Development of lightweight micro-porous materials for acoustic applications

A novel processing technique is developed and demonstrated for fabrication of micro-porous materials using different size hollow microbubbles for acoustic applications. Physical, mechanical, and thermal properties of this material were measured and reported. The materials processed and fabricated were repeatable, consistent in quality, lightweight (bulk density less than 0.45 g/cc) and reasonably strong for the acoustic applications. 25 mm thick specimens were made of three different size group microbubbles: small (10–180 μm), medium (10–500 μm) and large size (150–850 μm), these are naturally available from coal burned electric power plants. These bubbles were selected because of near spherical shape, surface roughness, possibility of interconnectivity of particles and pore sizes. The specimens were tested in impedance tube and sound absorption and transmission were measured as a function of frequency (250–1600 Hz). The absorption coefficient increased with the increase in microbubble size which gave rise to a material with a simultaneously greater pore size and a greater porosity. Similarly, the transmission loss increased with the decrease in microbubble size, because of the decrease in the pore size, and porosity (low permeability). Therefore, an acoustic system could be designed and fabricated for any geometry by proper selection of hollow microbubble size and their distribution to achieve a targeted acoustic response.