Principles of losses
The mechanisms responsible for the dissipation of an acoustic wave are mainly viscosity and thermal conduction.
The general principle of losses are presented below:
Viscous losses. In a wave guide, the particles oscillate (particular displacement) on a direction parallel to that of the main wave guide axis. As the wall velocity is zero, there is a transition zone between the wall and the oscillating fluid, called the boundary layer, in which the particular velocity increases rapidly. In this boundary layer, the viscosity of the fluid is opposed to the tangential movement of the fluid along the wall. The energy lost due to viscosity is transformed into heat.
Thermal losses. In a wave guide, the fluid particles compress and decompress thus varying the temperature of the fluid around the average value. As the wall temperature is contant over time (due to the thermal inertia), there is a difference (gradient) in temperature between the fluid and the wall which creates a transistion zone between the wall and the fluid called the thermal boundary layer.
