Dynamic measurements of the elastic constants of glass wool

Abstract

The sound wave in the air between the fibers of glass wool exerts an oscillatory viscous drag on the fibers and excites a mechanical wave in the fiber skeleton. Accurate calculations of sound attenuation in glass wool must take the mechanical wave in the fiber skeleton into account, and this requires knowledge of the dynamic elastic constants of the fiber skeleton. The mechanical properties of glass wool are highly anisotropic. Previously only one of the elastic constants has been measured dynamically, but here all the elastic constants are reported. The measurement method is well known. But a new mechanical design, which reduces mechanical resonance, is described. The measurements were carried out in atmospheric air at normal pressure, and this causes an oscillatory airflow in the sample. To obtain the elastic constants, the influence of the airflow was subtracted from the data by a new formula. The elastic constants were measured in the frequency range 20–160Hz for glass wool of mass density 30kg∕m3. The elastic constant C11 depended on the frequency; at 20Hz it was 1.5+0.01iMPa, and at 160Hz it was 2.6+0.06iMPa. The constant C33=12+0.6ikPa did not depend on frequency. The shear constant C44=40+2ikPa was constant. The two constants C12,C13 were zero.