Vibration with a frequency higher than the upper limit of human hearing range, that is, greater than about 20 kHz. The word sound wave applies to ultrasonic waves with very high amplitude. Ultrasound, sometimes called praetersound or microsound, is a sound wave with a frequency greater than 10 13 Hz. At such a high frequency, it is difficult for sound waves to spread effectively; In fact, above the frequency of about1.25×1kloc-0/3 Hz, it is impossible for longitudinal waves to propagate, even in liquid or solid, because the substance molecules in which waves propagate cannot transmit vibration fast enough.
Many animals can hear sounds in the range of human ultrasonic frequency. The table compares some hearing ranges of mammals, insects and humans. The supposed sensitivity of cockroaches and rodents to frequencies in the range of 40 kHz has led to the manufacture of "pest controllers", which emit loud sounds in this frequency range to drive away pests, but they do not seem to work as advertised.
transducer
Ultrasonic transducer is a device for converting other types of energy into ultrasonic vibration. There are several basic types, which are classified according to energy sources and media that generate waves. Mechanical devices include gas-driven or pneumatic sensors, such as whistles, and liquid-driven sensors, such as hydrodynamic oscillators and vibrating blades. These devices are limited to low ultrasonic frequency and have many industrial applications, including drying, ultrasonic cleaning and injecting fuel into burners. Electromechanical transducers are more versatile, including piezoelectric and magnetostrictive devices. Magnetostrictive transducer uses a magnetic material, in which the atoms of the material are squeezed together by an oscillating magnetic field, resulting in periodic changes in the length of the material, thus generating high-frequency mechanical vibration. Magnetostrictive sensors are mainly used in low frequency range, and are commonly used in ultrasonic cleaning machines and ultrasonic machining applications.
Up to now, the most popular and universal ultrasonic transducer is piezoelectric crystal, which converts the oscillating electric field applied to the crystal into mechanical vibration. Piezoelectric crystals include Shi Ying, Rochelle salt and some types of ceramics. Piezoelectric sensors are easy to use in the whole frequency range and all output levels. For a specific application, a specific shape can be selected. For example, a disc shape provides a plane ultrasonic wave, while bending the radiating surface into a slightly concave or bowl shape will generate ultrasonic waves focused at a specific point.
Piezoelectric and magnetostrictive transducers are also used as ultrasonic receivers to pick up ultrasonic vibrations and convert them into electrical oscillations.
Application in research
Cavitation is an important scientific research field in which ultrasound has a great influence. When water boils, bubbles form at the bottom of the container, rise in the water and then burst, resulting in the sound of water boiling. The boiling process and the resulting sound have aroused people's interest since they were first observed. They are the objects of extensive research and calculation by British physicists Osborne Reynolds and Lord Rayleigh, who applied the term cavitation to the bubble formation process. Ultrasound is a useful tool to study this process because it can be used carefully to control cavitation. The study of cavitation also provides important information about intermolecular forces.
Various aspects of cavitation process and its application are being studied. A contemporary research topic involves the light emission when the cavity collapses caused by high-intensity ultrasound. This effect, called sonoluminescence, is thought to produce an instantaneous temperature that is hotter than the surface of the sun.
The propagation speed of ultrasonic wave depends largely on the viscosity of the medium. This property is a useful tool in studying the viscosity of materials. Because different parts of living cells are distinguished by different viscosities, acoustic microscopes can use this characteristic of cells to "see" living cells, which will be discussed in medical applications below.
Ranging and navigation
Sonar (acoustic navigation and ranging) has a wide range of marine applications. By emitting a sound or ultrasonic pulse and measuring the time it takes for the pulse to reflect from the echo source of a distant object, the position of the object can be determined and its motion can be tracked. This technology is widely used to locate and track submarines at sea and locate mines under water. Two ships in a known position can also use triangulation to locate and track a third ship or submarine. The use distance of these technologies is limited by the temperature gradient in water, which will make the light beam deviate from the surface and produce shadow areas. In underwater applications, one advantage of ultrasonic wave over acoustic wave is that the former will travel a longer distance with less diffraction due to its higher frequency (or shorter wavelength).
Ranging is also used to map the seabed and provide depth maps that are usually used for navigation, especially near the coast and shallow waterways. Now, even small boats are equipped with acoustic ranging devices, which can determine and display the water depth, so that navigators can prevent ships from being stranded on underwater sandbars or other shallow water points. Modern fishing boats use ultrasonic ranging devices to locate fish schools, which greatly improves efficiency.
Even in the absence of visible light, bats can guide their flight by using acoustic ranging, and even locate insects in flight (they consume these insects in flight). Ultrasonic echo location is also used in traffic control and counting and classification on assembly lines. Ultrasonic ranging provides the basis for the robot's eyes and vision system, and it has many important medical applications (see below).
