An object makes a sound when it vibrates. Scientists call the number of vibrations per second the frequency of sound, and its unit is hertz. The frequency of sound waves that our human ears can hear is 20HZ ～ 20000Hz. Therefore, when the vibration of an object exceeds a certain frequency, that is, above the upper limit of the human hearing threshold, people cannot hear it. Such sound waves are called "ultrasonic waves". The frequency of ultrasound usually used for medical diagnosis is 1 to 5 MHz.
Although humans can't hear ultrasound, many animals have this ability. They can use ultrasound to "navigate", hunt down food, or avoid dangerous objects. You may have seen many bats flying back and forth in the courtyard on a summer night. Why are they flying without light and not disoriented? The reason is that bats can emit ultrasonic waves of 20,000 to 100,000 Hz, which is like an active "radar station". Bats use this "radar" to determine whether an insect or obstacle is ahead of the flight.
We humans did not learn to use ultrasonic waves until the First World War. This is the principle of "sonar" to detect underwater targets and their status, such as the position of submarines. At this time, people send a series of ultrasonic waves of different frequencies to the water, and then record and process the reflected echoes. From the characteristics of the echoes, we can estimate the distance, shape and dynamic changes of the detected object. The earliest use of ultrasound in medicine was in 1942, the Austrian doctor Dusick used ultrasound technology to scan the brain structure for the first time; in the 1960s, doctors began to apply ultrasound to the detection of abdominal organs. Nowadays, ultrasound scanning technology has become an indispensable tool for modern medical diagnosis.
The working principle of medical ultrasound examination is similar to that of sonar, that is, the ultrasound is emitted into the human body. When it meets the interface in the body, it will reflect and refract, and 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 and refraction and absorption of ultrasonic waves are also different. Doctors are distinguishing by the characteristics of the wave mode, curve, or image reflected by the instrument. they. In addition, combined with anatomical knowledge, normal and pathological changes, it can be diagnosed whether the organ under examination is diseased.
Sound waves with a frequency higher than 20000Hz (Hertz). The branch of acoustics that studies the generation, propagation, and reception of ultrasound, as well as various ultrasound effects and applications, is called ultrasound. The ultrasonic generating devices include mechanical ultrasonic generators (such as air whistle, whistle and liquid whistle, etc.), electric ultrasonic generators made using the principle of electromagnetic induction and electromagnetic action, and the electrostrictive effect and ferromagnetic using piezoelectric crystals Electroacoustic transducers made of the magnetostrictive effect of substances.
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