Features Of Ultrasound

 The propagation laws of ultrasonic waves in the medium such as reflection, refraction, diffraction, and scattering are not fundamentally different from the laws of (audible) sound waves. But the wavelength of ultrasound is very short, only a few centimeters, or even a few thousandths of a millimeter. Compared with (audible) sound waves, ultrasound has many exotic properties:

1. The wavelength of the ultrasonic wave is very short, and the size of the usual obstacle is many times larger than the wavelength of the ultrasonic wave, so the diffraction ability of the ultrasonic wave is very poor, but it can propagate in a directional straight line in a homogeneous medium. characteristics are more pronounced. Therefore, when the ultrasonic wave propagates, the directionality is strong, and the energy is easy to concentrate.

2. Ultrasound can propagate in a variety of different media and can travel far enough distances.

3. The interaction between ultrasound and the sound transmission medium is moderate, and it is easy to carry information about the state of the sound transmission medium (diagnosis or effect on the sound transmission medium). Ultrasound is a form of wave, which can be used as a carrier or medium for detection and load information (such as B-ultrasound used for diagnosis); ultrasound is also a form of energy, when its intensity exceeds a certain value, it can pass and The medium through which the ultrasonic wave is transmitted interacts, affects, alters and destroys the state, properties and structure of the latter (used for therapy).

The ultrasonic wave interacts with the medium during the propagation process, and the phase and amplitude change, which can change the state, composition, structure, function and properties of the medium. This type of change is called the ultrasonic effect. The interaction between ultrasonic and medium can be divided into thermal mechanism, mechanical mechanism and cavitation mechanism.

(1) Thermal mechanism: When the ultrasonic wave propagates in the medium, its vibration energy is continuously absorbed by the medium and converted into heat, which increases the temperature of the medium. This effect of increasing the temperature of the medium is called the thermal mechanism of ultrasound. (2) Mechanical mechanism: When the frequency is low, the absorption coefficient is small, and the ultrasonic action time is very short, the ultrasonic effect is not accompanied by obvious thermal effect. At this time, the ultrasonic effect can be attributed to the mechanical mechanism, that is, the ultrasonic effect originates from the contribution of the mechanical quantity characterizing the sound field. Ultrasound is also a form of transmission of mechanical energy, and parameters such as origin displacement, vibration velocity, acceleration and sound pressure in the fluctuation process can express the ultrasonic effect.

(3) Cavitation mechanism: One of the main mechanisms of ultrasonic sonochemical effects is acoustic cavitation (including the formation, growth and collapse of bubbles, etc.). The phenomenon includes two aspects, that is, the strong ultrasound produces bubbles in the liquid and the special movement of the bubbles under the action of strong ultrasound.

Ultrasound is a high-frequency mechanical wave with the characteristics of concentrated energy and strong penetrating power. Ultrasound is composed of a series of dense and dense longitudinal waves, and propagates around through the liquid medium. When the acoustic energy is high enough, the attraction between molecules in the liquid phase is broken during the loose half-cycle, forming a cavitation nucleus. The lifetime of the cavitation nucleus is about 0.1μs, it can generate a local high temperature and high pressure environment of about 4000-6000 K and 100MPa at the moment of explosion, and generate a microjet with a speed of about 110m/s with a strong impact force, this phenomenon is called Ultrasonic cavitation.