Some part of the sound energy incident on a noise control material gets reflected, some get absorbed and the rest get transmitted, as shown in Fig. 6.4. This phenom­enon of acoustical interaction of the sound energy with the material is very strongly dependent on the physical structure and physical properties of the material. The sound absorption coefficient is defined as the amount of energy absorbed by the noise control material to the energy incident on the material. The materials which have high sound absorbing coefficients are usually used as sound absorbers. A good barrier material “blocks” the sound and reflects it back to the incident medium. A good sound absorbing material has a poor reflecting capability. The sound transmis­sion coefficient of an acoustical panel is the fraction of sound power in the incident airborne that appear in the transmitted airborne wave on the opposite or rear side of the acoustical panel. These coefficients are a function of the frequency of the sound wave, and thus need to be known for the noise control materials in the frequency or a frequency band at which noise has to be controlled15,16. Traditional sound absorbers are open cell porous polyurethane foam, fiber glass and naturally occurring materi­als like coir, cotton, hemp and jute. Traditionally used sound barrier material for industrial noise control are heavy concrete, steel, lead and so forth.

The mechanism of sound absorption in a material is due to the viscous loss be­tween the sound pressure waves while interacting with the walls of the pores in the material. The sound energy is thus dissipated as heat at the pores. At low frequen­cies this energy exchange is isothermal and at high frequencies it is adiabatic17. The sound absorption coefficient of a material depends upon its acoustical impedance at the surface. Several experimental techniques exist for the measurement of the same18,19. The fiber size, porosity, tortuosity, flow resistivity affect the impedance of the material and some of these values for the jute derivatives are measured by experiments and some of them can be estimated from analytical formulations. Many a times during numerical simulations of computer aided engineering models using techniques like the finite element method and the boundary element method the knowledge of impedance of these biocomposite materials are required to estimate the noise reduction obtained20. The sound absorption coefficients of recycled fibrous materials and effects of thickness, surface facing and compression on the sound absorption coefficients of some fibrous material are available in the literature.21,22

The research on natural fibers shows that Egyptian cotton can be used as an acoustical material in different forms23. It is an effective, cheap product and pos­sesses a high sound reducing capability. Also, it is easy to use and creates no health risks compared to common commercial acoustical materials. Work has been done on natural fibers such as jute, coir and sisal where the structure-property relation­ship of these fibers including fracture modes has been determined. Attempts to in­corporate them in polymers and characterization of these new composites, with and without subjecting them to environmental conditions, have also been reported24. Relationship between the sound absorption coefficients of a cover made of woven cotton fabric with its intrinsic parameters has been determined25. The effect of air space, behind and/or between the layers of new sample made of local textile ma­terial (100% cotton) which is produced from a specially woven structure on the absorption coefficient has been studied for use as a sound absorbing curtain26. Stud­ies show that natural fiber composites are likely to be environmentally superior to glass fiber composites in most cases for the following reasons: (1) natural fiber production has a lower environmental impact compared to glass fiber production; (2) natural fiber composites have higher fiber content for equivalent performance, reducing the more polluting base polymer content; (3) the light-weight natural fiber composites improve fuel efficiency and reduce emissions, especially in automobile applications; and (4) end of life incineration of natural fibers results in recovered energy and carbon credits27.

Sound proofing properties such as absorption coefficient and transmission loss index of natural organic multilayer coir fiber has been studied28. The effect of perfo­rated size and air gap thickness on acoustical properties of coir fiber has also been studied. Comparison of acoustical properties between coir fiber and oil palm fiber has also been reported. The results obtained show that the coconut coir fiber gives an average noise absorption coefficient of 0.50. It shows a good noise absorption coefficient for higher frequencies but less for the lower frequencies. The oil palm fiber gives an average noise absorption coefficient of 0.64. The oil palm fiber shows a good noise absorption coefficient for higher frequency region compared to lower frequency region. Both fibers have a high potential to be used as sound absorber ma­terials. The potential of using coconut coir fiber as sound absorber is also there. The effects of porous layer backing and perforated plate on sound absorption coefficient of sound absorber using coconut coir fiber were studied and implemented using woven cotton cloth as a layer type porous material in car boot liners in automobile industry29. Another type of natural sound absorbing material such as industrial tea — leaf-fiber waste material for its sound absorption properties has been investigated30. The experimental data indicate that a 1 cm thick tea-leaf-fiber waste material with backing provides sound absorption which is almost equivalent to that provided by six layers of woven textile cloth.

The technology based on the synthetic fiber composites made up of glass, Kev­lar or carbon has played a vital role in noise reduction applications in the aerospace industry since 1950. The advancement in the composites design after reaching the aerospace requirements is targeted for the general industrial and domestic sectors. In contrast, the increased usage of electrical and mechanical appliances at home and industries has created a concern for noise pollution. Even though synthetic compos­ites possess specific properties like light-weight, high strength-to-weight ratio and stiffness, they are not much applicable for the industrial and domestic sectors due to the high cost of the raw materials. Further these materials are harmful when kept exposed in the open environment. Though recently rice hull has been added to open cell polyurethane foam and its acoustical properties evaluated31. On the basis of these aspects, the new direction in industrial application on sound proofing materi­als based on natural biocomposites reinforced with fibers of sisal, coir, jute, etc. is steadily developing in the past few years.

There are many theoretical models available for predicting the sound absorption coefficients of sound absorbing materials32. Though the sound absorption coefficient can be measured, the knowledge of certain acoustical parameters help the material developer to estimate the sound absorption coefficients well before the material is made. Some such parameters are density, fiber size, porosity, tortuosity, flow resis­tivity and characteristic lengths.