The secret of sound

  ”Who’s steps are approaching, whoever’s steps are far, I have my ears up…” Eyes are the windows of the soul, and ears allow us to hear the sounds of the world, mechanical, natural, beautiful, and harsh… What is that voice? How does sound spread? How do we use sound waves? Let us walk into the scientific world of sound together with this famous song “Ear” sung by Li Ronghao.
What is the sound

  Sound is a sound wave generated by the vibration of an object. It is a fluctuating phenomenon that is transmitted through a medium (air or solid, liquid) and can be sensed by the auditory organs of humans or animals. The object that initially vibrates (vibrates) is called the sound source. Sound travels in the form of waves (vibrations).
  Sound is a kind of wave, which has general wave parameters such as frequency, wavelength, wave number, etc. The frequency range that humans, different animals and many instruments can receive is different. The human ear can hear the sound frequency range from 20 to 20000 Hz. The human ear will not hear any sound below or beyond this range. Therefore, people call sound waves with a frequency higher than 20000 Hz as ultrasound, and those with a frequency lower than 20 Hz It is called infrasound.
Application of ultrasound effect

  Although ultrasound and infrasound waves are not accepted by the human ear, the ultrasound effect has been widely used in our actual lives.
Ultrasound imaging

  Ultrasound has a shorter wavelength than ordinary sound waves, has better directivity, and can penetrate opaque materials. This feature has been widely used in ultrasonic flaw detection, thickness measurement, distance measurement, remote control and ultrasonic imaging technology.
  Ultrasound imaging is a technique that uses ultrasound to present the internal structure of opaque objects. Focus the ultrasonic wave emitted from the transducer on the opaque sample through the acoustic lens, and the ultrasonic wave transmitted from the sample carries the information of the illuminated part (such as the ability to reflect, absorb and scatter sound waves), and is converged by the acoustic lens On the piezoelectric receiver, the obtained electrical signal is input to the amplifier, and the image of the opaque sample can be displayed on the fluorescent screen by the scanning system. The above device is called an ultrasound microscope.
  Ultrasound imaging technology has been widely used in medical examinations. The working principle of medical ultrasound is similar to that of sonar. Ultrasound is emitted into the human body. When it encounters an interface in the body, it will reflect and refract, and it may be absorbed and attenuated in human tissue. Because the shape and structure of various tissues of the human body are different, the degree of reflection, refraction, and absorption of ultrasound are also different. Doctors distinguish them by the characteristics of the waveform, curve, or image reflected by the instrument. In addition, combined with anatomical knowledge, normal and pathological changes, the examined organs can be diagnosed for disease.
Ultrasonic treatment

  Ultrasound has energy. Using the mechanical, cavitation, thermal and chemical effects of ultrasound, it can perform ultrasonic welding, drilling, solid crushing, emulsification, degassing, dust removal, scale removal, cleaning, sterilization, and chemical reaction promotion. And conduct biological research, so ultrasound has been widely used in various sectors such as industry and mining, agriculture, and medical care.

Ultrasonic and infrasonic frequency range
Ultrasonic cleaning

  The application principle of ultrasonic cleaning is that the high-frequency oscillation signal sent by the ultrasonic generator is converted into high-frequency mechanical oscillation by the transducer and transmitted to the medium cleaning solvent. Ultrasonic waves radiate forward in the cleaning liquid in dense and dense phases, causing the liquid to flow and produce 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 suddenly close. When the bubbles close, a shock wave is generated, and thousands of atmospheres are generated around it. Pressure destroys insoluble dirt and disperses them in the cleaning solution. When the group particles are wrapped in oil and adhere to the surface of the cleaning part, the oil is emulsified and the solid particles are separated, so as to achieve the purpose of cleaning the surface of the cleaning part.
Ultrasound surgery

  With the development of science and technology and the advancement of medical technology, traditional surgical procedures have also undergone advancements again and again. In particular, the focused ultrasound surgery independently developed by China has achieved the ability to operate in vivo without surgery or scarring.
  For many people, focused ultrasound surgery is new, and people may think that traditional open surgery or surgical procedures are safer and more reliable. So, will focused ultrasound surgery bring harm to patients? the answer is negative.
  Focused ultrasound surgery is to focus harmless low-energy ultrasound through the body to the affected area in the body. The temperature of the focused focus can reach 60-100 ℃ in an instant, causing coagulation and necrosis of cells and tissues in the affected area to achieve the therapeutic effect. Will such a high focal temperature cause harm to the human body?
  For example, when the ultrasound passes through a human hand, the hand will only feel a slight vibration, but when the ultrasound is focused to a point, its temperature will rise instantly, and the tumor cells in the body will be completely killed. The focus has high accuracy, so other tissues of the body will not be harmed. Therefore, focused ultrasound surgery can only persecute the tumor, while protecting other body tissues from harm.

