The difference between ultrasound and ultrasound
What is ultrasound? The human ear can hear the fluctuation, and its frequency is about 16Hz to 20KHz. What if? If the frequency of fluctuation is higher than this range, human beings can't hear it, which is called ultrasound.
Ultrasonic wave refers to sound wave with frequency higher than 20000Hz, which has good directivity, strong penetration ability, easy to obtain relatively concentrated acoustic energy and long propagation distance in water, and can be used for ranging, speed measurement, cleaning, welding, lithotripsy, sterilization and disinfection. It has many applications in medicine, military, industry and agriculture. Ultrasound is named because its lower frequency limit is about equal to the upper hearing limit of human beings.
Scientists call the number of vibrations per second the frequency of sound, in hertz. The frequency of sound waves that our human ears can hear is 20 ~ 20~20000Hz. When the vibration frequency of sound wave is greater than 20KHz or less than 20Hz, we can't hear it. So, we call sound waves with frequencies higher than 20000 Hz? Ultrasound? . The ultrasonic frequency usually used for medical diagnosis is 1 ~ 5 MHz.
Ultrasonic wave and ultrasonic wave are only different in name, in fact, they both mean that the frequency of vibration is beyond the range of sound that people can hear (20 Hz-20 Hz).
The difference between ultrasonic wave and sound wave
Sound wave is a fluctuation of pressure and density; The frequency that people are most sensitive to sound waves is 2000 Hz. Above or below 2000Hz, people's hearing will decline. According to this characteristic of human hearing, sound waves above 20,000 Hz are called ultrasonic waves, sound waves below 20Hz are called infrasound waves, and sound waves between 20 and 20,000 Hz are called audio sound waves. The higher the frequency, the stronger the directivity, the higher the reflection performance and the weaker the diffraction and diffraction performance. For example, ultrasound can be used for defect detection and diagnosis. The lower the frequency, the weaker the directivity, the lower the reflection performance and the stronger the diffraction and diffraction performance. For example, infrasound waves generated by earthquakes can travel far and can be detected several times around the earth;
Electromagnetic waves are mainly introduced
Just as people have been living in the air but their eyes can't see the air, people can't see the ubiquitous electromagnetic waves. Electromagnetic wave is such a man who has never met before? Friends? . Electromagnetic wave is a form of movement of electromagnetic field. In the case of high-frequency electromagnetic oscillation, a part of energy radiates from space to form electric waves and magnetic waves, collectively referred to as? Electromagnetic waves? . In the low-frequency electric oscillation, the mutual change between magnetoelectricity is relatively slow, and almost all of its energy is reflected back to the original circuit, and there is no energy radiation. However, in high-frequency electric oscillation, magnetoelectricity changes rapidly, and it is impossible for all energy to return to the original oscillation circuit, so electric energy and magnetic energy spread to space in the form of electromagnetic waves with the periodic changes of electric and magnetic fields. Electromagnetic waves are shear waves. The propagation directions of magnetic field, electric field and electromagnetic wave are perpendicular to each other. The propagation of electromagnetic waves includes ground waves along the ground and air waves from the air. The longer the wavelength, the smaller the attenuation. The longer the wavelength of electromagnetic waves, the easier it is to bypass obstacles and continue to spread. Medium-wave or short-wave carrier waves are propagated by repeated reflection between the ionosphere around the earth and the ground (the ionosphere is between 50 and 400 kilometers from the ground). The amplitude alternates periodically along the vertical direction of the propagation direction, and its intensity is inversely proportional to the square of the distance. The wave itself drives energy, and the energy power at any position is proportional to the square of the amplitude. Its speed is equal to the speed of light (3? 10 10 cm). Light waves are electromagnetic waves, and radio waves have the same characteristics of refraction, reflection, diffraction, scattering and absorption as light waves when passing through different media. The wavelength of electromagnetic wave propagation in space is the distance between two points with the same intensity direction and the largest distance from the nearest electric field (magnetic field). The frequency of electromagnetic waves? That is, the frequency of electric oscillation current, the unit used for radio broadcasting is kilohertz, and the speed is C. According to =c, what is obtained? =c/？ .
