The first major step in noise control is to determine the source of the noise be­fore designing the sound absorber. To give an automotive-related example where biocomposites find applications; the engine, the tire-road interaction and the wind are the three major factors that affect the acoustics of the passenger department of a vehicle. Hence, the floor coverings, the headliner, and the hood insulation are the critical contributions to the acoustic performance of vehicles.2 The other most important characteristic to be sought in the design of absorption materials are the noise absorption capacities in the audible frequency range of interest. For example, medium range (1200-4000 Hz) is of interest for the interior of passenger vehicles.45 Also, cost effectiveness and other conditions should be considered including dura­bility in hostile environments such as high temperature, contamination, and high speed turbulent flow, etc.17 Recyclability, lightweight, thermal comfort and contri­bution to passive safety systems are also desirable characteristics of noise absorbers in vehicles.2

Different researchers give importance to different material parameters as influ — encers of sound absorption. Among the material parameters, which are applicable to porous absorbers, Bies and Hansen29 cite only the flow resistance/resistivity, Cox and D’antonio14 consider flow resistivity and porosity, whereas Fahy,13 and Atten­borough and Ver17 refer to porosity, and structure factor (tortuosity) in addition to flow resistance/resistivity as the primary parameters that affect the sound absorption properties.

Several authors give various parameters as factors affecting the sound absorp­tion of fibrous structures. Cox and D’antonio14 report that sound absorption effi­ciency of fibrous structures can be achieved by manipulating:

• Material density

• Fiber composition

• Fiber orientation

• Fiber dimensions.

Banks-Lee et al.36 reported that the mass per unit area, thickness and porosity of the material and fiber fineness to be of significant importance to the airflow resis­tance and sound absorption of needled fibrous materials.

The factors affecting sound absorption behavior of fibrous materials, biocom­posites in particular, are classified as fiber parameters, macroscopic physical pa­rameters, process parameters of the porous material production and treatments as shown in Table 5.1 and explained below. It is important to note, beforehand, that the parameters to be described are not independent from each other.3

TABLE 5.1 (Continued)

Source: Yilmaz (2009).3