NAZIRE DENIZ YILMAZ and NANCY B. POWELL

ABSTRACT

Biocomposites, provided that they are produced in porous form, that is, unconsoli­dated structure, act as noise control elements in a wide range of applications as they present a cost-effective, light-weight, and environmentally friendly alternative to conventional sound absorbers. This chapter presents an overview of biocompos­ites as rigid, porous sound absorbers. Sound absorption mechanisms that take place in porous biocomposites are explained. Methods of measuring sound absorption performance are presented. Some models to predict sound absorption capacity are described. Based on these models, factors that affect sound absorption behavior of biocomposites are given. An overview of biocomposite sound absorbers developed by researchers is reviewed. Suggestions for future research are listed.

5.1 INTRODUCTION

Advances in new technologies are often accompanied by noise pollution, besides air, soil, and water pollution.1 To give an example, transportation is a major source of noise pollution with the ever-increasing number of more powerful and larger vehicles on the road. Vehicle passengers are affected by the noise generated by vehi­cles as much as the people outside the car. In addition to affecting the comfort of the passengers, it has negative effects also on the driver such as fatigue and distraction; hence, reduces the safety of the occupants. Not only the comfort and safety of pas­sengers, but also the quality perception of the vehicle is deteriorated by unwanted sound.2

While progress in technologies has resulted in higher standards of living, devel­opment of advanced materials should be carried out with responsible environmental practices. In this respect, ever-tightening regulations, together with growing public awareness, force the manufacturing industry to select environmentally friendly ma­terials and processes.3 Biocomposites, which contain bio-based or biodegradable

components, offer a viable alternative to their conventional counterparts: glassfi — ber based synthetic polymer composites. Automotives is a promising market seg­ment for fiber-reinforced biocomposites with increasing product quantity, quality and variety. More than 40 automobile components including trunk and hood liners, floor mats, carpets, padding and door panels are conventionally made of fibrous structures and composites. This fact presents the significant potential for the use of bio-fibers as substitution for conventional petro-based fibers.4 Bio-fiber based composites, that is, biocomposites have already found commercial uses by major vehicle producers since the 90s, in automobile components including door linings and panels, package shelves, and seatback linings,5 for all of which, noise control is a requirement.

In today’s conditions, environmentally friendly industrial practices cannot be carried out at the expense of quality performance.6 Within this context, acoustic biocomposites should be able to compete with conventional sound absorbers such as glassfiber composites.3 Glassfiber presents some critical disadvantages in terms of human and environment ecology, including being unsafe to handle and posing health risks when inhaled, in addition to being nonrecyclable.2-7’8 Due to these afore mentioned drawbacks, bio-fibers are gaining increased attention in a variety of en­gineering fields to replace glassfibers.9 Natural plant fibers offer some advantages compared to glassfibers. The specific gravity of plant fibers (~1.5 g/cc) is lower than that of glassfibers (~2.5 g/cc). If used in transportation, this, in turn, leads to lower gas consumption, that is, higher mileage per gallon and lower greenhouse gas emis­sions. Other advantages can be listed as lower cost, lower weight, and better heat insulation and noise reduction characteristics.1

There are three major methods to reduce unwanted noise. Primary methods consider modifications at noise and vibration sources. Secondary methods include alterations along the sound propagation path, and tertiary methods engage in sound receivers. Primary methods are restrained by economical and technical parameters to a great extent; while tertiary methods have to deal with each receiving person separately. This situation renders the secondary methods relatively advantageous in a number of applications.310 The secondary methods concerning the control of air­borne noise include the use of sound barrier and absorbers.11 This chapter is focused primarily on sound absorbers.

Sound absorbers are porous materials. In this context, sound absorber biocom­posites should be allowed to have pores as shown in Fig. 5.1. Noise is attenuated in tortuous channels of pores present in the porous materials due to viscosity and heat conductivity of the medium.1213 Porous sound absorbers can be classified into three groups: cellular, granular and fibrous materials.10 Among sound absorbers, fibrous materials are promising materials for noise reduction applications. Fibrous materi­als are advantageous in that they absorb more sound over a broader frequency range compared to other materials.14 Fibrous materials may also be more environmentally friendly in terms of production and after-service life practices.115

Among parameters of porous materials, air flow resistivity, porosity, and tortu­osity are the main factors that affect sound absorption.13,17 In fibrous materials, flow resistivity increases with decreasing pore dimensions and fiber diameter. In addition to fiber size, fiber orientation,18 web density, porosity, tortuosity,19 mean pore size, pore size distribution, and absorber surface characteristics also affect flow resistiv- ity.20 Fiber reinforced composites offer some advantages compared to conventional sound absorber materials, including the economical price of the raw materials, effient thermo-processing, and lower specific weight.21

A thorough knowledge of sound propagation through fibrous materials is of prime importance for evaluating the noise absorption capacities of biocomposites, which are designed to serve as noise control elements in a wide range of applica­tions. Sound absorber biocomposites are mostly produced by natural fiber nonwo — vens bonded by some means to produce three-dimensional rigid materials.

In this work, the term “biocomposite” refers to a material made up of distinct parts such as fibers or resin either of which is of biological origin. As the topic is related to noise control in terms of sound absorption, most of the biocomposite ex­amples given in the open literature are in their unconsolidated form as they include pores to allow for sound wave dissipation.

This chapter presents an overview of biocomposites as rigid, porous sound ab­sorbers. Sound absorption mechanisms that take place in porous biocomposites are explained. Methods of measuring sound absorption performance are presented. Some models to predict sound absorption capacity are described. Based on these models, factors that affect sound absorption behavior of biocomposites are given. An overview of biocomposite sound absorbers developed by researchers is re­viewed. Suggestions for future research are listed.