What is the relationship between ultrasound and sonar?

Objects make sounds when they vibrate. 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 16 ~ 20000Hz. Therefore, when the vibration of an object exceeds a certain frequency, that is, it is higher than the upper limit of human hearing threshold, people can't hear it. This sound wave is called "ultrasonic wave". The ultrasonic frequency usually used for medical diagnosis is 1 ~ 5 MHz.

Although humans can't hear ultrasound, many animals have this ability. They can use ultrasound to "navigate", chase food or avoid dangerous things. You may have seen many bats flying back and forth in the yard on summer nights. Why don't they get lost when they fly without light? The reason is that bats can emit ultrasonic waves of 20 ~ 65438+ megahertz, just like a mobile "radar station". Bats use this kind of "radar" to judge whether there are insects or obstacles flying ahead.

We humans didn't learn to use ultrasound until World War I, that is, we used the principle of "sonar" to detect underwater targets and their status, such as the position of submarines. At this time, people emit a series of ultrasonic waves with different frequencies into the water, and then record and process the reflected echoes. From the characteristics of echo, we can estimate the distance, shape and dynamic change of the detected object. The earliest application of ultrasound in medicine was in 1942. Austrian doctor Dusik used ultrasound technology to scan brain structure for the first time. Later, in the 1960s, doctors began to apply ultrasound to the detection of abdominal organs. Nowadays, ultrasonic scanning technology has become an indispensable tool for modern medical diagnosis.

The working principle of medical ultrasonic examination is similar to sonar, that is, when ultrasonic waves are emitted into the human body, they will be reflected and refracted when they meet the interface in the body, and may be absorbed and attenuated in the human tissue. Due to the different shapes and structures of various tissues in the human body, the degree of reflection, refraction and absorption of ultrasonic waves is also different. Doctors distinguish it by the characteristics of the wave pattern, curve or image reflected by the instrument. In addition, combined with anatomical knowledge, normal and pathological changes, we can diagnose whether the examined organ is sick or not.

At present, the ultrasonic diagnosis methods used by doctors have different forms, which can be divided into four categories: A, B, M and D.

Type A: It is a method of displaying tissue characteristics with waveforms, which is mainly used to measure the diameter of organs to determine their size. It can be used to identify some physical characteristics of diseased tissue, such as substance, whether there is liquid or gas, etc.

B-type: display the specific situation of the probed tissue in the form of plane graphics. During inspection, the reflected signal of human interface is first converted into light spots with different intensities, which can be displayed on the fluorescent screen. This method is intuitive and repeatable, and can be used for before and after comparison, so it is widely used in the diagnosis of gynecological, urinary, digestive and cardiovascular diseases.

M-type: it is a method used to observe the time change of active interface. It is most suitable for checking the heart activity. The dynamic change of its curve is called echocardiography, which can be used to observe the position, activity and structure of each layer of the heart, and is mostly used to assist in the diagnosis of heart and great vascular diseases.

D-mode: It is a special ultrasonic diagnostic method for detecting blood flow and organ activity, also known as Doppler ultrasonic diagnosis. Can determine whether the blood vessels are unobstructed, whether the lumen is narrow, 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 with different colors under the instruction of anatomical markers of echocardiography, and the depth of color represents the speed of blood flow. At present, ultrasound techniques such as stereo ultrasound imaging, ultrasound CT and ultrasound endoscope are constantly emerging, which can be combined with other inspection instruments to greatly improve the diagnostic accuracy of diseases. Ultrasonic technology plays a great role in the medical field. With the progress of science, it will be more perfect and will benefit mankind better.

Sound waves with frequencies higher than 20000 Hz. A branch of acoustics that studies the generation, propagation and reception of ultrasonic waves, as well as various ultrasonic effects and applications, is called ultrasound. produce

Ultrasonic devices include mechanical ultrasonic generators (such as air whistle, whistle and liquid whistle). ), an electro-ultrasonic generator based on the principle of electromagnetic induction and electromagnetic action,

And an electroacoustic transducer made of electrostrictive effect of piezoelectric crystal and magnetostrictive effect of ferromagnetic material.

Ultrasonic effect When ultrasonic waves propagate in the medium, the interaction between ultrasonic waves and the medium causes physical and chemical changes in the medium, which leads to.

A series of mechanical, thermal, electromagnetic and chemical ultrasonic effects, including the following four effects:

① Mechanical effect. The mechanical action of ultrasonic wave can promote the emulsification of liquid, the liquefaction of gel and the dispersion of solid. When standing waves are formed in the ultrasonic fluid medium, the tiny particles suspended in the fluid condense at the nodes due to the mechanical force, and form periodic accumulation in space. When ultrasonic waves propagate in piezoelectric materials and magnetostrictive materials, induced polarization and induced magnetization occur due to the mechanical action of ultrasonic waves (see dielectric physics and magnetostrictive).

