Experimental study and modelling of date palm fibre composite acoustic behaviour using differential evolution algorithm

Background and aims: In recent years, in most countries of the world, the provision of a calming environment without disturbing noise has become a need, which has subsequently led to significant growth in noise control techniques. It is now well documented that prolonged over-exposure to the excessive levels of environmental noise not only induces disabling hearing impairment but also contributes to numerous adverse health effects such as stress, irritability, hypertension, cardiovascular diseases, annoyance, and sleep disturbance. Working in noisy environments has also been reported to be linked with increased workplace accidents, aggression and anti-social behaviours. Based on the World Health Organization (WHO), occupational noise accounts for 16% of the disabling hearing loss in adults (more than four million DALYs), with estimates of disease burden ranging from 7% in developed countries to 21% in underdeveloped and developing nations. There are several methods to reduce the noise pollution and control the sound of the working environment; one of the methods of controlling sound propagation path is the use of insulation and sound absorption. Sound control improvements with using sound absorption materials have provided a suitable opportunity to study noise reduction and acoustic attenuation by using a variety of porous materials. Increasing concerns about the adverse effects of the use of synthetic sound absorbing materials on the health of individuals has provided a favorable ground for the development of research on the use of natural fibers as insulators and absorbents. During the last two decades, there has been considerable interest in the use of lignocellulosic fibres for various purposes such as sustainable acoustic absorbers.  Bio-based composites of acoustic absorbers are mainly biodegradable and non-toxic and have low weight, low density and low cost and can be used as alternatives to the absorbers made of synthetic fibres. Every year, in Iran, significant amounts of agricultural waste containing lignocellulosic fibres are burnt or disposed of improperly due to the lack of a proper recycling system and reusing mechanisms. However, a large proportion of such waste can be used for various uses. Iran as the second largest producer of dates in the world, and having large date palm plantations, faces the scourge of harvesting and pruning palm trees each year, which are largely suitable for use. Therefore, the purpose of the present study was to investigate the acoustic behaviour of composite samples made of date palm natural fibres by predicting their sound absorption coefficients with the related experimental models and comparing the results with the data obtained from the experimental tests.  Methods: In this study, the natural date palm waste fibers were procured from Tabas city in southern Khorasan province in Iran. Having transferred to the laboratory, the raw DPFs were rinsed with distilled water before being placed inside an oven at 70 °C for 24 hours to dry and get a fixed weight. The DPFs were then cut into smaller pieces by a pair of scissors and crushed and pressed through a 2 mm mesh sieve to make their size fairly homogenous. In order to bind the fibers together and form a composite, polyvinyl alcohol (Sigma-Aldrich) was utilized. PVA is a polymeric chemical binder with high solubility in water and is known to be a biodegradable material reportedly used as natural fiber binder in previous studies. In order to prepare 5% PVA concentration, 5 g of the substance was first weighed by a scale and later dissolved in 100 ml of distilled water. The solution was then stirred with a magnet at 80 °C for 3 hours. Having prepared the solution, the fibers were soaked with it to bind. The fibers were then shaped by an aluminum mold (by compression molding) to be fitted inside the impedance tube (internal diameter of 100 mm) and form samples with thicknesses of 20, 30 and 40 mm with a constant density of 200 kg/ m3. After the molding process, the samples were left at room temperature for 12 hours to dry completely before being transferred to the laboratory for absorption coefficient measurement. SEM images showed that outer diameter of the DPFs ranged from 200 to 720 μm and the mean diameter, the density and porosity of them were 420 μm, 930 kg/m3 and >80% respectively. The measurement of normal incidence absorption coefficients of the samples with a constant density and three different thicknesses a different air gaps was performed by an impedance tube and based on ISO 10534-2. Afterwards, by creating the appropriate codes in MATLAB software, using the differential equation algorithm, the predicted sound absorption coefficients for the Delany-Bazley, Micky, and Johnson-Champoux-Allard models were calculated. In fact, such algorithm searches for the three parameters until the best fit (minimized difference) is observed between the experimental and theoretical absorption curves. Therefore, the experimental sound absorption curve for the samples of the corresponding date palm fibers was first determined by the impedance tube system. We followed the same method proposed by Atalla and Panneton. Results: The results from the laboratory data for the three thicknesses of 20, 30 and 40 mm of the sample absorbers of DPF and the impact of the air gaps on their sound absorption coefficients are presented. Using the mathematical model, the sound absorption coefficient in the frequency range of 125-6300 Hz was next coded by MATLAB software. As shown in this paper, the predicted values of the physical parameters of the DPF were measured using differential equation algorithm in the MATLAB software through the available laboratory data such as the thickness, density, airflow resistivity and absorption coefficient. Later, the prediction error rate determined by the JCA model - shown on the right axes is compared to the data obtained from the experiments. The results showed that the sound absorption coefficients of the samples fabricated from date palm fibres, significantly increased with increasing the frequency. Moreover, the increase in the thickness of the samples with a constant density has a major role in attenuating the sound waves, especially in lower frequencies (less than 1000 Hz). Comparison of experimental data and models output showed that by increasing the thickness, the predicted values for the acoustic absorption coefficient of the samples approach the values obtained from the experimental tests. As can be seen, the JCA model has a fair accuracy in predicting the absorption coefficient in different thicknesses compared to the Delany-Bazley and Micki models. Thus, this model at 20, 30- and 40-mm thickness and lower frequency range (63-1600 Hz) showed different prediction accuracies of 23%, 6% and 11% respectively, while at higher frequency range (1600-6300 Hz) came up with 9%, 6% and 4% prediction accuracies. Thus, such agreement is considered as significant in the higher frequency range. These findings also indicate that the outputs of the JCA model, rather than two other models, are closer to the data obtained from the experimental tests for setting the sound absorption coefficients. It is therefore concluded that the JCA model (compared to the Delaney-Basile and Mickey models) has included more dominant parameters that might impact the physical properties of the samples such as thickness, mass density, sample resistance against airflow, tortuosity, viscous and thermal characteristic length. Introduction of air gap behind the DPF samples in the impedance tube transfers the maximum values of sound absorption coefficient from the upper to the lower frequency range. The results indicate that, as the sample distance from the rigid surface of the backing (up to 30 mm) increases, the sound absorption coefficient at frequencies lower than 1000 Hz will rise as well. As a result, used in this study absorbing materials with an air cavity behind them seems to play a significant role in lower production cost, and thinner layers of absorbing materials yield better absorption coefficients. The reason for this behavior is probably due to the increased impedance of absorbing material. In this case, the acoustic resonance transferred towards lower frequencies and thus improved the values of absorption performance in that range. As a result, it seems that the use of absorbing material, while having a layer of air behind them, can reduce the cost and material consumption. Thus, thinner samples made of coir fibers would provide superior values of sound absorption coefficients. Conclusion: Date palm fibres have a great potential for attenuating the energy of sound waves. Elevated levels of sound absorption can be attributed to a longer dissipative process of viscous and thermal conduction between air and the absorber.  Therefore, increasing the thickness of the sample will contribute to higher amounts of sound absorption coefficient. On the other hand, natural fibers are still not as popular as synthetic fibers due to the properties such as low adhesion, high fiber diameter, low resistance to moisture and vulnerability to fungi. Therefore, it is obvious that these factors can affect the absorption coefficient of sound and the quality and durability of acoustic panels made of such natural fibers. In order to eliminate these defects, nanotechnology can be employed to achieve superior properties and better conditions by utilizing the best of these natural materials. Evidently this is the issue that should be taken into account in developing sound absorbers which are originated from the natural fibers.