The natural vibration frequency of human organs is called human alpha rhythm, and the range is 3 ~ 17 Hz, such as 8 ~ 12 Hz for the head, 4 ~ 6 ~ 9 Hz for the chest, 5 Hz for the heart and 6 ~ 9 Hz for the abdominal cavity. When the external infrasound coincides with the natural frequency of the human body, the muscles and internal organs will vibrate, and the stimulation will be transmitted to the relevant parts of the human central nervous system, causing a series of functional and morphological changes, which will eventually affect the molecular structure, biological oxidation and energy metabolism of tissues, thus causing harm.
When infrasound waves reach a certain level, people will have uncomfortable symptoms such as fear, nausea and dizziness. Among them, infrasound around 5Hz is the most harmful, which can lead to insanity, brain injury, blurred vision, elevated blood pressure, numbness of limbs and even rupture of five internal organs. The reason why people get carsick and seasick is also caused by the infrasound generated by the vibration of machines and the friction between air and waves. The harm of infrasound to human body is also related to its sound intensity.
Usually, the eardrum can withstand the sound wave intensity of 20 ~ 80dB (decibel). Once the intensity exceeds this limit, it will be harmful to health. Scientists generally believe that when infrasound intensity is about 140dB, even if the action time is short, it will cause changes in the function of human internal organs. When infrasound intensity rises to 150dB, it will cause pathological changes of some organs in human body. If the infrasound intensity rises again, it will not only produce obvious physiological and pathological changes, but also lead to casualties. Therefore, the sound intensity level of 150dB is defined as the safe limit for human body to bear infrasound.
Infrasound weapons studied at present can be divided into two categories according to their emission frequency and different effects on human parts, namely "nerve-type" infrasound weapons and "organ-type" infrasound weapons. The frequency of infrasound emitted by "neural" infrasound weapon is close to the natural frequency of human brain (8 ~ 12hz). When this infrasound acts on human body, it will stimulate human brain and make it vibrate, which will have a certain impact on human consciousness and psychology. In the light, people feel uncomfortable, their attention drops, they feel uneasy, and they feel dizzy and nauseous. In severe cases, people are insane, delirious, shocked and fainted, and lose their combat ability. The infrasound frequency emitted by "organ-type" infrasound weapon is close to the natural frequency of human viscera (4 ~ 8 Hz). When it acts on the human body, the internal organs of the human body will produce strong vibration, muscle spasm, whole body trembling and dyspnea. In severe cases, blood vessels rupture, visceral injury, and even rapid death.