InfrasoundAs you already know, sound consists of elastic waves creating vibrations in a medium. The latter can be air, water, or anything else. Humans can perceive these vibrations if they are within the frequency range between 20 Hz and 20 kHz.Some people hear a bit more. Others have a narrower range. This depends on physical abilities of each person.But it's absolutely clear that any sound with a frequency of less than 16 Hz can't be perceived by any human. This frequency is the upper limit of the sound range called the infrasound.Infrasound is vibrations in air, liquids or hard media with frequencies of less than about 16 Hz and, for convenience, greater than one thousandth of a Hertz. These are sound vibrations of extremely low frequencies.Infrasound obeys the same physical laws as regular sound. The two differ only in their frequency and wavelength. However, infrasound does have some unique properties. For instance, its waves are not as easily absorbed by a medium.Infrasound has a pronounced diffraction as its wavelength is greater than that of regular sound we can hear. It can travel longer distances. For instance, we can't hear the sound of a running jet engine from a distance of a couple of hundred metres. It will attenuate. Infrasound of the same level will travel for miles. However, we won't be able to hear it neither near nor far from the source.However, even though we can't hear it, it doesn't mean we can't feel it. It has been proved that people may get extremely unpleasant sensations, when near strong infrasound sources. The symptoms include headaches, sleepiness, and loss of concentration. In some cases people feel anxiety and even have panic attacks.This is why sometimes various unexplainable and mysterious cases are linked with the influence of infrasound. Often there are rumours of new types of weapons using infrasound and capable of demoralising and even killing people.One thing is definitely true. Infrasound does affect people, especially what concerns mental work. This is why there are sound pressure standards (approximately no more than 100 dB). They are designed to protect people from the negative impact of infrasound when it is possible.Sometimes these measures are really successful. Most infrasound sources are man-made. Infrasound is emitted by certain mechanisms. The rotation frequency of some parts may fall into the infrasound range. Infrasound is produced by all types of vehicles, gunshots, and explosions. The most common ...