Experimental and mathematical survey of sound absorption performance of date palm fibers

Application of Scirpus grossus fiber as a sound absorber

The application of Scirpus grossus (SG) fiber as a sound absorber is important to reduce the level of noise affected the physical and mental wellbeing of people. The sound absorption coefficient (SAC) and noise reduction coefficient (NRC) of the SG specimen were evaluated based on a typical model-based design using the data analysis with MATLAB. The results showed that SG specimen with a thickness of 20 mm coated with the perforated aluminum sheet (PAS) compared to that without coating can improve the capability of sound absorption by 14% at the frequency of 4000 Hz. SG specimen coated with PAS that has a NRC value of 0.39 can absorb 39% of sound and thus reflects 61% of sound wave while SG specimen without coating that has a NRC value of 0.23 absorbs 23% of sound and can reflect 77% of sound wave. The sound absorption class of D for SG specimen coated with PAS should be better that of E for SG specimen without coating, which permits us to get better understanding on the applications of SG fiber as a sound adsorber in the future.

Theoretical and Experimental Analyses on the Sound Absorption Coefficient of Rice and Buckwheat Husks Based on Micro-CT Scan Data

In this study, the sound absorption coefficients of rice and buckwheat husks were estimated. Computed tomography (CT) images were processed to determine the circumference and surface area of voids in the granular material, and the normal incident sound absorption coefficients were derived. In addition, the tortuosity, which expresses the complexity of the sound wave propagation through the structure, was measured for each material. The theoretical sound absorption coefficients were then compared to the measured sound absorption coefficients with and without consideration of the tortuosity. A correction factor was used to bring the surface area of the granular material closer to the actual surface area and observed that the tortuosity obtained theoretical values that matched the trend of the measured values. These results indicate that using CT images to estimate the sound absorption coefficient is a viable approach.

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.

The morphological and anatomical variability of the stems of an industrial hemp collection and the properties of its fibres

Industrial hemp (Cannabis sativa L.) is identified as a leading fibre crop and there is increasing interest in C. sativa fibre due to its new range of industrial applications. However, the complexity of hemp germplasm resulted in insufficient information on the effect of genotypes on fibre quality and quantity. In this study, 16 fibre and non-fibre type hemp genotypes were evaluated to compare the morpho-anatomical differences of stems and physico-mechanical fibre properties under three retting methods and to understand the effect of stem colour on the properties of hemp fibres. Morphological markers were scored and stem anatomy was examined using live and herbarium collections. Stems were retted using chemical, enzymatic, and microbiological methods. The resulting fibres were tested for tensile strength, moisture retention, colour, bast and hurd dry weights. Hemp genotypes showed morphological variations that affect fibre processing and a unique pattern of fibre wedges in cross-sections of the basal internode. Fibre yield, tensile strength, colour, and moisture retention significantly varied among the genotypes. The hemp collection used in this study formed three clusters in principal component analysis and traits such as internodal length, node number, hurd yield, and tensile strength highly contributed to the total variability. Additionally, non-fibre type hemp genotypes that showed important fibre properties were identified. The hemp genotypes that were selected based on our approaches can be tailored towards the specificities of the end-usage of choice. Our methods will enable the exploration of hemp genetic diversity pertaining to fibre properties and contribute to the preliminary identification of genotypes as a supplement to genetic analyses.

Effect of 3D-Printed PLA Structure on Sound Reflection Properties

3D printing technique is currently one of the promising emerging technologies. It is used in many areas of human activity, including acoustic applications. This paper focuses on studying the sound reflection behavior of four different types of 3D-printed open-porous polylactic acid (PLA) material structures, namely cartesian, octagonal, rhomboid and starlit structures. Sound reflection properties were evaluated by means of the normal incidence sound reflection coefficient based on the transfer function method using an acoustic impedance tube. In this study, various factors affecting the sound reflection performance of the investigated PLA samples were evaluated. It can be concluded that the sound reflection behavior of the tested PLA specimens was strongly affected by different factors. It was influenced, not only by the type of 3D-printed open-porous material structure, but also by the excitation frequency, the total volume porosity, the specimen thickness, and the air gap size behind the tested specimen inside the acoustic impedance tube.

