Eyeballs 19Hz, heart 5Hz... Adjust the resonance frequency correctly, and infrasound will hurt people invisibly

Humans not only live in a colorful world, but the environment is also filled with a variety of sounds. With the existence of sound, we can listen to the melodious birdsong and enjoy moving music. And more importantly, information can be transmitted through sound.

In addition, in Stephen Chow's movie "Kung Fu", the landlady even used the terrifying lion roar to repel the invading Qin Demon killer duo. After watching the movie, the question arises, what sound has such a powerful killing power?

The frequency of sound waves that the human ear can perceive is between 20 and 20,000 Hz. Above 20,000 Hz, the sound waves that the human ear cannot hear belong to ultrasound; and below 20 Hz, they belong to the category of infrasound. This classification standard is similar to the visible light in electromagnetic waves, and is divided according to the subjective standards of human perception. Among the sound waves, the one with the most potential to hurt people like the Lion's Roar is undoubtedly the protagonist of this article - infrasound.

—— Characteristics of infrasound ——

Sound is generated by the vibration of an object and propagated through a medium. It is a typical mechanical wave. The essence of its propagation is the transfer of mechanical energy in the medium.

It is not difficult to see from the frequency definition of infrasound that its wavelength is several orders of magnitude higher than that of ultrasound and audible sound. This characteristic makes infrasound not easily absorbed by media such as water and air, and it can also bypass certain large obstacles and diffract, and of course it is not easy to attenuate (the rate of energy decay is inversely proportional to the square of its frequency). In acoustics, the two important properties of measuring sound waves are frequency (Hz) and intensity (dB decibels), and the attenuation rate of infrasound is very low. For example, the attenuation rate of a 10 Hz sound wave is only 0.011 decibels/km (in an air environment of 30 degrees Celsius and 10% humidity).

In nature, many geographical and geological phenomena are typical sources of infrasound, such as earthquakes, volcanic eruptions, tsunamis, meteorite falls, mudslides, etc., and infrasound waves accompanying the events can often be recorded.

In the biological world, although humans cannot hear infrasound, some animals can not only perceive it, but also make good use of it. This function is concentrated in larger animals such as elephants, giraffes, and whales. Due to the large ear distance caused by their wide skull size, their hearing range can generally reach down to the infrasound frequency band.

—— Infrasound and the human body ——

In many literary and artistic works, there have been descriptions of infrasound "killing people", which is a manifestation of the resonance phenomenon. The inherent vibration frequencies of many organs in the human body are within the infrasound frequency range, such as 8-12Hz for the head, 19Hz for the eyeballs, 5Hz for the heart, 4-6Hz for the chest, 6-9Hz for the abdomen, and 6Hz for the pelvis. When an organism is in an infrasound environment and the sound pressure reaches a certain intensity, a resonance reaction will occur when its action frequency is the same as the inherent frequency of the organism's tissues and organs, and the stimulation produced by the infrasound waves is the greatest at this time.

If you are facing a 19Hz speaker, you will not be able to hear the sound even if you adjust the volume to 100dB. However, since 19Hz is the resonance frequency of the human eye, you will find that your eyeballs are shaking and your vision is blurred. If you are exposed to 5Hz, 177dB sound waves, you will find it difficult to breathe, and you will have symptoms such as nausea and vomiting, and your heart and lungs will be damaged to a certain extent.

Among the mechanisms of action of infrasound on the human body, physical resonance is undoubtedly the most important factor. However, contrary to our common understanding, infrasound can also cause biochemical damage, such as damaging tissue structure and affecting the normal metabolic and secretory functions of cells.

The blood-orange sky and grotesque twisted lines in the famous painting "The Scream" fully demonstrate the character's inner depression and fear. In recent years, some scholars have verified historical materials and believe that it is actually a work with a strong realistic component. The blood-orange sky is not an illusion in the mind of the author Munch, but a byproduct of the eruption of the Krakatoa volcano in Indonesia. The huge amount of smoke and dust from the eruption rushed straight into the stratosphere and almost spread all over the world. Many places in Europe clearly recorded the blood-red sunset at that time. The infrasound waves generated by the volcano circled the earth three or four times before disappearing. Fortunately, Munch was tens of thousands of kilometers away from Indonesia. If he encountered the infrasound waves at a close distance, the pain of the characters in the oil painting would become a real scene.

——Are infrasonic weapons feasible?——

In nature, extremely high-intensity infrasound waves may cause harm to the human body. If used to manufacture weapons, they can also have the advantages of strong penetration, high concealment, and long effective range.

