Do you know the propagation characteristics of ultrasonic waves?

(1) Waveform and speed of sound of ultrasonic waves

Ultrasonic waves are mechanical waves that can propagate in gases, liquids and solids, in other words, ultrasonic waves can propagate in any medium with elastic properties. With the difference of the sound source, medium and boundary conditions, the wave patterns of the propagating ultrasonic waves are also different. waves, cylindrical waves, etc. Only a few of the most commonly encountered wave modes and sound velocities are described here.

① Longitudinal waves. When the ultrasonic wave propagates in the medium, the wave in which the vibration direction of the medium particle is parallel to the wave propagation direction is called the longitudinal wave. In the area where the longitudinal wave passes, the medium particles are periodically sparse and dense, as shown in Figure 2-3. Longitudinal waves can propagate in gaseous, liquid and solid media.

② Transverse wave. When a shear wave propagates in a medium, the direction of the medium particle vibration is perpendicular to the direction of wave propagation (Figure 2-4). Shear waves can only propagate in shear elastic media, so it exists only in highly viscous liquids and solids.
③ Surface wave. When a sound wave propagates in a medium, the sound wave whose energy is concentrated in the free surface layer of the medium or near the interface between the two interfaces is called a surface wave. There are many types of such waves, such as Rayleigh waves on the solid surface, Stoneley waves near the interface between two solids, and so on. When the surface wave propagates, the trajectories of the medium particle vibration are elliptical. The long axis of the ellipse is perpendicular to the propagation direction of the sound wave, and the short axis is parallel to the propagation direction of the sound wave, as shown in Figure 2-5.

④ Board wave. When a sound wave propagates in a plate-like medium, its wave mode is called a plate wave. There are many types of plate waves, the main one is Lamb waves. The so-called plate wave usually refers to the Lamb wave, which is an elastic wave propagating in a plate whose thickness is comparable to the wavelength of the acoustic wave in it, and its sound field spreads through the entire thickness of the plate. In the process of plate wave propagation, the vibration of the particle is as follows: on the two surfaces of the solid, the particle vibrates along an elliptical trajectory, which is the same as the Rayleigh wave; inside the plate, the vibration of the particle varies with the wave pattern. When the wave type is symmetric, the trajectory of the particle vibration is the same as that of the longitudinal wave; when the wave type is antisymmetric, it is similar to the transverse wave.

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