Many times we can't hear the sound.

The sound that people can't hear is infrasound and ultrasound, and their characteristics are completely different.

Sound waves with frequencies below 20Hz are called infrasound waves. Infrasound will interfere with the normal function of human nervous system and endanger human health. A certain intensity of infrasound can make people dizzy, sick, vomiting, out of balance and even depressed. Some people think that motion sickness and seasickness are caused by infrasound waves accompanied by cars and ships. People who live in buildings with more than ten floors often feel dizzy and nauseous in windy weather, which is also because strong winds make high-rise buildings shake and produce infrasound waves. Stronger infrasound waves can also make people deaf, unconscious, insane and even die.

Since the 1950s, the development of nuclear weapons has greatly promoted the establishment of infrasound, and great progress has been made in the research of infrasound reception, anti-jamming methods, positioning technology, signal processing and propagation, and the application of infrasound has gradually attracted people's attention. In fact, the application prospect of infrasound is very broad, including the following aspects:

1. Study the characteristics and mechanism of natural infrasound and predict natural disaster events. For example, infrasound generated by the friction between typhoon and sea waves, because its propagation speed is much faster than that of typhoon, people can use an instrument called "jellyfish ear" to monitor infrasound generated by storm, so as to give an alarm before storm warning. Similar methods can also be used to predict natural disasters such as volcanic eruptions and thunderstorms.

2. By measuring the propagation characteristics of natural or artificial infrasound in the atmosphere, we can detect the properties and laws of some large-scale meteorological processes, such as sandstorms, tornadoes and electromagnetic waves disturbing the atmosphere.

3. By measuring the characteristics of weak infrasound emitted by some organs of human beings and other organisms, we can know the activities of the corresponding organs of human beings or other organisms. For example, the infrasound diagnostic instrument developed by people can check whether human organs are working normally.

4. The military application of infrasound, using the strong penetrating power of infrasound to make weapons that can penetrate tanks and armored vehicles, infrasound weapons-generally only hurt people and will not cause environmental pollution.

Sound waves with frequencies higher than 20000 Hz are called "ultrasonic waves". Ultrasonic wave has the following characteristics:

1. Ultrasonic wave can propagate effectively in gas, liquid, solid, solid melt and other media.

2. Ultrasonic wave can transmit powerful energy.

3. Ultrasonic waves will produce reflection, interference, superposition and * * * vibration.

4. When ultrasonic wave propagates in liquid medium, it will have strong impact and cavitation on the interface.

Ultrasonic wave has the characteristics of good directivity, strong penetration, easy to obtain concentrated acoustic energy and long propagation distance in water. It can be used for ranging, measuring speed, cleaning, welding, breaking stones, etc. It has many applications in medicine, military, industry and agriculture.

If you need something profound, you can ask me.