Electrorheological Fluids as an Electrically Controllable Acoustic Medium: II. Medium Properties and Description by the Biot Theory

Electrorheological fluids (ERF) can, under the influence of an electric field, be reversibly transformed from a fluid-like to a solid-like state. This transition affects also their acoustic properties which are presented in this paper. The sound velocity and absorption in an ERF without and with electric field have been measured in a newly developed impedance tube. It turns out that the acoustic loss of an ERF is electrically controllable. As a theoretical model the Biot theory has been applied. The theory is well-suited for the description of both highly concentrated suspensions and fluid-saturated sediments in a wide range of porosity and is therefore capable of modeling both: the fluid-like and the solid-like state of an ERF. Some minor changes to the theory had to be applied, and the bulk modulus of the particles must be assumed complex. Most of the Biot parameters of the ERF are initially known, but the frame moduli, used to model the field influence, are difficult to obtain.