INFRASOUND (Latin infra under + a sound) — the low-frequency fluctuations similar to sound waves, but not heard by the person. The lower bound of infrasonic range is not certain — can be located in the area to thousand shares of hertz. Usually take frequencies within 16 — 25 Hz for the upper bound though various researchers allow a possibility of acoustical perception of fluctuations with a frequency from 1 to 15 Hz.
Source And. there can be many natural phenomena — the wind, lightning discharges, processes proceeding in crust, e.g. earthquakes, collapses, explosions, etc. And. contains also in the noise accompanying operation of plants and vehicles (see. Noise ). And., as a rule, accompanies vibration (see). Feature And. its small absorption is various environments and in this regard ability to extend to long distances by air, water and the land surface. Use is based on it And. for definition of the place of strong explosions, earthquakes, and also a research of a condition of the atmosphere, aqueous medium and arrays of the earth. Distribution And. on long distances by the sea gives the chance to predict natural disaster — a tsunami.
Depending on duration of action and intensity And. can make various impact on a human body. And. rather low and average intensity can cause a complex of unpleasant feelings: dizziness, nausea, difficulty of breath, pain in a stomach, feeling of mental oppression, depression and fear. At increase in radiation intensity the feeling of dryness in a throat, cough, sometimes feeling of suffocation appears; these symptoms are shown at intensity And. it is higher than 150 dB. Especially adversely repeated, long action And. on a human body. In these conditions harmful effects can be shown at much smaller intensity And.
Along with the general impact on an organism And. has local action on an acoustic organ. It is shown by a hyperemia of a tympanic membrane, emergence of pressure sense in an ear, acoustical discomfort and pain, increase in hearing thresholds. Degree of manifestation of these manifestations depends on intensity, frequency and duration of action of I. Obychno they arise at intensity And. more than 150 dB, frequency of 1 — 7 Hz and duration of influence about 8 — 10 min. Pilot studies on animals testify to a possibility of perforation of a tympanic membrane at increase in intensity And. (St. 170 dB). Unlike fluctuations in the sound range, And. has no so sharply expressed effect on function of a vestibular mechanism and, obviously, is not the reason of vestibular frustration. Prevention of harmful action And. on a human body it is provided with restriction or elimination of the factors promoting emergence And., and measures of medical prevention. For elimination or decrease in intensity And. in sources of its emergence change of a design of the devices, cars or separate nodes generating is recommended And., and also reorganization of a techno of l. processes. For early detection of precursory symptoms of the pathology caused by influence And., regular carrying out preliminary and periodic surveys of workers in the terms established by state standard specification is necessary. It is also necessary to consider that harmful action And. under production conditions it is usually shown in a complex with action of noise of various spectral structure, vibration and other factors.
Bibliography: see bibliogr, to St. Sound .
B. M. Sagalovich.