Electricity can produce magnetism, and magnetism can also bring electricity. The changing electric field and the changing magnetic field constitute an inseparable and unified field, which is the electromagnetic field. The changing electromagnetic field propagates in space to form electromagnetic waves, so electromagnetic waves are often called radio waves. 1864, Maxwell, a British scientist, established a complete electromagnetic wave theory on the basis of summarizing previous studies on electromagnetic phenomena. He concluded that electromagnetic waves exist and concluded that electromagnetic waves and light travel at the same speed. 1887 German physicist Hertz confirmed the existence of electromagnetic waves through experiments. After that, many experiments were carried out, which not only proved that light is electromagnetic wave, but also found more forms of electromagnetic wave. Their essence is exactly the same, but there are great differences in wavelength and frequency. Arranging these electromagnetic waves in the order of wavelength or frequency is the electromagnetic spectrum. If the frequencies of each band are arranged from low to high, they are power frequency electromagnetic wave, radio wave, microwave, infrared ray, visible light, ultraviolet ray, X ray and R ray.
The wavelength used is between 10 and 3000m, which can be divided into long wave, medium wave, medium-short wave and short wave. The wavelength for fax (TV) is 3 ~ 6 meters; The wavelength used for radar is shorter, 3 meters to several centimeters. Electromagnetic waves include infrared, visible light, ultraviolet, x-ray, and? Lei et al. All kinds of light and rays are also electromagnetic waves with different wavelengths. Among them, the wavelength of radio is the longest and the wavelength of cosmic rays is the shortest.
Radio waves are 3000m ~ 0.3mm. ..
The infrared ray is 0.3 mm ~ 0.75 micron.
Visible light is 0.7 micron to 0.4 micron.
Uv 0.4 μ m ~ 10 nm
X-ray 10 nm ~ 0. 1 nm
? Light 0. 1 nm ~ 0.00 1 nm
Cosmic rays are less than 0.00 1 nanometer.
electromagnetic radiation
Electromagnetic radiation in a broad sense usually refers to the electromagnetic spectrum. In a narrow sense, electromagnetic radiation refers to the radiation wave generated by electrical equipment, usually referring to the part below infrared.
Harm of electromagnetic radiation to human body
The mechanism of electromagnetic radiation endangering human body is mainly thermal effect, non-thermal effect and accumulation effect.
Thermal effect: More than 70% of human body is water, and water molecules rub against each other after being radiated by electromagnetic waves, which causes the body to heat up, thus affecting the normal work of other organs of the body.
Non-thermal effect: There are weak electromagnetic fields in human organs and tissues, which are stable and orderly. Once disturbed by external electromagnetic waves, the weak electromagnetic field in equilibrium will be destroyed, and the normal circulation function of human body will be destroyed. Cumulative effect: after thermal and non-thermal effects act on human body, if the damage to human body is radiated by electromagnetic waves again before it can repair itself, the damage degree will accumulate and become permanent pathological or life-threatening. For groups exposed to electromagnetic radiation for a long time, even if the power is very small and the frequency is very low, unexpected lesions will be induced, so be alert! Scientists from all over the world have proved through long-term research that long-term exposure to electromagnetic radiation will lead to decreased immunity, metabolic disorder, memory loss, premature aging, arrhythmia, decreased vision, abnormal blood pressure, acne and rough skin, and even lead to various cancers. The reproductive capacity of men and women is declining, and women are prone to menstrual disorders, abortion, teratogenesis and other diseases.
With the improvement of people's living standards, household appliances such as TV, computer, microwave oven, electric blanket, refrigerator and so on. With the increasing popularity, electromagnetic radiation is more and more harmful to human body. However, because electromagnetic waves are invisible, intangible and imperceptible, and their destruction is slow and hidden, it has not attracted widespread attention. For example, electric blanket is the most widely used household appliance, which has the closest contact with human body, the longest contact time and the most serious harm to human body. At present, the electromagnetic radiation intensity of most electric blankets used by people exceeds the safe value by 20 ~ 100 times, which is extremely harmful to human health. But it has not attracted people's attention and is still widely used. Thousands of electric blanket users in Qian Qian are still suffering from electromagnetic radiation from electric blankets, which should arouse people's attention!
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