples F and F-C. In addition, the mathematical models of Miki and Attenborough can predict the acoustic behavior of the samples to some extent, with the Attenborough model providing higher accuracy in its predictions. Conclusion: The results of this study demonstrate that a composite composed of natural bamboo fibers can be used as a sound-absorbing material for noise control purposes, while also contributing as a green technology replacing natural fibers with synthetic fibers.

Modelling of railway ballast as a poro-elastic medium and its effects on sleeper sound radiation

Conventional railway track is laid in a layer of coarse stones known as ballast. Due to the gaps between the stones, the railway ballast behaves as a porous acoustic material, with absorptive properties that are important for the noise produced by the railway system. Sound radiated by the bottom of the sleepers may also be transmitted through the ballast. Moreover, during a train pass-by the ballast can vibrate, which may contribute to the radiated noise. To consider all three effects simultaneously, the ballast is treated here as a poro-elastic medium. To obtain the properties of the ballast as a porous medium, transfer function measurements are performed in a vertical impedance tube to determine the surface impedance and absorption coefficient of different depths of reduced scale ballast. The complex wavenumber is also determined through transfer function measurements within the ballast in the vertical tube. Porosity and flow resistivity are determined by non-acoustic measurements, and the Johnson-Champoux-Allard model for porous materials is then fitted to the measured wavenumbers, allowing the remaining parameters to be determined. Comparison is made with the measured impedance and normal incidence absorption coefficient. The model is then used to predict the diffuse field absorption coefficients of both the reduced scale ballast and full-size ballast, which are compared with measurements, showing acceptable agreement. Finally, the effects on the sleeper radiation of introducing the poro-elastic ballast model are estimated and the sensitivity of the results to a practical range of ballast properties is assessed. The ballast vibration increases the sleeper radiation at low frequency, and especially between 120 and 280 Hz. This is caused by the structural vibration of the ballast driven by the sleepers, which has a resonance around 250 Hz. The acoustic velocity within the ballast is 180° out of phase with the structural velocity, and this leads to a small reduction in the sound radiation around 100 Hz. These effects could not be predicted using a surface impedance model for the ballast. Compared with the effect of the ballast vibration and sound transmission, the absorption of the ballast has a much smaller influence on the sleeper radiation.

Wood chip sound absorbers: Measurements and models

Normal incidence absorption coefficient spectra of samples made from glued wood chips have been measured for various mesh sizes, bulk densities, thicknesses, and air gaps. Increasing thickness introduces additional layer resonance peaks and shifts the initial peak towards lower frequencies. The wood chip samples composed of the smallest mesh sizes were found to offer the highest sound absorption, comparable with that of the same thickness of materials made from synthetic fibers. Measured absorption spectra are compared with predictions of four models for the acoustical properties of rigid porous media. These include a model for slanted parallel identical uniform slits (SS), the Johnson-Champoux-Allard (JCA) and Johnson-Champoux-Allard-Lafarge (JCAL) models for arbitrary pore structures, and model for a non-uniform pore size distribution (NUPSD). Porosity and flow resistivity values have been determined non-acoustically. However, the tortuosity and characteristic lengths required for the JCA model have been obtained by fitting the measured absorption spectra. The thermal permeability required for the JCAL model has been deduced indirectly from the fitted tortuosity through a relationship with standard deviation of the pore size distribution due to the NUPSD model. JCAL and JCA models give the best agreement overall, but predictions of the SS and NUPSD models that use only the fitted tortuosity in addition to measured porosity and flow resistivity are found to give comparable agreement with data for many samples. SS and NUPSD predictions are improved by increasing the tortuosity values compared with those obtained by fitting the JCA model. The study should encourage the creation of sustainable sound-absorbing materials from wood chip wastes.

Relation between Acoustic Absorption Performance and Flow Field around Perforated Plate with Cross Flow

The present paper focuses on the relationship between the acoustic absorption performance and the flow field around a perforated plate with cross-flow. The normal incidence absorption coefficient was calculated from the measured transfer function using transfer function methods. As the flow velocity passing through the orifices increased, the peak level of the acoustic absorption coefficient increased in some cases. We observed the flow field around a perforated plate by flow visualization using a laser. The vortices were clearly formed downstream of the orifices due to the incident sound of pure tone. We discussed the relationship between the vortex and the acoustic absorption performance of perforated plate with a cross flow.

