Process parameters during absorbent material formation have an important impact on sound absorption due to their effects on the characteristics of the absorbent mate­rial. Process parameters have been classified into production and treatment param­eter categories here. Different production and treatment processes used in biocom­posite sound absorber formation are given in Table 5.2.

5.3.3.1 PRODUCTION

The fiber component of biocomposites is mainly manufactured by web formation and bonding methods before combining with other components. Web formation can be classified into three categories:

• Drylaid system (carding or airlaying)

• Wetlaid system (similar to paper production from pulp)

• Polymer-laid system (spunbonding, meltblowing, etc.).79

• There are also three classes of web bonding processes:

• Chemical bonding (use of binders)

• Thermal bonding (calendaring, through-air blowing, or ultra-sonic impact)

• Mechanical bonding (needling, stitching, water-jet entangling).27

Among web forming types, Jayaraman et al.47 report higher sound absorption for fibrous structures formed by airlaying compared to carded ones irrespective of the fiber content. This finding was agreed by the findings of Parikh et al.22. This might be due to relatively random placement of fibers, and thus, higher tortuosity, higher number of pores with smaller sizes, higher number of fiber-to-fiber contact points, and gradient in porosity due to gravity.

Among web bonding methods, for needle-punching, the factors which affect noise reduction properties are given as the number of needling passes,136 and punch­ing density80.

Genis et al.80 found that the absorption coefficient reaches its maximum at mate­rial density rw = 100 kg-m-3 for thermo-bonded polypropylene, and punching den­sity P = 28 cm2 for needle-punched polypropylene and polyamide materials in the sound frequency range of 63 to 8000 Hz, for fiber diameters between 10 to 40 pm, and material thicknesses of 3 to 20 mm. They found the absorption coefficient of their needle-punched samples to have more dependency on the frequency range compared to thermo-bonded ones; thus they have a narrower absorption efficiency frequency range. The absorption in needle-punched materials was also more depen­dent on the diameter of fibers compared to thermally bonded webs. Jayaraman et al.47 did not report a significant difference between needled and needled plus ther­mally bonded fibrous structures.