Sound wave working principle

Update:18 Dec 2017

Sounds when the object vibrates. Scientists refer to the frequency of vibration per second as the frequency of the sound, and its unit is Hertz. The frequency of sound we can hear in our human ears is 16 to 20,000 Hz. Therefore, when the object's vibration exceeds a certain frequency, which is higher than the upper limit of human hearing threshold, people can not hear it. Such sound waves are called "ultrasonic waves." Ultrasound frequencies commonly used for medical diagnosis are 1 to 5 MHz.
Although humans can not hear ultrasound, but many animals have this ability. They can use ultrasound to "navigate," chase food, or avoid dangerous objects. You may have seen many bats fly back and forth in the courtyard during the summer nights. Why do they fly without light and lose their way? The reason is that a bat can send out 2 to 100,000 hertz of ultrasound, which is like an active "radar station." The bats use this "radar" to determine whether an insect or obstacle is in front of the flight.
We human beings until the first world war to learn to use ultrasound, which is the use of "sonar" principle to detect underwater targets and their status, such as the location of submarines and so on. At this time, people send out a series of ultrasonic waves of different frequencies to the water, then record and process the reflected echoes. From the characteristics of the echoes, we can estimate the distance, shape and dynamic changes of the probe. The earliest medical use of ultrasound was in 1942 when Austrian doctor Dusick scanned the brain using ultrasound for the first time. By the 1960s, doctors began using ultrasound in the detection of abdominal organs. Now ultrasound scanning technology has become an indispensable tool for modern medical diagnosis.
The principle of medical ultrasound is similar to sonar in that ultrasonic waves are emitted into the body and reflect and refract when they encounter an interface in the body and can be absorbed and attenuated in human tissues. Because of the different forms and structures of various tissues in the human body, the degree of reflection and refraction and the absorption of ultrasound are also different. Doctors precisely identify the wave patterns, curves, or images reflected by the instruments they. In addition combined with anatomical knowledge, the normal and pathological changes, we can diagnose the organ under examination for disease. The company is located in:
Acoustic waves at frequencies higher than 20000 Hz (Hertz). Study of ultrasound generation, propagation, reception, as well as a variety of ultrasonic effects and applications of acoustic branch called ultrasound. Ultrasonic devices include mechanical ultrasonic generators (such as gas whistles, whistles and liquid whistles, etc.), the use of electromagnetic induction and the principle of electromagnetic effects made of electric ultrasonic generator, and the use of piezoelectric crystal electrostrictive effect and the ferromagnetic Magnetostrictive material made of electro-acoustic transducers and other effects.