High-frequency measurement method of normal-incidence sound absorption coefficient with a square cross-section impedance tube using eight microphones

The authors have proposed a high-frequency measurement method of normal-incidence absorption coefficient with a circular cross-section impedance tube using eight microphones. This method enables measurement at frequencies about three times higher than the conventional two-microphone method. This study investigates the case of using an impedance tube with a square cross-section. Four microphones are placed at intersections of the (2, 2) acoustic mode nodal lines in one cross-section. When the signals of these microphones are summed, the effects of higher-order acoustic modes whose cut-on frequencies are lower than that of the (4, 0) acoustic mode are canceled. In this case, normal-incidence sound absorption coefficients can be measured at four times higher frequency than the conventional one. Numerical and experimental results are presented to demonstrate the validity and feasibility of the method.

Hand-made sound absorber using familiar materials for nursery facilities

We have been proposing a hand-made sound absorber using familiar materials to develop a comfortable sound environment for children. By installing the hand-made absorber, anyone can easily add sound absorption to nursery rooms. In this study, as one of the familiar materials, we focused on bubble wrap, which is used as a packaging material. We measured the normal incidence absorption coefficient of bubble wrap arranged in layers and found that bubble wrap had reasonable sound absorption. As an example of a hand-made absorber, we created round-shaped fabric balls filling bubble wrap and measured the sound absorption performance in a reverberation room. As a result, the fabric ball absorber had sufficiently high sound absorption performance. Additionally, we clarified the sound absorption mechanism of bubble wrap by predicting based on simple assumptions. Therefore, it could be possible to design sound absorbers with higher sound absorption performance at low frequencies by stacking bubble wrap horizontally or vertically.

Design Of aesthetic acoustic metamaterials window panel based on Sierpiński triangle for sound-silencing with free airflow

Design of high-efficiency, low frequency (<1000Hz) soundproof window or wall absorber which is transparent to airflow is presented. Due to the massive rise in human population and modernization, environmental noise has significantly risen globally. Prolonged noise exposure can cause severe physiological and psychological symptoms like nausea, headaches, fatigue, and insomnia. There has been continuous growth in building construction and infrastructure like offices, bus stops, and airports due to urban population. Generally, a ventilated window is used for getting fresh air into the room, but at the same time, unwanted noise comes along. Researchers used traditional approaches like noise barrier mats in front of the window or designed the entire window using sound-absorbing materials. However, this solution is not aesthetically pleasing, and at the same time, it is heavy and not adequate for low-frequency noise shielding. To address this challenge, we design a transparent hexagonal panel based on Sierpiński fractal triangle which is aesthetically pleasing, demonstrates normal incident sound absorption coefficient more than 0.96 around 700 Hz and transmission loss around 23 dB, while maintaining air circulation through triangular cutout. Next, we present a concept of fabrication of large acoustic panel for large-scale applications which lead to suppressing the urban noise pollution

Extension of frequency range of the sixteen-microphone method in normal-incidence sound transmission loss measurement.

This study deals with development of a high-frequency measurement method of normal incidence sound transmission loss using acoustic impedance tube. On ordinary transfer-function method, measurable frequency of the method is limited by the diameter of the tube. Authors have developed 8 microphone method to measure normal incidence absorption coefficient. This technique lays 4 microphones on the same cross section and cancels rotational acoustic modes. This makes it possible to measure normal incidence absorption coefficient up to 3 times the upper frequency of the ordinary 2 microphone method. In this study, authors apply the same technique to normal incidence transmission loss measurement.

Estimation of the Acoustic Properties of the Random Packing Structures of Granular Materials: Estimation of the Sound Absorption Coefficient Based on Micro-CT Scan Data

In this study, the sound absorption properties of randomly packed granular materials were estimated. Generally, it is difficult to construct a general mathematical model for the arrangement of randomly packed granular materials. Therefore, in this study, an attempt was made to estimate the sound absorption coefficient using a theoretical analysis by introducing data from computed tomography (CT) scans, as the tomographic images of CT scans correspond to the slicing and elemental division of packing structures. In the theoretical analysis, the propagation constants and characteristic impedances in the voids were obtained by approximating each tomographic image as a void between two parallel planes. The derived propagation constants and characteristic impedances were then treated as a one-dimensional transfer matrix in the propagation of sound waves, and the transfer matrix method was used to calculate the normal incident sound absorption coefficient. The theoretical value of the sound absorption coefficient was derived using the effective density to which the measured tortuosity was applied. As a result, for the theoretical values considering the tortuosity, in many cases, the theoretical values were close to the measured values. For the theoretical values, when both the surface area and tortuosity were considered, the peak sound absorption frequency moved to a lower frequency and was in general agreement with the measured values.

Mathematical Models and Experiments on the Acoustic Properties of Granular Packing Structures (Measurement of Tortuosity in Hexagonal Close-Packed and Face-Centered Cubic Lattices).

