Sound Absorption of Lotus-type Porous Magnesium

摘要

When sound wave enters porous materials, viscous friction occurs in the air adjacent to the pore wall, and a part of the sound energy is converted into thermal energy, leading to the absorption of sound by the porous materials. Lotus-structured porous magnesium, which is a new type of light-weight structured materials, is also expected as a sound absorption material. In the present work, the sound absorbing of this porous magnesium was investigated. The Lotus-structured porous magnesium was fabricated by unidirectional solidification of the melt dissolving hydrogen in a high pressure mixture gas of hydrogen and argon. The sound absorption coefficient of porous magnesium with various pore size, porosity and thickness was measured by standing-wave method as a function of the frequency of sound. The absorption coefficient of specimens of thicknesses 10 mm and 20 mm, which have rather uniform pore diameter of 490 and 310 mu m, decreases with decreasing porosity from 53 to 31 percent . The sound absorption requires the incident wave to enter the pores of the material. When the porosity decreases for a given pore diameter, the pore number per unit area of the specimen reduces and more of the incident sound wave is reflected, resulting in the decrease of the sound absorption. The absorption coefficient of a specimen increases with increase of specimen thickness from 10 to 30 mm. This is because as the specimen thickness increases the distance the sound travels becomes long and more sound is absorbed.