Acoustical Properties of Homogeneous, Isotropic Rigid Tiles and Flexible Blankets

Abstract

Studies on rigid acoustical tiles and soft blankets are described in this paper. It is shown that two waves travel through the material—one primarily airborne and the other primarily structure‐borne. From a knowledge of the density of the sample, the volume coefficients of elasticity of the air and of the skeleton of the material, the porosity, the air flow resistance, the inter‐fiber frictional resistance, and the structure factor, the propagation constants of each of these waves can be calculated. The experimental results indicate that the theory is useful in calculating the performance of the flexible, airplane type of blankets over the entire audible frequency range. For rigid tiles, however, the theory appears to fail at frequencies above 1000 c.p.s. if the flow resistance is high, and it fails at all frequencies for materials with low flow resistance. Experiment shows that the condensations and rarefactions of the gas in blankets take place isothermally at low frequencies and adiabatically at high. The transfer from one state to the other occurs gradually in the 100 to 2000 c.p.s. region. A more complete theory is required to explain the effects of thermodynamic and viscous losses on the propagation constant of rigid materials.