In this study, the sound absorption characteristics of hexagonal close-packed and face-centered cubic lattices were estimated by theoretical analysis. Propagation constants and characteristic impedances were obtained by dividing each structure into elements perpendicular to the incident direction of sound waves and by approximating each element to a clearance between two parallel planes. Consequently, the propagation constant and the characteristic impedance were treated as a one-dimensional transfer matrix in the propagation of sound waves, and the normal incident sound absorption coefficient was calculated by the transfer matrix method. The theoretical value of the sound absorption coefficient was derived by using the effective density applied to the measured tortuosity. As a result, the theoretical value was becoming closer to the measured value. Therefore, the measured tortuosity is reasonable.

Simplified Limp Frame Model for Application to Nanofiber Nonwovens (Selection of Dominant Biot Parameters).

This study aimed to discover an easy and precise prediction model for the acoustic properties of nanofiber nonwoven fabrics. For this purpose, a prediction model focusing on the two dominant parameters in the Limp frame model—bulk density and flow resistivity—was suggested. The propagation constant and characteristic impedance was generated from the effective density and effective volume modulus generated by the predictive model and treated as a one-dimensional transfer matrix. The sound absorption coefficient was then estimated using the transfer matrix approach. The trend of the normal Incident sound absorption coefficient measured and the sound absorption coefficient obtained from the predictive model were consistent. Thus, it is suggested that the predictive model for the proposed nanofiber nonwoven composite sheet is valid.

Effects of urea–formaldehyde and polyvinyl acetate adhesive on sound absorption coefficient and sound transmission loss of palmyra palm fruit fiber composites

Composite materials made from palmyra palm fruit fiber (PPFF) were formed using urea–formaldehyde (UF) and polyvinyl acetate (PVAc). We prepared two sets of five different cylindrical samples with varying PPFF contents. The PPFF composites’ normal-incident sound absorption coefficient (SAC) and transmission loss (TL) were measured by using the impedance tube method. The sample with higher PPFF content shows a lower SAC spectrum. It is the opposite for the TL, where a sample with high PPFF content demonstrates a higher TL spectrum. We conducted the least-square fitting method on the experimental SAC and TL spectra utilizing the Johnson-Champoux-Allard (JCA) equivalent fluid model. Non-acoustic parameters were acquired from the fitting. The optimized porosity (ϕ), viscous characteristic length (Λ), and thermal characteristic length (Λ′) are inversely proportional to the PPFF content. The airflow resistivity (σ) and tortuosity (α∞), on the other hand, demonstrate a direct correlation with the PPFF content. Even though the UF samples have an average density of 14.7 % higher than the PVAc samples, their σ,Λ′, and α∞ are just 7.7 %, 4.5 %, and 0.39 % higher than PVAc samples. On the other hand, PVAc samples show higher average Λ and ϕ of 1.4 % and 0.73 %, respectively. The optimized porosity values obtained from the JCA model (ϕJCA) are coherent with ones from the direct estimation method assuming adhesive-coated fiber (ϕfa). It can be concluded that the adhesive’s quantity and density contribute to the composites’ porosity value, ultimately affecting material acoustic properties. Researchers can control and predict how the SAC and TL of fibrous sound absorbers would behave by varying the quantity and density of an adhesive.

Experimental study of the level-dependent softening of carbon particle stacks

Recently, it has been observed that a particle stack’s elastic modulus and damping change with incident levels. Hence, the measured normal incidence absorption coefficient also varies: i.e., the quarter-wave resonance peak shifts to a lower frequency and grows broader under progressively higher incident sound levels. Therefore, models with strain-dependent modulus and damping have been developed for sand particles based on a series of cyclic triaxial tests, while more recently, a velocity-dependent modulus was proposed to model the acoustically induced elastic softening for the hollow glass beads. To investigate the nature of this nonlinear behavior, the absorption coefficients of a 30 mm carbon particle stack were measured under four types of band-passed input signals (i.e., 500–1000 Hz, 500–2000 Hz, 500–4000 Hz, and 500–8000 Hz), each with 15 levels in steps of 1 dB. The input signals were designed to span the resonance frequency and to extend beyond to various degrees. The integrated sound pressure level, velocity, and displacement at the particle stack surface were calculated and were used to normalize the changing modulus and damping. It was found that only the total rms surface displacement can align the changing properties for all measurements.

