ABSTRACT
This chapter gives a brief description of the regularities of oscillatory and wave motion as well as the principles of the propagation of low-amplitude waves in fluids and solids. However, it should be noted that if the forces acting on the volume element in fluids are determined only by pressure, then in solids the relation between the forces and the strains they produce is more complex. Since various particles of a complex mechanical oscillating system subject to a force can oscillate with different velocities, the impedance is defined as the ratio of the applied force to the vibrational velocity of that point to which this force is applied. The elastic forces are defined as the pressure per unit volume Absorption in a solid is due to physical processes occurring during the propagation of an elastic wave. Propagation of ultrasonic waves in real media is accompanied with their attenuation dependent on geometrical factors, scattering, and energy dissipation.