② Cavitation. When ultrasonic wave acts on liquid, it will produce a large number of small bubbles. One reason is that the local tensile stress in the liquid forms negative pressure, and the decrease of pressure makes the gas originally dissolved in the liquid supersaturate and escape from the liquid into small bubbles. Another reason is that the strong tensile stress "tears" the liquid into a cavity, which is called cavitation. The cavity is filled with liquid vapor or another gas dissolved in the liquid, even vacuum. Small bubbles formed by cavitation will suddenly move, grow up or burst with the vibration of the surrounding medium. When it bursts, the surrounding liquid suddenly rushes into bubbles, producing high temperature, high pressure and shock wave. Internal friction related to cavitation can form charge, and discharge in bubbles can produce luminescence. Ultrasonic treatment technology in liquid is mostly related to cavitation.

③ Thermal effect. Because of the high frequency and high energy of ultrasonic wave, it can produce significant thermal effect when it is absorbed by medium.

④ Chemical action. The action of ultrasound can promote or accelerate some chemical reactions. For example, pure distilled water generates hydrogen peroxide after ultrasonic treatment; Nitrite is produced by ultrasonic treatment of water dissolved with nitrogen; Dye aqueous solution will change color or fade after ultrasonic treatment. These phenomena are always accompanied by cavitation. Ultrasound can also accelerate the hydrolysis, decomposition and polymerization of many chemicals. Ultrasound also has obvious influence on photochemical and electrochemical processes. After ultrasonic treatment, the characteristic absorption bands of various amino acids and other organic aqueous solutions disappeared, showing uniform general absorption, indicating that cavitation changed the molecular structure.

Ultrasonic application ultrasonic effect has been widely used in practice, mainly in the following aspects:

① Ultrasonic examination. Ultrasonic wave has shorter wavelength than ordinary sound wave, better directivity and can penetrate opaque substances. This characteristic has been widely used in ultrasonic flaw detection, thickness measurement, distance measurement, remote control and ultrasonic imaging technology. Ultrasonic imaging is a technology that uses ultrasonic waves to present the internal image of opaque objects. The ultrasonic wave emitted by the transducer is focused on the opaque sample through the acoustic lens, and the ultrasonic wave emitted by the sample carries the information of the irradiated part (such as the ability of reflecting, absorbing and scattering sound waves), and is concentrated on the piezoelectric receiver through the acoustic lens, and the obtained electric signal is input into the amplifier, and the image of the opaque sample can be displayed on the fluorescent screen by using the scanning system. The device above is called an ultrasonic microscope. Ultrasonic imaging technology has been widely used in medical examination. It is used to test large-scale integrated circuits in the manufacture of microelectronic devices, and to display the regions and grain boundaries of different components in alloys in materials science. Acoustic holography is an acoustic imaging technology that records and reproduces the stereoscopic image of opaque objects by using the interference principle of ultrasonic waves. Its principle is basically the same as that of light wave holography, but the recording means are different (see holography). The two transducers placed in the liquid are excited by the same ultrasonic signal source, and they respectively emit two coherent ultrasonic waves: one beam becomes an object wave after passing through the studied object, and the other beam is used as a reference wave. Acoustic hologram is formed by the coherent superposition of object wave and reference wave on liquid surface. The acoustic hologram is irradiated with a laser beam, and the reconstructed image of the object is obtained by using the diffraction effect produced when the laser is reflected on the acoustic hologram. Usually, real-time observation is done by a camera and a TV set.

② Ultrasound therapy. Ultrasonic welding, drilling, solid crushing, emulsification, degassing, dust removal, cleaning, sterilization, promoting chemical reaction and biological research can be carried out by using the mechanical action, cavitation, thermal effect and chemical effect of ultrasound, which has been widely used in various departments such as industry, mining, agriculture and medical treatment.

③ Basic research. After the ultrasonic wave acts on the medium, the acoustic relaxation process occurs in the medium. The acoustic relaxation process is accompanied by the transport process of energy between molecular degrees, which is macroscopically manifested as the absorption of sound waves (see sound waves). The characteristics and structure of matter can be explored through the law that matter absorbs ultrasonic waves, which constitutes the branch of acoustics of molecular acoustics. The wavelength of ordinary sound wave is larger than the atomic spacing in solid, and under this condition, solid can be regarded as a continuous medium. However, for ultrasonic waves with a frequency above 10 12 Hz, its wavelength can be comparable to the atomic spacing in solids, so solids must be regarded as a lattice structure with spatial periodicity. The energy of lattice vibration is quantized and called phonon (see solid state physics). The effect of ultrasound on solids can be attributed to the interaction between ultrasound and thermal phonons, electrons, photons and various quasi-particles. The generation, detection and propagation of ultrasonic waves in solids and the study of sound phenomena in quantum liquid-liquid helium constitute a new field of modern acoustics-

Quantum acoustics.