NOWADAYS, sound absorbing materials used for building, transportation, electrical and electronics, packaging, information and communication technology and biomedical applications are expected to be sustainable. This requirement is becoming more important with the growing interest in sustainability, which aims to reduce greenhouse gas emissions.
With the rapid advancement of technology, particularly in industries and transportation sectors, noise pollution has been on the rise and a source of excessive disturbance for humans as well as animals. It has now become a major causal agent of environmental contamination and a concern for personal health.
Noise control and its principles play an important role in creating an acoustically fair environment. Noise control can be accomplished when the loudness of sound is brought down to a point that is not harmful to human ears. Achieving a quiet environment can be obtained by employing diverse techniques that utilise different materials.
Noise reduction materials are divided into two broad categories. One is the sound-absorbing material and the other is acoustic material. Many of the fibre reinforced composites are used as sound absorbing materials and only a small number of them are used as acoustic materials.
Acoustic absorption depends on the frequency of the sound waves. For a given noise source, the manner of the sound it emits and the noise levels it produces within a chamber are largely dependent on the sound absorbing properties of the materials in the chamber.
The sound absorption coefficient of a material is the fraction or percentage of incident sound energy that is absorbed by the material. Sound absorption coefficients of materials are sometimes expressed as percentages and recently, many studies have been carried out in this field and new findings have uncovered the potential use of new materials for sound absorption applications.
A wide range of sound absorbing materials exist in which they provide absorption properties, dependent upon frequency, composition, thickness, surface refinement and method of mounting. Research on composite materials and natural fibres was done on acoustical panels. Usually acoustical panels are made from synthetic fibres which are hazardous to human health and the environment, not to mention quite expensive for trivial needs.
Synthetic fibres are also called man-made fibres and can be created industrially. There are several hundred types of synthetic fibres in the world. Synthetic fibres are often manufactured with cellulose as the starting materials. The sound absorption materials which are commercially available for acoustic treatment include glass or mineral fibre material.
The sound absorption characteristic values of rock wool were measured and found to be comparable to those of glass fibre. Nevertheless, considering health and safety, these fibres can adversely affect human health when brought into contact. Because of this health risk, other alternatives have been investigated such as the use of natural renewable fibres rather than synthetic fibres.
With the recent increasing attenuation towards sustainability and environmental awareness, there is a large need to find clean, green, and sustainable materials that can be used as replacements. These events explore an opportunity to look for substitute materials from organic fibres to be grown as a noise absorption material.
Lignocellulosic fibre, also known as natural fibre has gained much interest over the past few decades in terms of research and engineering due to its extraordinary properties. Natural fibres are commonly retrieved from plants, animals as well as minerals. Plant fibre such as cotton, kenaf, oil palm, pineapple leaf fibre, jute and so on can be obtained from the plants stem, leaves, seed and its fruits, while animal fibres consists of mainly protein are extracted from animal’s hair, silk, and wool.
Apart from having vast benefits in enhancing materials’ mechanical properties, natural fibres are also known to be biodegradable, inexpensive, easy to process, renewable, recyclable as well as environmentally friendlier as compared to synthetic fibres. Organic fibres as basic material for sound absorber have various benefits. They are renewable, nonabrasive, cheaper, in abundance and have less potential health hazards.
Natural fibre composite materials are superior to synthetic fibre composites in certain properties like sustainability, renewability, recyclability, triggered biodegradability and combustibility. In addition, natural fibres have lower density than synthetic fibres.
These advantages of acoustical and mechanical properties rank the Lignocellulosic composite sound absorbing materials among high performance composites which have economic and environmental advantages.
Dr Elammaran Jayamani is a senior lecturer and coordinator for the mechanical engineering programme at the Faculty of Engineering, Computing and Science, Swinburne University of Technology Sarawak Campus.