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5.4 Sound and vibration stimuli

Sound is longitudinal undulation which causes vibration in the air, water, glass or other medium. The eardrum starts to vibrate with the same frequency which creates hearing perception. The higher the pitch is the higher the frequency of sound is. The human ear is able to hear sound of about 20 - 20 000 Hz. Outside the range of hearing there are sounds which in high volumes and/or with long-term  predisposition are dangerous to people as you can't protect yourself against them because you can't hear them. A sound with a frequency over 20 000 Hz is called ultrasound which is used e.g. in medicine to examine the inner organs in the human body or in industry to drill, cut, weld or to monitor the quality of products. (Levävaara, H. 1987).

Low-frequency sound under 20 Hz is caused in nature by e.g. wind, storms and earthquakes. Many animals (e.g. whales, giraffes, alligators) communicate with the low frequency tones. The human being uses infrasound to explore the ground and some phenomena in it (e.g. rock and oil deposits and earthquakes). Manmade devices and machines also create infrasound. The stronger diesel engines, wind farms or loudspeakers are the more harmful to people they are. According to prof. Simon Carlile (2015) infrasound affects some people's senses of balance and creates symptoms that resemble those of seasickness in areas around wind farms. He is concerned about the fact that they are built in wrong places and with wrong technique.

In the USA  Alec Salt, PhD (2010) has studied the function of the inner ear and found out that infra sound effects it but in a different way from the sound within the range of hearing. In the cochlea of the inner ear inner hair cells respond to soundwaves within the range of hearing and convert them into electrical signals. They are sent to the brain and create a hearing perception. Instead, outer hair cells are very sensitive to infrasound outside the hearing range, but the outer hair cells don't transmit it to the inner hair cells. This is how the outer hair cells protect the inner ones from infrasound and that's why the sound is not detected. Dr. Salt wants to challenge the idea of "what we can't hear won't hurt us" with his research. He has heard from people living near wind farms about many "wind farm symptoms" such as lack of sleep, headaches, difficulty in concentration, irritability, fatigue and pain and pressure in the ear.

The human being reacts to different sound stimuli both with the sense of hearing and the sense of balance even if they didn't create any hearing perception outside the range of hearing. No stimulus is too weak to be sensed by the sense of balance even if it is unrecognizable tremble, quivering, vibration or the whirl of electrically charged particles (electrons and protons) in the atmosphere. All this is possible because the sense of balance is the physical, electric sense of the human body. Its task is to protect the body from any disturbing substances whether they are organic (living) or inorganic, solid or liquid or gas. The sense of balance recognizes their unfavorable, strange or harmful properties by their oscillatory frequency patterns.

In the best case a well-functioning / treated sense of balance is able to protect the body from many disturbances by keeping the salt-fluid metabolism in balance and thereby the magnetic field ("the shield") of the body unbroken. If the sense of balance is disturbed too much it always reacts in the same way regardless of what stimulus affects any part of the body. The intensity of the stimulus always influences the intensity of the reaction. The sense of balance tenses muscles and increases (adrenaline and other) hormone secretion to keep the vital functions in balance and to eliminate the disturbing factor. This process happens repeatedly when there has originally been dysfunction in the sense of balance. If there are several additional simultaneous disturbing factors, some troublesome symptoms may appear. What, how many or how severe symptoms the DSB causes depend on numerous things. See 3 How does the DSB affect the body? See 5 Factors affecting the sense of balance

The sense of balance has only one task to keep the body in a favorable position in relation to the gravity field of Earth. According to this force it controls the body and its muscles, and in case of disturbance it activates the brain to maintain balance with conscious control.