The sound wave generated by the tuning fork in the above picture causes the water surface to vibrate.
Similar to the fact that human eyes can only see specific wavelengths (electromagnetic waves within 300 nm to 700 nm), human ears can only hear sound waves with a frequency of about 20 Hz to 20,000 Hz, and sound waves with a frequency exceeding 20,000 Hz are called ultrasonic waves.
How was ultrasound discovered? People can't hear ultrasonic waves, but some animals can. In 1794, Spallanzani discovered that bats navigate through inaudible sounds.
1876, Gao Erdun invented the dog whistle, which is a special whistle. This whistle can emit sound waves with a frequency of 20000 Hz to 50000 Hz. This kind of sound wave has exceeded the hearing limit of human beings, but both cats and dogs can hear it, because the upper limit of the hearing range of dogs is about 40000Hz and that of cats is about 60000Hz.
So ultrasound was discovered about 200 years ago, and real ultrasound can be applied. Madame Curie's husband, pierre curie, and his brother discovered the piezoelectric effect in 1880, and they could emit and detect ultrasonic waves according to the piezoelectric effect, thus opening the door to the application of ultrasonic waves.
The characteristic of ultrasonic wave is that the wavelength is inversely proportional to the frequency, and the ultrasonic frequency is relatively high, so the wavelength is short, that is to say, the ultrasonic wave has good directivity, and the energy is high because of its short wavelength, high frequency and strong vibration.
Ultrasound also has good penetrability, so it can travel a long distance in matter. Based on these characteristics, the application of ultrasound is also divided into two major areas.
The application field of ultrasound has two main application fields because of its good directivity and strong penetration. The first is detection and detection. For example, B-ultrasound, which is commonly used in medicine, probes into the internal structure of human body according to the reflection law of human body to ultrasonic waves, and does little harm to human body. It is an indispensable diagnostic method in clinical medicine.
There is also a sonar system, whose principle is also ultrasonic, which is widely used in navigation and aviation fields and can be used to detect obstacles in front. In addition, there are many similar applications, such as ultrasonic flaw detector, which specializes in detecting the surface production of precision parts.
The second field is ultrasonic treatment, which is realized by the powerful energy of ultrasonic waves, such as glass cleaning, dust removal in factories, ultrasonic welding and so on. All this is done by the strong vibration of ultrasonic waves.