Ultrasonic has many characteristics. At present, ultrasound has been widely used. In this paper, the characteristics and applications of the American version of ultrasound for home appliances are analyzed.
1、 Ultrasonic description
Mechanical vibration propagates in the medium and forms mechanical waves. The mechanical waves that can be heard by the human ear are called sound waves, whose frequency range is roughly between 20Hz and 2000Hz. The mechanical waves whose frequency is lower than 20Hz are called infrasound waves, and the mechanical waves whose frequency is higher than 20000Hz are called ultrasonic waves. Neither infrasound wave nor ultrasonic wave can cause human hearing. They have the same essence as sound wave, follow the common motion law, and can spread in solid, liquid and gas. They have the same sound speed and propagate in the form of longitudinal waves in the fluid. At present, humans have been able to obtain 1012Hz ultrasound. With the development of ultrasound technology, ultrasound has been widely used in medicine, industry, national defense, agriculture and other fields. Ultrasonic technology has been used in medicine for more than half a century and has become one of the indispensable clinical diagnostic means in medicine. Ultrasonic waves used in medical diagnosis are generated by ultrasonic probes (transducers) made of materials such as piezoelectric crystals.
2、 Ultrasonic characteristics
As the frequency f increases, the wavelength λ Shorter, the ultrasonic wave has its particularity, which is similar to some characteristics of light, such as beam radiation, refraction and reflection from one medium into another; At the same time, it has strong absorption and attenuation, with strong energy. The following briefly introduces several main characteristics of ultrasonic:
1. Beam emittance of ultrasonic
The sound that can be felt by the human ear is a non directional spherical wave, that is, it spreads around in a sphere with the sound source as the center, and the sound can be heard around. The ultrasonic frequency is very high and the directivity (i.e. beam emittance) is strong. When the diameter of the ultrasonic generator piezoelectric crystal is much larger than the ultrasonic wave length, the ultrasonic wave generated by the crystal is similar to the characteristics of light. The larger the diameter of the piezoelectric crystal or the higher the frequency, that is, the wavelength λ The shorter the length of the near-field region is, the better the beam emittance of this ultrasonic field will be.
2. Transmission, reflection, refraction and aggregation of ultrasonic waves
When a beam of light is cast into a transparent glass beaker filled with water in a dark environment, we will clearly see the transmission, reflection and refraction of light on the water surface. Due to the high frequency, the transmission, reflection and refraction phenomena similar to light will appear on the interface of two different media when the ultrasonic wave propagates directionally.
The aggregation of ultrasonic waves is the same as that of light. The ultrasonic beam can be converged to one point by using the ultrasonic gathering device, so that the ultrasonic sound intensity can be increased several times or even thousands of times. With such a huge sound intensity, many meaningful works can be done, such as ultrasonic cutting, ultrasonic drilling, ultrasonic polishing, etc.
3. Absorption and attenuation of ultrasonic
When sound waves propagate in various media, because the media will absorb part of its energy, the intensity of sound waves will gradually decrease with the increase of propagation distance.
In a square, a national string music is performing in the street for the masses. When you approach the band from a distance, the first thing you hear is the low pitched drum sound. Slowly approaching the band, you will gradually hear the sound of locking, flute, erhu, etc; Finally, when you walk around the band, you will hear the ringing bell with high pitch.
This example vividly shows that sound waves with different frequencies are absorbed to different degrees when they travel in the air. The higher the frequency of the sound wave, the stronger the air will absorb it, so the distance of the ultrasonic wave propagation is shorter.
4. Huge energy of ultrasonic
One of the important reasons why ultrasound plays a unique and huge role in industry, national defense and medical treatment is that it has extremely powerful power than sound wave. According to the experimental measurement of acoustics workers, the energy of general speech sound is very small. If we want to use the energy of ordinary speech to boil a pot of water, we must mobilize more than 7 million people to speak for 12 hours continuously. Ultrasound has much more energy than sound. According to relevant acoustic experiments, the energy of ultrasonic waves with a frequency of 1 million Hz is 1 million times greater than that of sound waves with a frequency of 1000 Hz at the same amplitude. It can be seen that having huge energy is an important feature of ultrasound.
5. Ultrasonic pressure characteristics
The so-called "sound pressure" refers to the intensity of the additional pressure on the objects in the sound field due to the vibration of sound waves. The unit is kg/cm2. The sound pressure of sound waves is very small, and its value is about 1 × 10-6 kg/cm2 - 1 × 10-6 kg/cm2. Such a small sound pressure generally does not attract people's attention. However, the number of ultrasonic sound pressure is generally large. For example, when ultrasonic waves of general intensity are injected into water, the sound pressure generated can reach several atmospheres. The fundamental reason why ultrasound can generate such strong sound pressure is that the frequency of ultrasound is very high, so when entering the medium, high-density molecules stretch quickly, resulting in the formation of an instantaneous vacuum and compression of high-density areas, resulting in huge pressure difference. When the amplitude reaches a certain degree, the energy of ultrasonic wave is very huge.
