Infrasonic is used to refer to
sounds with waves of
frequency that are lower than about 20
hertz, the lower end of the range humans can hear. Sounds that occur in this range are natural phenomena such as
earthquakes,
volcanoes,
waterfalls,
wind, and
thunder. In addition machines such as
airplanes,
cars, or anything else that moves at high speed can produce infrasonic sounds. A few instruments, including the
string bass and the
contrabassoon can also produce such low frequency sounds.
Exposure to infrasonic radiation can cause nausea, blurred vision, and dizziness in humans. In some other species, such as birds and elephants, infrasonic radiation is used to provide warnings about natural disasters and aid in navigation and migration. Earthquakes are one of the most important examples of naturally produced infrasonic radiation. The P waves and the radiation they produce are very useful for pinpointing the epicenter of an earthquake as the sound can travel long distances through a number of different mediums. Such waves can also be produced artificially to pinpoint the location of oil and natural gas deposits.
People living in an urban environment can be exposed to unhealthy levels of infrasonic radiation. Constant exposure to low frequency noise intensifies the symptoms mentioned above and can make it almost impossible to function normally and damage hearing over time. For people in urban environments or even just areas with a great deal of infrasonic radiation from things such as planes, it is advisable for noise reducing measures to be taken. Even having heavy curtains or thicker walls can make a difference in day-to-day life. Noise canceling headphones or white noise machines can help as well. The Occupational Safety and Health Act of 1970 and the Noise Control Act of 1972 limit industrial noise to certain levels and durations. It states that using this formula:
D = (C1/T1) + (C2/T2) + (C3/T3)...
where C is the time exposed and T is the permitted time of exposure for the noise level according to the act, D cannot be greater than one for any day or any person. Today, most buildings are built with the intention to block as much noise as possible and loud areas such as highways are noise shielded from residential areas and houses that don't meet certain noise-canceling standards aren't permitted to be built.
Sources:
"Ultrasonics." Encyclopędia Britannica. 2003. Encyclopędia Britannica Online.
25 Feb. 2003 <http://search.eb.com/eb/article?eu=117558>.