Determination of Acoustic Characteristics of Melamine foam withExperimental Validation

Space launch vehicles experience severe dynamic loadings throughout the flight. Acoustic loads are one such load that is very critical to the launch vehicle at the time of ignition and take off. The amplitudes of acoustic loadings are generally very high and required to be diminished to save electronic components. Typically, one of the most common and efficient insulating materials is Melamine Foam (MF). This paper analyses the acoustic and environmental noise reduction using MF panels of variable thicknesses (25, 50, and 75 mm). Commercial FEA software is used to estimate the acoustic parameters, which are experimentally, validated using an impedance tube based on the transfer matrix method. The impedance tube can measure the normal incident sound absorption coefficient and transmission loss for the frequency range of 64 Hz to 6.2 kHz. A conventional, two-microphone impedance tube is connected to a sample holder downstream of the first microphone pair and a section downstream of the sample holder that accommodates a second pair of microphones. Two separate arrangements of impedance tubes are used to measure the absorption coefficient and transmission loss. The FEA and experimental results are compared and found in good agreements. Furthermore, the optimized insulation foam thickness is obtained based on required acoustic parameters.

The Effect of Deviation Due to the Manufacturing Accuracy in the Parameters of an MPP on Its Acoustic Properties: Trial Production of MPPs of Different Hole Shapes Using 3D Printing

In this study, we discuss the effect of the manufacturing accuracy of a microperforated panel (MPP) produced by 3D printers on acoustic properties through measured and calculated results as a pilot study. The manufacturing costs of MPPs have long been one of their shortcomings; however, with recent developments in the manufacturing process, low-cost MPPs are now available. In a further attempt at reducing the cost, 3D printing techniques have recently been considered. Cases of trial production of MPPs manufactured by 3D printing have been reported. When introducing such new techniques, despite the conventional microdrill procedure, manufacturing accuracy can often become an issue. However, there are few studies reporting the effect of manufacturing accuracy on the acoustic properties in the case of 3D-printed MPPs. Considering this situation, in this pilot study, we attempted to produce MPPs with circular and rectangular perforations using a consumer 3D printer of the additive manufacturing type. The hole sizes of the specimens were measured, and the accuracy was evaluated. The normal incidence absorption coefficient and specific impedance were measured using an impedance tube. The measured results were compared with the theoretical values using Guo’s model. Through these basic studies, the MPPs produced by an additive manufacturing 3D printer demonstrated good sound absorption performance; however, due to the large deviations of parameters, the agreement with the theoretical values was not good, which suggests that it is difficult to predict the acoustic properties of MPPs made by a consumer-grade additive manufacturing 3D printer.

Investigation of the acoustic properties of corrugated micro-perforated panel backed by a rigid wall

Extensive studies have been carried out on the acoustic performance of a micro-perforated panel (MPP) absorber which typically consists of a flat MPP fitted in front of a rigid backing wall. The present study concerns its acoustic properties when the conventionally flat perforated panel is replaced with a corrugated one that finds many applications in architecture. A three-dimensional finite element model is used to simulate the acoustic performance of a such corrugated MPP absorber at normal incidence, oblique incidence and diffuse field. Results show that the absorption performance of corrugated MPPA can be very different from conventional flat MPPA when the acoustic wave length is smaller than the corrugation depth. At low frequencies, the corrugated MPP performs similarly as a flat one of the same bulk perforation ratio. The absorption levels of the corrugated MPPA are considerably enhanced at the dips of absorption curves, which is favorable for reverberation control or broadband random noise reduction in large spaces and buildings. Further modal analysis shows that multiple modes of the corrugated configuration are excited at the peak and dip frequencies, and the acoustic response by the non-resonating modes give rise to the performance improvement. Absorption coefficients are also measured experimentally using an impedance tube. The measured normal incidence absorption coefficients compare well with the numerical predictions, which validates the numerical model and, in part, the numerical findings.

Study on the acoustic characteristics of natural date palm fibres: Experimental and theoretical approaches

The present study deals with the acoustic performance of natural fibres originated from the date palm empty fruit (DPEFB) fibres which is mainly considered as agricultural waste. The fibres were processed and fabricated to be sound absorber samples with two different densities of 100 kg/m3 and 200 kg/m3 and with thicknesses of 10–40 mm. The normal incidence absorption coefficients of the sound absorbers were measured using an impedance tube based on ISO 10534-2. The effects of fibre density and sample thickness are discussed. The findings reveal that for density of 100 kg/m3 the absorption coefficient is 0.6–0.8 above 1.5 kHz for the samples with the thickness of 20 mm and 30 mm. For the thickness of 40 mm, the values even reached the value of 0.9. The values can reach 0.7–0.8 above 1 kHz for the density of 200 kg/m3. Mathematical model using the optimized Delaney-Bazley model with Nelder-Mead simplex method is shown to successfully predict the sound absorption coefficient of the fibre samples. The Johnson-Champoux-Allard model follows the trend of the absorption coefficient, but underestimates the measured data at high frequencies above 2.5 kHz.

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.

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.