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605
Rubric: Live and Learn
Section: Technology
Solovey Razboinik and Kot Bayun in Your Car

Solovey Razboinik and Kot Bayun in Your Car

Each car driver has a favorite speed of his own. Motion at this speed gives inexplicable pleasure. Drivers normally believe that the reason is excitement aroused by the feeling of insecurity. At the same time, driving at the edge of a precipice or in a traffic jam, though insecure, is tiresome rather than pleasant.

Why does the phenomenon of a favorite speed exist at all? Why is this speed individual for every driver? Why does it depend on the type of the car? Why is it noisy in the tunnel? Why do you feel insecure in a long tunnel?

Aeolus, Sirens, Stribog, and others...

People have always been interested in sounds, both audible and inaudible. It is not accidental that ancient mythology has a pantheon of creatures related to sounds. In Greek mythology, these are the god of wind Aeolus and mellisonant Sirens whose singing tempted sea travelers to face certain death. In Slavonic mythology, these are the god of wind Stribog, Solovei Razboinik, and Kot Bayun.

People learned from experience that sounds could be not only pleasant but also dangerous and that they should protect themselves from the latter. Thus, Ulysses ordered his companions to fill their ears with wax when passing the island of Sirens, so that Sirens’ singing would not drive the whole crew crazy. In another legend, Orpheus drowned Sirens’ voices by his own singing and saved the Argonauts.

This shows that people have always understood that they can feel the action of acoustic waves and can be controlled by means of sound. Sometimes a person can realize that, but in most cases the sound affects the human mind “on the quiet.”

Sound and pseudosound

Ask anyone to define sound is and he will tell you that this is something we hear. Your interlocutor may suspect a trick because you seem to be asking copy-book maxims. Nevertheless, his reply will not be correct because sound is not something we hear; actually, this is something described by equations of acoustics. All the rest is pseudosound.

Examples of pseudosound are periodic spatial pressure perturbations “frozen” into an air flow. Normally, these perturbations are caused by a periodic chain of vortices. If the air moves with respect to a person, the person can hear these pressure perturbations. When pseudosound meets an obstacle in the air flow, it generates sound.

In contrast to pseudosound, sound always propagates in the air with the velocity approximately equal to 330 m/sec under standard conditions (in a quiescent air flow). If a car moves toward (away from) the source of sound, the velocity of sound increases (decreases) by a value equal to the car speed. The increase (decrease) in the audible frequency of the source of sound is called the Doppler effect.

Low-frequency sound and one’s mood

Not all sounds can be heard. The sound should be neither too quiet nor too loud, neither too high nor too low; and these thresholds are specific for each person.

Pressure oscillations in the air which we can hear range from 16 to 16,000 Hz (1 Hz corresponds to one oscillation per second). Oscillations with a frequency below 16 Hz are called infrasound. The wavelength of the lowest audible frequency is more than 20 meters, which significantly exceeds the size of a human body. This means that, in terms of infrasonic oscillations, a person can be considered as a point with periodic changes in pressure.

The effect of infrasound on people and animals was noticed long ago, which was reflected in myths and legends. Today they often use low frequencies to control the mood of the public in various musical shows. Low-amplitude infrasound increases our emotional receptivity, whereas high-amplitude infrasound arouses the feeling of fear and, moreover, can provoke panic.

This fact was proved by the severe experiment carried out by Ingmar Bergman, a Swedish film director. He made a short film showing a simple story of a girl humming a song and picking flowers, and ran it several times. The first performance was accompanied by ordinary music, whereas the following two performances were accompanied by infrasound, first with a low and then with a high amplitude. The spectators were asked what they thought about the film. The people who attended the first performance had either light and pleasant feelings or were bored. The spectators who watched the film accompanied by low-amplitude infrasound admired the director’s art. During the third show, the one with strong infrasound, spectators left the cinema hall in panic.

Despite such a powerful influence of infrasound on the human mind, it does not make sense to use it for military purposes: it would produce an identical effect on both parties, and protection against it would be ineffective because of the large wavelength of acoustic waves.

Possibly, the reasons for such a strong response to infrasound date back to the past of the human society and of the animal kingdom. Infrasonic oscillations are precursors of earthquakes, tornadoes, and hurricanes; the feeling of insecurity drives people to search intuitively for a safer place, which increases their chances to stay alive.