Doppler effect
If an ultrasonic wave is reflected from a moving obstacle, the frequency of the generated wave will change or Doppler shift. More specifically, if the obstacle is moving towards the sound source, the frequency of the reflected wave will increase; If the obstacle is moving away from the sound source, the frequency of the reflected wave will decrease. The frequency shift can be used to determine the speed of moving obstacles. Just as the Doppler frequency shift of radar (an electromagnetic wave) can be used to determine the speed of moving cars, the speed of moving submarines can also be determined by the Doppler frequency shift of sonar beams. An important industrial application is ultrasonic flowmeter, in which ultrasonic waves reflected from flowing liquid cause Doppler frequency shift, which is calibrated to provide the flow rate of the liquid. This technique is also applied to blood flow in arteries. Many household and commercial burglar alarms adopt the principle of ultrasonic Doppler frequency shift. This alarm can't be used where pets or moving curtains may trigger them.
material test/testing
Non-destructive testing includes the use of ultrasonic echo location to collect information about the integrity of mechanical structures. Because the change of materials will lead to impedance mismatch, which will reflect ultrasonic waves, ultrasonic detection can be used to identify defects, holes, cracks or corrosion in materials, check welds, determine the quality of pouring concrete, and monitor metal fatigue. Due to the propagation mechanism of sound waves in metals, ultrasonic waves can detect deeper than any other form of radiation. Ultrasonic program is used for in-service inspection of structures in nuclear reactors.
Structural defects in materials can also be studied by applying stress to materials and looking for acoustic emission when materials are stressed. Acoustic emission, a general term for this type of nondestructive research, has developed into a unique acoustic field.
High intensity application
High intensity ultrasound has achieved various important applications. Perhaps the most common is ultrasonic cleaning. Ultrasonic vibration is set in a small liquid tank, in which objects are placed for cleaning. Cavitation and vibration of liquid caused by ultrasonic waves produce turbulence in liquid and lead to cleaning. Ultrasonic cleaning is very popular in jewelry industry, and it is also used in IT equipment such as dentures, surgical instruments and small machinery. Degreasing is usually enhanced by ultrasonic cleaning. Large ultrasonic cleaning machines have also been developed for assembly lines.
Ultrasonic machining uses the high-intensity vibration of the transducer to move the machine tool. If necessary, slurry containing emery can be used; Diamond tools can also be used. A variant of this technology is ultrasonic drilling, which uses pneumatic vibration at ultrasonic frequency to replace the standard rotary drill. Almost any shape hole can be drilled in hard and brittle materials such as glass, germanium or ceramics.
Ultrasonic welding is becoming more and more important, especially for welding unusual or difficult materials and very clean applications. Ultrasonic vibration performs the function of cleaning the surface, and even removes the oxide layer on aluminum for welding materials. Because the surface can be made very clean and there is no normal thin oxide layer, flux becomes unnecessary.
Chemical and electrical applications
The chemical effect of ultrasonic wave comes from the discharge accompanying the cavitation process. This forms the basis of ultrasound as a catalyst in some chemical reactions, including oxidation, reduction, hydrolysis, polymerization and depolymerization, and molecular rearrangement. With ultrasound, some chemical processes can be carried out faster and more effectively at lower temperatures.
Ultrasonic delay line is a thin layer of piezoelectric material, which is used to produce short and accurate delay in electrical signals. The electrical signal generates mechanical vibration in the piezoelectric crystal, which passes through the crystal and is converted into an electrical signal. By constructing a crystal with an appropriate thickness, a very accurate time delay can be achieved. These devices are used in fast electronic timing circuits.
Medical application
Although ultrasound competes with other forms of medical imaging (such as X-ray technology and magnetic resonance imaging), it has some ideal characteristics that other technologies cannot provide, such as Doppler motion research. In addition, ultrasound equipment is the cheapest among various modern technologies for internal organ imaging. Ultrasound is also used to treat joint pain and to treat certain types of tumors, which require local heating. A very effective use of ultrasound is to eliminate kidney calculi and bladder stones, which is produced by ultrasound as a mechanical vibration.
diagnose
Many medical diagnostic imaging are performed by X-rays. Because of the high photon energy of X-rays, this type of radiation is highly ionized-that is, X-rays can easily break the molecular bonds in the body tissues through which they pass. This kind of destruction will lead to the change of the function of related organizations, or in extreme cases, the extinction of organizations.
An important advantage of ultrasound is that it is a mechanical vibration, so it is a non-ionizing form of energy. Therefore, it can be used in many sensitive environments where X-rays may cause damage. In addition, the resolution of X-ray is limited because of its strong penetrating power and subtle differences between soft tissues. On the other hand, ultrasound provides good contrast between various types of soft tissues.
