Masomeh Ghassem

Enhancement of coir fiber normal incidence sound absorption coefficient

Coir fiber from coconut husk is an important agricultural waste in Malaysia. The porous structure of fiber makes it an eligible material for acoustical absorption. In previous studies at Universiti Kebangsaan Malaysia, single layer coir fiber showed low acoustical absorption in medium and low frequencies; e.g. absorption coefficient of a 20 mm industrial prepared coir fiber sample was below 0.3 for frequencies less than 1.5 kHz. Current research was initiated to improve the shortcoming by mixing industrial prepared coir fiber with air gap layers. Analyzes were based on two approaches, namely; Delany–Bazley with acoustic transmission analysis (ATA) and Allard elastic model with transfer matrix analysis. Experimental measurements were conducted in impedance tube to validate the analytical results. Outcomes described that Delany–Bazley-ATA technique was an approximate solution showing overall absorption path without giving any accurate information about the peaks and resonances. Allard method took the elasticity of material into account and transfer matrices were able to characterize the whole structure as a combination of single layers. Results were close to experimental values and predicted the path and resonances very well. Further analyzes were conducted by Allard method and derived that having a fiber layer backed by an air gap was better than leaving the same gap in between that layer. The explanation was that, in the first arrangement, the sound field impacted a solid layer with higher thickness and followed a longer transmission path which caused a higher acoustical absorption. Furthermore, it was noticed that a reasonable thickness of backing air gap improved the overall sound absorption. Increase in the gap thickness produced more peaks and moved them to lower frequencies which caused better absorption in low frequencies. The cause of this phenomenon was increase in impedance of the panel due to rise in the air-gap thickness. This moved the acoustical resonances to lower frequencies and improved the sound absorption in this frequency span. Finally, it was concluded that other acoustic absorption techniques such as adding perforated plate may be combined with coir fiber-air gap structure to improve the low frequency acoustic absorption coefficient without any need to highly increase the air gap thickness.

Implementation of Coir Fiber as Acoustic Absorber Material

Coir fiber is one of the natural fibers that can be easily found in tropical countries like Malaysia. This study illuminates the acoustical absorption of coir fiber throughout the frequency spectrum. The empirical Delany-Bazley method together with Allard elastic model were the two analytical approaches implemented in this research. Outputs of analytical techniques were validated by impedance tube measurements. Both methods were able to show the general pattern but existing frame resonance/resonances could only be predicted by Allard model as Delany-Bazley method does not take modulus of elasticity into consideration. Absorption of fresh coir fiber was satisfactory; more than 70% at f>600 Hz for a 30 mm thickness sample with resonance peak center frequency at 1 kHz. The industrial prepared coir fiber is mixed with binder; has higher stiffness and contains less amount of moisture compared to fresh samples. Therefore the absorption of industrial prepared fibers with the same thickness were generally less than 70% except for a small region around its resonance peak which also happened at higher frequency of 2.6 kHz. It is suggested that absorption of the industrial samples should be improved further in order to be used in real world applications.

Novel implementation of natural fibro-granular materials as acoustic absorbers

In this study, a new innovative fibro-granular composite was prepared as a natural acoustic material by combining fibrous and granular materials. Delany–Bazley model and Biot–Allard techniques were utilized to estimate the absorption coefficient of the developed composite material. The predicted values were validated through an analytical outcome employing Johnson–Champoux–Allard technique and an experimentally measured value which were conducted in an impedance tube. The outcomes showed the reflection of the good agreement between the analytical and experimental methods. The current research concluded that the introduction of granular materials provides significant contribution in enhancing the surface area within the composite, thereby achieving a promising acoustic absorption in the low-frequency region which is below 1 kHz. In addition, the current research also reports that, like Johnson–Champoux–Allard model, Delany–Bazley and Biot–Allard models are also two efficient analytical tools for predicting the significant low-frequency acoustic absorption performance of a fibro-granular composite.

Improvement of Fire Retardant Property of Natural Fiber

Excessive exposure to noise is harmful for human health. Noise-induced hearing loss is one prevalent disorder resulted from above case. One root solution that converts the unnecessary sound waves to dissipated heat energy is acoustic absorption panel. Previous studies had looked upon potential sound-absorbing resources corresponding to natural fiber. However, several characteristics of these biodegradable supplies such as stiffness, anti-fungus and flammability are still yet to be improved. Hence, this research was undertaken to enhance the fire retardant performance of coir fiber for the production of high quality yet low cost acoustic absorption panel. Three types of additives, borax, Di-Ammonium Phosphate (DAP), and urea were investigated to perform chemical treatment for coir fiber. Experimental measurements were executed to validate the results by referring standard of ASTM E6902 (Standard Test Method for Combustible Properties of Treated Wood) by using the Fire-Tube Apparatus. Final results showed that DAP-treated fiber has the lowest percentage loss in mass of 6.67% compared to that of borax and urea-treated fiber with values of 7.60% and 9.48% respectively. This outcome clarified that DAP-treated fiber possesses higher self-extinguishing ability. Further evaluations in term of economic values, degree of hazards against health, flammability as well as reactivity supported that DAP is the best choice since its potency was ahead of the other two chemicals.