Infrasonic weapons are generally divided into two categories based on design theory: organ-type infrasonic weapons and neural-type infrasonic weapons.
The former uses infrasound waves with a frequency close to that of human organs, causing the organs to resonate strongly, leading to muscle spasms, organ damage, and even death; while the latter's working frequency coincides with the human nervous system, which can affect the target person's mental state and distract their attention at the very least, or cause loss of consciousness, shock, and fainting at the worst.

But why has this legendary killing weapon not been put into actual combat so far? It should be said that there is no problem with infrasound weapons in theory, but the effect may not be ideal. It can indeed penetrate protective armor, even solid reinforced concrete buildings. However, as defined above, infrasound can propagate very far, which means that its efficiency in applying energy to objects is very low, and it is difficult to maintain a narrow beam width at long distances. Considering the cost of input power and the effect of realization, the cost-effectiveness of infrasound weapons is undoubtedly low, and it is difficult to put them into actual combat on a large scale.

In the military, the application of infrasound waves is generally seen in nuclear explosion monitoring and artillery positioning. The United Nations Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) has established a network of infrasound monitoring stations around the world, covering frequencies from 0.01 to 20 Hz, to monitor nuclear tests in various countries. The Institute of Acoustics of the Chinese Academy of Sciences has also joined forces with many institutions to form a widely distributed atmospheric infrasound monitoring array network in China.

As early as World War II, the prototype of modern infrasound positioning technology had already appeared. If the enemy's artillery positions are deployed in hidden valleys or bunkers, radar and even laser detection technologies based on electromagnetic waves are difficult to detect, but the infrasound waves generated by the muzzle during firing cannot escape, and the location of the launch point can be obtained after analysis.

—— Where infrasound comes in handy ——

Infrasound is widely used in earthquake prevention and disaster reduction. For example, in the two typical scenarios of earthquakes and volcanoes, it has irreplaceable indicative and predictive significance.

Infrasound and earthquakes. There is a strong correlation between atmospheric infrasound and earthquake phenomena. Not only will earthquakes trigger infrasound, but infrasound anomalies may also occur before earthquakes. Researchers used a wide-band sensor array to form a wide-area distribution network, and observed and analyzed the waveforms and spectra of infrasound signals. They found that it is closely related to earthquake events, and usually appears in one or more groups of intermittent regularities, with a frequency mainly between 0.001Hz and 0.02Hz.

However, only when the energy released by an earthquake is extremely large and has a very large radiation area can it be sufficient to produce infrasound waves with abnormal signal characteristics. In addition, due to factors such as geological structure and formation mechanism, there are individual differences in each earthquake event. Infrasound waves are not yet sufficient to become an accurate conventional prediction method. But infrasound wave monitoring undoubtedly has practical significance for earthquake analysis, especially in large earthquakes. It is a powerful measure to avoid secondary damage caused by aftershocks.

The contribution of infrasound to seismic exploration goes far beyond the above-mentioned methods. For example, research by the American Geophysical Union (AGU) shows that infrasound generated by earthquakes can cause disturbances in the total electron mass in the atmospheric ionosphere. The model has a good fit with seismic events, revealing the corresponding relationship between the two.

Infrasound and volcanic eruptions. Generally speaking, the more violent the volcanic eruption, the greater the amplitude of the abnormal infrasound before the earthquake. By monitoring seismic waves (infrasound) and the duration of earthquakes, the internal structure of the volcano can be determined to a certain extent, and the time of the volcanic eruption can be predicted. In 2010, 57 eruptions of Mount Etna in Italy were successfully predicted by relying on the infrasound sensor array. Nanyang Technological University in Singapore has also networked the infrasound array to monitor abnormal volcanic activity in Southeast Asia.

Infrasonic technology not only plays an irreplaceable role in earthquake prevention and disaster reduction, but also frequently appears in scientific research and industry, from the exploration and analysis of the lithosphere, oceanosphere and even the atmosphere, to the detection and positioning of oil and gas pipelines, bridges and tunnels. In production, it can also be commonly used for infrasonic dust removal for boiler cleaning and infrasonic cooling of steel wire. These methods generally have the advantages of low energy consumption and no pollution.

Infrasound has many other applications. In a nutshell, it has a unique ability due to its long wavelength and non-attenuation characteristics. With the continuous integration and evolution of technology, we may not necessarily see the shocking infrasound weapons, but the role of infrasound in the field of engineering science will undoubtedly become more and more extensive.