On the Dependence of Acoustic Pore Shape Factors on Porous Asphalt Volumetrics

The sound absorption of a road pavement depends not only on geometric and volumetric factors but also on pore shape factors. In turn, pore shape factors mainly refer to thermal and viscous factors (i.e., thermal and viscous effects that usually occur inside porous materials). Despite the presence of a number of studies and researches, there is a lack of information about how to predict or estimate pore shape factors. This greatly affects mixture design, where a physical-based or correlation-based link between volumetrics and acoustics is vital and plays an important role also during quality assurance and quality control (QA/QC) procedures. Based on the above, the objective of this study is to link mixture volumetrics and pore shape factors. In particular, 10 samples of a porous asphalt concrete were tested in order to estimate their thickness, air voids content (vacuum-sealing method, ASTM D6857/D6857M), sound absorption coefficient (Kundt’s tube, ISO 10354-2), airflow resistivity (ISO 9053-2), and permeability (ASTM PS 129). Subsequently, two models (herein called STIN and JCAL) were used to derive both volumetrics and pore shape factors from the estimated parameters listed above, and statistical analysis was carried out to define correlations among the parameters and models performance. Results confirm the complexity of the tasks and point out that estimates of the pore shape factors can be derived based on mixture volumetrics. Results can benefit researchers (in acoustic and pavement mixtures) and practitioners involved in mix design and pavement acceptance processes.

Morphological, acoustical, mechanical and thermal properties of sustainable green Yucca (Y. gloriosa) fibers: an exploratory investigation

BACKGROUND

The objective of this study was to evaluate the acoustical, morphological, mechanical and thermal properties of fibers extracted from the leaves of Yucca (Y. gloriosa) shrub.

METHODS

Several tests were performed on either untreated or alkali-treated (5% NaOH) fibers. The chemical analysis of the fibers was performed to determine their lignin, cellulose, hemicellulose, wax and moisture content. Fourier transform infrared (FTIR) and thermogravimetric analysis were respectively employed to chemically and thermally analyze the fibers. The microscopic examination was also carried out using scanning electron microscope (SEM). INSTRON universal testing machine and an impedance tube system were employed for measuring the tensile properties of the fibers and the sound absorption coefficient values of the samples fabricated from the same fibers, respectively.

RESULTS

The results from the experiments revealed that the fibers have low density (1.32 g/cm3) and higher cellulose content (66.36 wt.%,). The mechanical characterization of these fibers also confirmed they are similar to the other lignocellulose fibers used for the reinforcement of polymer matrix composites. The tensile strength test conducted on Yucca fibres showed that mechanical properties of alkali treated fibers are superior to the untreated fibers. The thermal analysis also demonstrated that the alkali treated fibres can thermally withstand temperatures of up to 364 °C which confirms the fact that the thermal stability of fibers was improved by alkali treatment.

CONCLUSION

Fibers extracted from the leaves of Yucca demonstrated fair amounts of mechanical and thermal resistance and strength. The samples fabricated from Yucca fibers demonstrated fair levels of sound absorption coefficients particularly at higher frequency ranges which are typical to the other natural fibers. The possible use of Yucca fibers as either a reinforcing agent for composites or a sound absorbing medium is highly promising.

Elucidating the Sound Absorption Characteristics of Foxtail Millet (Setariaitalica) Husk

The current study deals with the analysis of sound absorption characteristics of foxtail millet husk powder. Noise is one the most persistent pollutants which has to be dealt seriously. Foxtail millet is a small seeded cereal cultivated across the world and its husk is less explored for its utilization in polymer composites. The husk is the outer protective covering of the seed, rich in silica and lingo-cellulose content making it suitable for sound insulation. The acoustic characterization is done for treated foxtail millet husk powder and polypropylene composite panels. The physical parameters like fiber mass content, density, and thickness of the composite panel were varied and their influence over sound absorption was mapped. The influence of porosity, airflow resistance, and tortuosity was also studied. The experimental result shows that 30-mm thick foxtail millet husk powder composite panel with 40% fiber mass content, 320 kg/m³ density showed promising sound absorption for sound frequency range above 1000 Hz. We achieved noise reduction coefficient (NRC) value of 0.54. In view to improve the performance of the panel in low-frequency range, we studied the efficiency of incorporating air gap and rigid backing material to the designed panel. We used foxtail millet husk powder panel of density 850 kg/m³ as rigid backing material with varying air gap thickness. Thus the composite of 320 kg/m³ density, 30-mm thick when provided with 35-mm air gap and backing material improved the composite's performance in sound frequency range 250 Hz to 1000 Hz. The overall sound absorption performance was improved and the NRC value and average sound absorption coefficient (SAC) were increased to 0.7 and 0.63 respectively comparable with the commercial acoustic panels made out of the synthetic fibers. We have calculated the sound absorption coefficient values using Delany and Bezlay model (D&B model) and Johnson-Champoux-Allard model (JCA model) and compared them with the measured sound absorption values.