When an object vibrates, it produces sound. Scientists call the number of vibrations per second the frequency of the sound, and the unit is Hz. The frequency of sound waves that our human ears can hear is 20Hz～20kHz. When the vibration frequency of the sound wave is greater than 20kHz or less than 20Hz, we can't hear it. Therefore, we call the sound wave with a frequency higher than 20kHz as ultrasound. The ultrasonic frequency usually used for medical diagnosis is 1~5MHz.
Ultrasound has the characteristics of good directionality, strong penetrating ability, easy access to concentrated sound energy, and long transmission distance in water. It can be used for distance measurement, speed measurement, cleaning, welding, stone crushing, etc., in medicine, military, industry, and agriculture. There are many applications on it.
Theoretical research shows that under the same amplitude conditions, the energy of an object's vibration is proportional to the vibration frequency. When the ultrasonic wave propagates in the medium, the medium particle vibration has a high frequency, so the energy is very large. In the dry winter in northern my country, if ultrasonic waves are passed into the water tank, the violent vibration will break the water in the tank into many small droplets, and then use a small fan to blow the droplets into the room, which can increase the indoor air humidity. It is the principle of ultrasonic humidifier. For diseases such as pharyngitis and tracheitis, it is difficult for the medicine to reach the affected part. Using the principle of a humidifier to atomize the liquid medicine and let the patient inhale it, the curative effect can be improved. Using the huge energy of ultrasound can also make the stones in the human body undergo violent forced vibration and break, thereby alleviating the pain and achieving the purpose of healing.
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