Ultrasound imaging technology

  Since focused ultrasound surgery is to ablate the diseased tissue in the human body instead of directly removing it, will it cause trauma to the human body? In fact, human beings have inherently strong immunity and rejection functions. When trauma occurs, the human body will quickly activate its self-healing function. For example, when people inadvertently scratch the epidermis, they can recover to their original state in a few days, and even no signs of injury can be seen. This is due to the great function of human immune repair; when necrotic tissues such as uterine fibroids are killed by focused ultrasound high temperature , The body will automatically activate the self-healing system to identify and surround the necrotic tissue, and then decompose, swallow, and absorb it.
The power of infrasound

  The characteristics of infrasound waves are that they have a wide source, a long spread, and the ability to bypass obstacles and travel far. The sound wave frequency of infrasound is very low, below 20 Hz, but the wavelength is very long, and the propagation distance is also very long. It travels farther than ordinary sound waves, light waves and radio waves. For example, infrasound waves with a frequency lower than 1 Hz can travel thousands or even tens of thousands of kilometers away.
  In August 1883, the Krakato volcano between South Sumatra and Java erupted, producing infrasound waves that circled the earth 3 times, with a total length of more than 100,000 kilometers and lasted 108 hours.

  In 1961, the infrasound wave provoked by the Soviet Union’s nuclear test on Novaya Zemlya in the Arctic Circle circled the earth five times.
  Infrasound has a strong penetrating power, not only can penetrate the atmosphere, sea water, soil, but also can penetrate strong reinforced concrete buildings, even tanks, warships, submarines and aircraft.

Using ultrasound imaging technology to judge physical health

  The 7000 Hz sound wave can be blocked with a piece of paper, while the 7 Hz infrasound wave can penetrate more than ten meters of reinforced concrete. The infrasound waves produced by earthquakes or nuclear explosions can destroy houses. If infrasound resonates with surrounding objects, it can generate considerable energy. For example, 4-8 Hz infrasound can produce resonance in the human abdominal cavity, which can cause strong resonance in the heart and damage to the lung wall.
Attenuation of sound waves

  Sound propagates in the medium in the form of sound waves. Gas, solid, and liquid can all become the medium for sound propagation, but in a vacuum environment, sound cannot propagate. In addition, among the three different media, the propagation speed of sound waves in solids is the fastest, followed by liquids. Naturally, the propagation speed of sound waves in gases is the slowest.

Principle of ultrasonic cleaning

  Then why is it that closing the window can effectively reduce the noise when the noise outside the window is unbearable?
  It turns out that although sound travels the fastest in solids, when sound starts from the sound source and passes through the medium, it loses part of its energy. Therefore, when we close the window, part of the energy in the sound wave will be absorbed by the window, and even part of it will be reflected back. Therefore, the original sound wave (noise) loses a lot of energy. When we close the window, the energy outside the window The noise is significantly reduced.

Schematic diagram of facial medical focused ultrasound

Closing windows can reduce noise interference

  Therefore, sound waves will have different degrees of attenuation in different types of media. We can even use a simple small experiment to better understand this.
  Find a table, tap the leg of the table lightly, and feel the sound you hear. Then lay your head on the table with your ears close to the table, and then tap on the legs of the table. How about, is the sound loud and clear this time, far greater than the sound heard for the first time? I believe that after reading the above content, you can definitely explain the mystery by yourself!
Further reading

  Infrasound harm the
  normal function of infrasound can interfere with the nervous system and endanger human health. A certain intensity of infrasound waves can cause dizziness, nausea, vomiting, loss of balance and even mental depression. Some people believe that motion sickness and seasickness are caused by infrasound waves associated with vehicles and ships in operation.
  People living in buildings with more than ten stories often feel dizzy and nauseous when encountering windy weather. This is also because the high winds shake the tall buildings and produce infrasound waves. Stronger infrasound waves can also cause deafness, coma, mental disorders and even death.