The hypothesis that the legendary “Flying Dutchman” (a phantom ship without the crew) appeared owing to the action of a powerful infrasonic field caused by a crust shearing seems to be fairly well grounded. Captured by unspeakable horror generated by infrasonic oscillations, the crew had left the ship in panic. Certainly, if you prefer the more romantic hypothesis for the existence of mythic creatures, this phenomenon can be explained by the interference of the malicious Solovei Razboinik or Kot Bayun.

Infrasonic oscillations in the human body

It is absolutely clear that the action of periodic infrasonic waves on a person is related to his physiology, i. e., to low natural frequencies of the human body. In terms of long waves, a human being is a certain amount of water with weak inhomogeneities.

One of the basic natural frequencies of a human is the frequency of heart beatings, approximately equal to 1 Hz. There are also other oscillations such as intestinal peristalsis, -brain rhythm, natural oscillations of abdominal cavity and skull contents… In addition, the human mind is affected by oscillations of the fluid in the vestibular apparatus and oscillations of the vestibular apparatus itself.

Low-frequency oscillations in a car

Oscillations inside the car can be classified into two groups:

1) swinging of the car’s body;

2) infrasonic oscillations which are periodic changes in the pressure inside the car.

SWINGING OF THE CAR

The simplest mechanical model of a car is a platform resting on four springs. The natural frequencies of mechanical oscillations of the model are assumed to be the basic frequencies of car swinging. If the frequency of an external force is close to a certain frequency of the natural mechanical oscillations of the car, the car will swing at this frequency. The external force can be induced by a rough road or by aerodynamic forces: periodic shedding of vortices. Swinging mainly affects the vestibular apparatus and lulls a person. This is Kot Bayun purring in your car — time to sleep.

EUPHORIA OF SPEED

If the speed is sufficiently high, a zone of airflow separation is formed behind the car; still higher speeds induce periodic shedding of vortex macrostructures from the car. Incipience of vortex macrostructures and their pseudoacoustic interaction with the car body excite infrasonic oscillations inside the car.

When reaching a certain speed, the driver experiences intense positive emotions. We can now easily guess that they result from infrasonic oscillations. This speed is individual for each particular car and can be determined experimentally.

IS AN OPEN WINDOW A WAY OUT?

The natural frequencies of acoustic oscillations inside the car are quite high, because the linear size of the passenger compartment is always greater than one half of the wavelength of natural acoustic oscillations. Nevertheless, if the compartment communicates with the ambient medium through open air ducts or windows, it acquires its own low-frequency mode of acoustic oscillations called the Helmholtz mode. If the vortex-shedding frequency coincides with the frequency of the Helmholtz mode, certain speeds can produce intense infrasonic oscillations inside the car.

Such oscillations with low amplitudes always lead to emotional excitement. Many drivers compare this feeling to the action of drugs: almost every driver has his own speed which pleases him most. As the amplitude of oscillations increases, the driver experiences in succession feelings of uneasiness, fear, and finally panic: these are the tricks of Solovei Razboinik. The way out is very simple: the driver should reduce or increase the speed of the car.

IN THE TUNNEL

When entering a tunnel, a car always experiences a sonic boom. The elevated noise in a tunnel is caused by multiple reflections of sound from the tunnel walls, which act as acoustic mirrors. For this reason, the noise produced by a car in a tunnel equals the noise produced by the motion of two infinite rows of cars uniformly distributed across the tunnel in the horizontal and vertical directions, like on a chessboard.

If a tunnel is more than 10 meters long, it always has its own low-frequency infrasonic oscillations. Regardless of its shape, the tunnel is a resonator for longitudinal waves (waves directed along the tunnel). The greatest natural wavelength is approximately twice the tunnel length. Therefore, in addition to the sonic boom, drivers and passengers in long tunnels should also expect sudden excitement, fear, or panic.

Recommendations

If you make friends with these mythic characters, Kot Bayun and Solovei Razboinik, and learn to understand them, they can become your friends rather than malicious jokers. They will help you to detect malfunctions without any stethoscope, just from a changed noise made by your engine or gear.

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