3、 Application of ultrasonic wave
1. Ultrasonic diagnosis
At present, ultrasound diagnostic methods used in medicine have different forms, which can be divided into four categories: type A, type B, type M and type D.
Type A: It is a method of displaying tissue characteristics by waveform, mainly used to measure the diameter of organs to determine their size. It can be used to identify some physical characteristics of pathological tissue, such as the presence of substance, liquid or gas.
Type B: It displays the specific situation of the investigated organization in the form of plane graphics. During the examination, the reflected signal of the human body interface is first transformed into light spots with different intensity. These light spots can be displayed through the fluorescent screen. This method is intuitive, repeatable, and can be used for comparison before and after. Therefore, it is widely used in the diagnosis of gynecological, urinary, digestive, cardiovascular and other system diseases.
M type: It is a method for observing the time change of the active interface. It is most suitable for checking the activity of the heart. The dynamic change of its curve is called echocardiogram, which can be used to observe the position, activity status, structure status, etc. of the structure of each layer of the heart, and is mostly used to assist in the diagnosis of heart and major vascular diseases.
Type D: It is a kind of ultrasonic diagnostic method specially used to detect blood flow and organ activity, also called Doppler ultrasonic diagnostic method. It can determine whether the blood vessel is unobstructed, whether the lumen is narrow and occluded, and the lesion site. The new generation of D-mode ultrasound can also quantitatively measure the blood flow in the lumen. In recent years, scientists have developed a color coded Doppler system, which can display the direction of blood flow in different colors under the guidance of the anatomical signs of echocardiography. The color depth represents the flow rate of blood flow.
2. Common ultrasonic instrument
(1) Ultrasonic stone breaker
The ultrasonic lithotripter sends ultrasonic waves into the human body by the machine, and uses the huge energy of ultrasonic waves to make the stones in the human body resonate and be crushed by shock, so as to alleviate the pain and achieve the goal of cure.
(2) Ultrasonic humidifier
Theoretical research shows that under the condition of the same amplitude, the vibration energy of an object is proportional to the vibration frequency. When ultrasonic waves propagate in the medium, the vibration frequency of the medium particle is very high, so the energy is very large. In the dry winter in northern China, if the ultrasonic wave is introduced into the water tank, the severe vibration will break the water in the tank into many small droplets, and then blow the droplets into the room with a small fan to increase the indoor air humidity, which is the principle of the ultrasonic humidifier. For the treatment of pharyngitis, tracheitis and other diseases, it is difficult for drugs to flow through the blood to the diseased part. Using the principle of humidifier, atomizing the drug solution for patients to inhale can improve the efficacy.
(3) Ultrasonic cleaning
The principle of ultrasonic cleaning is that the ultrasonic generator sends out high-frequency oscillation signal, which is converted into high-frequency mechanical oscillation through the transducer and transmitted to the medium cleaning liquid. Ultrasonic waves radiate forward in a dense and alternating manner in the cleaning fluid to make the liquid flow and generate tens of thousands of tiny bubbles. The tiny bubbles (cavitation nuclei) existing in the liquid vibrate under the action of the sound field. When the sound pressure reaches a certain value, the bubbles grow rapidly and then close suddenly. When the bubbles close, shock waves are generated, generating thousands of atmospheric pressures around them, Destroy insoluble dirt and make them disperse in cleaning solution. When the group particles are wrapped by dirt and adhered to the surface of the cleaning part, the dirt is emulsified and the group particles are separated, so as to achieve the purpose of cleaning the surface of the cleaning part without directly contacting the surface of the part. This is the method used by opticians to wash glasses.
(4) Ultrasonic flaw detector
An instrument that uses ultrasonic waves to test materials is called an ultrasonic flaw detector. Its principle is: when the ultrasonic wave propagates in the tested material, the changes of the acoustic characteristics and internal organization of the material will have a certain impact on the ultrasonic wave propagation. Through the analysis of the extent and condition of the ultrasonic wave affected, the material properties and structural changes will be understood. Ultrasonic testing methods usually include penetration method, pulse reflection method, tandem method, etc.
The receiving and generating principle of ultrasonic wave is similar. When ultrasonic wave encounters discontinuous medium, it will generate reflection. The reflected ultrasonic wave will vibrate the piezoelectric chip, and then generate voltage at both ends of the piezoelectric chip. The voltage will be converted into the waveform on the flaw detector screen. The x direction of the screen is the voltage applied by the piezoelectric flaw detector on the piezoelectric chip, The y direction is the voltage generated by the vibration of the piezoelectric chip. This forms the waveform on the screen.
Now there are still ultrasonic technologies such as three-dimensional ultrasound imaging, ultrasonic CT, ultrasonic endoscope and so on emerging constantly, and they can also be used in combination with other inspection instruments, which greatly improves the accuracy of disease diagnosis. Ultrasonic technology is playing a huge role in the medical community. With the progress of science, it will be more perfect and will better benefit mankind.
