3 Theory of the Measurement Methods and the Quantities Considered

This chapter presents the theoretical basis for obtaining information about physical properties and the behavior of various parameters in a fluid system. The measurement techniques of ultrasonic spectroscopy and light scattering are also considered.

3.1 Ultrasonic Spectroscopy

A sound wave requires a medium in which to propagate. It describes changes in the state of that medium in space and time. The fundamental hydrodynamic equations relate density ρ, particle velocity v, pressure P, and external force f:

Equation (3.1) is the continuity equation, Eq. (3.2) the Euler equation, and Eq. (3.3) the equation of state.

In treating sound waves, an equilibrium solution is assumed (see [7]); ρ₀, v₀, and P₀ denote the values of these quantities in the absence of a sound field.