Ultrasonic wave is a kind of sound wave whose frequency is higher than 20000 Hz. It has good directionality, strong penetrability, easy to obtain concentrated sound energy, and can be used for ranging, velocity measurement, cleaning, welding, gravel, sterilization and disinfection, etc. It has many applications in medicine, military, industry and agriculture. Ultrasound is named because its lower frequency limit is greater than the upper limit of human hearing.
Scientists call the number of vibrations per second the frequency of sound, in Hertz (Hz). The frequency of sound waves that we can hear in human ears is 20hz-20000hz. Therefore, we call the sound wave with frequency higher than 20000 Hz as "ultrasonic wave". The frequency of ultrasound usually used in medical diagnosis is 1 mhz-30 MHz.
The theoretical study shows that the vibration energy of an object is directly proportional to the vibration frequency under the condition of the same amplitude. When the ultrasonic wave propagates in the medium, the vibration frequency of the medium particle is very high, so the energy is very large. In the dry winter of northern China, if the ultrasonic wave is put into the water tank, the water in the tank will be broken into many small droplets by violent vibration, and then the droplets will be blown by a small fan Enter the room, can increase indoor air humidity, this is the principle of ultrasonic humidifier.