Noise is becoming an increasingly signiﬁcant concern because of its adverse eﬀects on the lives many people in urban societies and in rural areas. Noise arising from diﬀerent sources such as vehicles, aircrafts, power plants and machinery is not only uncomfortable but also hazardous to health. These concerns have led to major developments in the ﬁeld of sound absorbing materials. For homogenous and isotropic materials, acoustic performance is deﬁned by a set of experimentally determined constants, namely: absorption coeﬃcient, reﬂection coeﬃcient, acoustic impedance, propagation constant and noise reduction coeﬃcient (NRC).
Fig. 2. SEM micrograph of broken cenosphere showing its hollow characteristics and porous walls.
The absorption of sound results from the dissipation of the sound energy as heat. The dissipation mechanisms are mainly due to one of two phenomena. The ﬁrst is the energy loss due to flexural vibrations in the specimen. The second is porosity eﬀects, where energy is dissipated due to multiple reﬂections of sound waves within the voids in the structure. For most porous materials like synthetic foam and mineral wool with interconnected pores, incoming sound is reﬂected within the pores, causing them to vibrate and convert sound energy into heat.
Fig. 1. Cross-section of impedance tube showing variation of pressure as a function of distance from the specimen.
The following conclusions were drawn from the study conducted by enriching the coating material with hollow ceramic cenospheres:
- The mass density decreases by 40% as the volume fraction of the cenospheres increases to 70%.
- Sound absorption increases with the addition of cenospheres up to 40% by volume.