But what about the sounds you cannot hear?
Very low frequency sounds, below the 20 Hertz level and generally inaudible to the human ear, are known as infrasounds. These “silent sounds” come from many sources, some natural, others man made. Earthquakes, hurricanes, major storms, volcanic eruptions send low level waves thousands of miles from the source. Missile launchings, construction machinery, explosions, defective electric motors, and aircraft also produce infrasonics. Previously these were assumed to be inconsequential; recent research has uncovered the strange effects they have on people.
In 1968 Dr. Floyd Dunn, a professor of engineering at the University of Illinois, found that auto accidents and school absenteeism rose to abnormal highs in Chicago on days of heavy infrasonic activity. Insur ance company records and school attendance figures provided the statistics. Dining the three week survey period, sophisticated monitoring equipment showed the area to be under bombardment by heavy infrasonic activity. Dr. Dunn concluded that the city wide deviation from normal behavior could only be attributed to the “silent sounds.”
John Green, a Bell Telephone electrical engineer, reports that infrasonics can easily disrupt people’s work habits. When a local company’s staff reacted badly after they were moved to new, modem, more comfortable quarters, he was called in to investigate.
Previously a cooperative and genial group, the employees now were edgy and quarrelsome. Absenteeism was running high, morale was low. Company executives could find no reason for the constant bickering and petty jealousies. The trouble was traced to an improperly installed air conditioning unit that was emitting infrasonics. Once the unit was fixed, the employees’ behavior returned to normal.
NASA researchers learned to take “silent sounds” seriously when they found that the infrasonics surrounding the launch platform during the first three minutes of a space probe seriously impaired the astronauts’ ability to do mathematical calculations, or translate verbal instructions, accurately.
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