Ultrasonic scanning in medical diagnosis uses the same principle as sonar. Usually, high-frequency ultrasonic pulses above 1 MHz are generated by piezoelectric transducers and introduced into the body. When ultrasonic waves pass through various internal organs, they will encounter changes in acoustic impedance, which will cause reflection. The number and time delay of various reflections can be analyzed to obtain information about internal organs. In the B-scan mode, a linear array of sensors is used to scan a plane in the body, and the obtained data is displayed as a two-dimensional picture on the TV screen. A-scan technology uses a single sensor to scan along a line in the body, and the echo is plotted as a function of time. This technique is used to measure the distance or size of internal organs. M-scan mode is used to record the movement of internal organs, such as in the study of cardiac dysfunction. By using higher frequency, that is, shorter wavelength, higher resolution can be obtained in ultrasonic imaging. One limitation of this characteristic of waves is that higher frequencies tend to be absorbed more strongly.
Because it is non-ionizing, ultrasound has become one of the main means of obstetric diagnosis. In the process of extracting amniotic fluid for birth defect detection, ultrasonic imaging is used to guide the needle, so as to avoid damage to the fetus or surrounding tissues. Fetal ultrasound imaging can be used to determine the date of conception, identify multiple fetuses, and diagnose fetal dysplasia.
Ultrasonic Doppler technology has become very important in diagnosing blood flow problems. In one technique, a 3 MHz ultrasonic beam is reflected by typical oncoming arterial blood, and the Doppler shift is several kilohertz-a frequency difference that doctors can directly hear. Using this technology, the fetal heartbeat can be monitored long before the stethoscope can hear the sound. Arterial diseases such as arteriosclerosis can also be diagnosed, and the healing of arteries can be monitored after surgery. The combination of B-scan imaging and Doppler imaging, called dual-function scanning, can identify arteries and measure their blood flow immediately; This has been widely used to diagnose heart valve defects.
Using an ultrasonic wave with a frequency as high as 2,000 MHz, which has a wavelength of 0.75 micron in soft tissue (in contrast, the wavelength of light is about 0.55 micron), an ultrasonic microscope with a resolution comparable to that of an optical microscope has been developed. The remarkable advantage of ultrasonic microscopes is that they can distinguish different parts of cells according to viscosity. In addition, because there is no need for artificial contrast agent to kill cells, acoustic microscope can study real living cells.
Treatment and surgery
Because ultrasound is a kind of mechanical vibration and can focus well at high frequency, it can be used to generate internal heating of local tissues without harmful effects on nearby tissues. This technique can be used to relieve joint pain, especially back and shoulder pain. In addition, research on the treatment of some types of cancer by local heating is currently under way, because focused intense ultrasound can heat the tumor area without significantly affecting the surrounding tissues.
Trackless surgery-that is, surgery that does not require an incision or trajectory from the skin to the affected area-has been developed for several situations. Focused ultrasound is used to treat Parkinson's disease by causing brain damage in areas that traditional surgery cannot reach. A common application of this technique is to use the shock wave formed by focused ultrasonic pulses to destroy kidney calculi. In some cases, a device called an ultrasonic lithotriptor focuses ultrasonic waves with the help of X-ray guidance, but a more common technique to destroy kidney calculi, namely endoscopic ultrasonic pulverization, uses a small metal rod that passes through the skin to transmit ultrasonic waves in the frequency range of 22 to 30 kHz.
infrasonics
The term "infrasound" refers to waves with frequencies below the range of human hearing, that is, below about 20 Hz. Such waves occur in nature in earthquakes, waterfalls, waves, volcanoes and various atmospheric phenomena, such as wind, thunder and weather patterns. It is one of the great challenges faced by modern high-speed computers to calculate the motion of these waves and predict the weather by using these calculations and other information.
Aircraft, cars or other fast-moving objects, as well as air handlers and blowers in buildings, also produce a lot of infrasound radiation. Studies have shown that many people have adverse reactions to high-intensity ultrasonic frequency, such as headache, nausea, blurred vision and dizziness. On the other hand, as shown in the table, many animals are sensitive to infrasound frequencies. Many zoologists believe that this sensitivity of animals such as elephants may help to provide them with early warning of earthquakes and weather disturbances. Some people think that birds' sensitivity to infrasound is helpful to their navigation and even affects their migration.
One of the most important examples of infrasound in nature is in earthquakes. There are three main types of seismic waves: S wave, a transverse body wave; P wave, a longitudinal body wave; And l waves propagating along the boundary of layered media. L-wave is very important in earthquake engineering. It propagates in a way similar to water wave, and its propagation speed depends on frequency. S-wave is a transverse body wave, so it can only propagate in rocks and other solids. P wave is a longitudinal wave similar to sound wave; They travel at the speed of sound and have a wide range.
When P waves propagating from the epicenter of an earthquake reach the earth's surface, they are converted into L waves, which may then destroy the surface structure. P-waves have a large range, which makes them helpful to identify earthquakes from observation points far away from the epicenter. In many cases, the most serious shock in an earthquake will be preceded by a smaller shock, which provides an early warning for a bigger shock. Underground nuclear explosions can also produce P waves. If the intensity is strong enough, P waves can be monitored from anywhere in the world.
The reflection of man-made earthquake helps to determine the possible location of oil and gas sources. The unique rock formations where these minerals may be found can be identified by acoustic ranging, mainly infrasound frequency acoustic ranging.