Infrasound waves, sound waves and ultrasonic waves are all mechanical waves that propagate in elastic media. The same waveform propagates at the same speed in the same medium. Their difference mainly lies in the frequency.
The various sounds that people hear every day are due to the vibration of various sound sources transmitted to the eardrum through elastic media such as air, causing the eardrum to vibrate, affecting the auditory nerve and producing hearing. However, not any mechanical vibration of any frequency can cause hearing. Only vibrations with frequencies within a certain range can cause hearing. Mechanical waves that can cause hearing are called sound waves, with frequencies between 20 and 20,000 Hz. Mechanical waves with a frequency lower than 20 Hz are called infrasound waves, and mechanical waves with a frequency higher than 20,000 Hz are called ultrasonic waves. Infrasound and ultrasonic waves cannot be heard by humans.
For ultrasonic detection of macro defects, the commonly used frequency is 0.5~25MHz, and for the detection of metal materials such as steel, the commonly used frequency is 0.5~10MHz. The frequency of ultrasonic waves is very high, which determines that ultrasonic waves have some important characteristics, making them widely used in non-destructive testing.
1. Ultrasonic waves have good directionality
Ultrasound is a mechanical wave with high frequency and short wavelength. The wavelength used in ultrasonic testing is on the order of millimeters. Ultrasonic waves have good directionality like light waves and can be emitted in a direction. Just like a beam of light from a flashlight, it can find the required items in the dark and find defects in the inspected materials.
Characteristics and Uses
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Detection and Monitoring: Infrasound is useful in monitoring natural events. For instance, it can detect volcanic eruptions and track their progress, providing early warnings. Similarly, it is employed in seismology to study earthquakes.
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Animal Communication: Some animals, such as elephants and whales, use infrasound for communication over long distances. These low-frequency sounds travel further than higher-frequency sounds, allowing communication across vast expanses.
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Research and Industry: Infrasound is used in atmospheric research to study phenomena like thunderstorms and to monitor nuclear tests for treaty compliance.
2. High ultrasonic energy
The frequency of ultrasonic detection is much higher than that of sound waves, and the energy (sound intensity) is proportional to the square of the frequency. Therefore, the energy of ultrasonic waves is much greater than the energy of sound waves. For example, the energy of a 1MHz ultrasonic wave is equivalent to 1 million times that of a 1kHz sound wave.
3. Can produce reflection, refraction, diffraction and waveform conversion on the interface
In ultrasonic testing, especially in pulse reflection method testing, some characteristics of ultrasonic geometric acoustics are used, such as straight propagation in the medium, reflection and refraction when encountering the interface, etc.
4. Strong ultrasonic penetration ability
When ultrasonic waves propagate in most media, the propagation energy loss is small, the propagation distance is large, and the penetration ability is strong. In some metal materials, its penetration ability can reach several meters. This is unmatched by other detection methods.
In addition to non-destructive testing, ultrasonic waves can also be used for mechanical processing, such as processing rubies, diamonds, ceramics, quartz, glass and other materials with particularly high hardness, and can also be used for welding, such as welding titanium, thorium and other difficult-to-weld metals. In addition, ultrasonic waves can be used in the chemical industry for catalysis and cleaning, in agriculture they can be used to promote seed germination, and in medicine they can be used for diagnosis, disinfection, etc.
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