Ultrasound generation

The "super" character of ultrasound comes from the fact that the lower bound of its frequency band exceeds human hearing, but if you analyze it in terms of wavelength, the wavelength of ultrasound is actually shorter. Scientists call the distance between two adjacent peaks or troughs of a wave as wavelength. The wavelength of mechanical waves that our human ears can hear is 2cm~20m (2cm~20m). Therefore, we call mechanical waves with a wavelength shorter than 2 cm as "ultrasound." However, in practical applications, mechanical waves with a wavelength below 3.4cm (above 10000hz) can be regarded as ultrasonic research. The wavelength of ultrasound usually used for medical diagnosis is 10μm~350μm.

Ultrasound is a kind of mechanical wave. It must rely on the medium to propagate and cannot exist in vacuum (such as space). Therefore, we cannot use ultrasound in vacuum, but we can still use electromagnetic wave-related equipment (including radio waves, microwaves, and infrared rays). , Visible light, ultraviolet rays, X-rays, gamma rays, etc.), using electromagnetic wave technology.

In the air, ultrasonic waves refer to mechanical waves with a wavelength of less than 2 cm (for example, 1.7 cm, 2 cm wavelength corresponds to 17000 Hz, 1.7 cm wavelength corresponds to 20000 Hz, in fact, there is no fixed standard, just a value that is easy to remember), and its wavelength is very short. Lower than the general lower limit of human hearing (2cm), people call this inaudible mechanical wave ultrasound, and the wavelength of infrasound waves is generally longer than 20 meters (for example, 17 meters, 20m wavelength corresponds to 17Hz, 17m wavelength corresponds to 20Hz), higher than The upper limit of the wavelength of hearing. In practical applications, ultrasonic waves often coincide with the range of short-wave audible sound waves, and mechanical waves with a wavelength shorter than 3.4cm (10000hz) can be regarded as ultrasonic research.

Its wavelength is much shorter than ordinary sound waves, so it can be used for cutting, welding, drilling, etc. Because of its short wavelength, it has many characteristics: firstly, the propagation anisotropy caused by the short wavelength, and also because of its short wavelength and poor diffraction ability. Although it has good anisotropy, it has a large loss in the air and cannot be transmitted. Far, poor penetration, easy to scatter. Ultrasound is commonly used in industry and medicine for ultrasonic detection. Ultrasound, infrasound, and audible sound are essentially the same. What they have in common is a mechanical wave, which usually propagates in elastic media in the form of longitudinal waves. It is a form of energy propagation. The difference is ultrasonic waves. Long and short, it can travel along a straight line within a certain distance with little diffraction, and has good anisotropy. However, compared with audible sound and infrasound, it has poor penetrating power and is easy to scatter.

The propagation laws of reflection, refraction, diffraction, and scattering of ultrasonic waves in the medium are not essentially different from the laws of infrasound and audible sound waves. But the wavelength of ultrasound is very short, only a few centimeters, even a few thousandths of a millimeter. Compared with other waves, ultrasonic waves have many characteristics: propagation characteristics—the wavelength of ultrasonic waves is very short, and the size of ordinary obstacles is many times larger than the wavelength of ultrasonic waves. Therefore, ultrasonic waves have poor penetration, poor diffraction capabilities, and easy scattering. . It can propagate in a straight line in a homogeneous medium but is difficult to diffract. The shorter the wavelength of the ultrasonic wave, the more obvious the characteristic. In addition, according to the Rayleigh scattering law, the intensity of the scattered wave is inversely proportional to the fourth power of the wavelength, and the wavelength of the ultrasonic wave is extremely short. , So the scattering is very serious and the penetrating power is not good. Cavitation ─ ─ When the ultrasonic wave propagates in the medium, there is a positive and negative alternating period. In the positive phase, the ultrasonic wave squeezes the medium molecules to change the original density of the medium and increase it; in the negative pressure phase When the medium molecules are sparse and further dispersed, the density of the medium is reduced. When a sufficiently strong ultrasonic wave is applied to the liquid medium, the average distance between the medium molecules will exceed the critical molecular distance that keeps the liquid medium constant. Fracture occurs and microbubbles are formed. These small cavities expand and close rapidly, causing violent collisions between the liquid particles, resulting in pressures of thousands to tens of thousands of atmospheres. This violent interaction between the particles has a good stirring effect, so that the two immiscible liquids (such as water and oil) emulsify and accelerate the dissolution of the solute. The various effects caused by the action of ultrasonic waves in the liquid are called ultrasonic cavitation.