Infrasound
At infrasonic sound is meant the frequency of which, however, is below about 16 Hz above the weather -induced variations in air pressure. The human ear is almost insensitive to infrasound.
Some animals such as elephants, giraffes and blue whales ( in the water have infrasonic waves a particularly high range) can perceive sound in a part of the frequency spectrum and use these sounds probably also for communication. Especially infrasonic waves of very low frequency spread well over large distances.
- 2.1 IMS Monitoring Network
- 2.2 German IMS stations
- 3.1 hairdryer
Physical and psychological effects
Even if people can hear without aids infrasound hardly, it is noticeable at high sound pressure levels. The threshold of hearing is, however, very high and is also frequency dependent. In addition, especially the low-frequency vibrations of the body at high sound pressures can be felt.
A harmful effect on hearing, balance organs, lungs or internal organs could - although often claimed - are not yet proven in relevant experiments below a sound pressure level of 170 dB. In the low-frequency vibrations that can occur along with infrasound, is by prolonged exposure and very high vibration acceleration, ie, when the amplitude of the vibration acceleration exceeds the acceleration of gravity, the possibility of isolated bleeding from internal organs.
Even below this level are extremely high, as with any sound exposure, psychological effects (particularly decrease in concentration or increased blood pressure ) is possible. Due to the different location of the hearing threshold in different people, a for some inaudible low sound other people seem loud.
That infrasound causes awe in people feelings or anxiety, persistent reports. Since it is not consciously perceived, it can give humans the vague impression, supernatural events are in progress.
Infrasonic - The 17 -Hz infrasound experiment
On 31 May 2003, a group of British scientists led by Richard Wiseman by a mass experiment where they sonicated 700 people with music. This was supplemented with a 17 -Hz sine wave of 90 dB, described as " on the edge of the audible ," and produced by a subwoofer with a Langhubmembran. The subwoofer is so placed in a 7 -meter-long plastic tube, as it is used in sewage systems, that he divided the total length of the tube in a 1:2 ratio. The experimental concert (entitled Infrasonic ) listed in the London Concert Hall Purcell Room, consisted of two performances, each with four pieces of music. Two pieces of music were backed by the described 17- Hz tone. To make the test results of the musical pieces independently, the 17- Hz tone in the second performance was placed just below those two pieces that were free of them in the first performance. The participants were not told which contained pieces of the sound. If the sound played, reported a significant number of respondents ( 22%) of anxiety, uneasiness, extreme sadness, irritability associated with nausea or fear, a "cold run down the back" and pressure on the chest. When these results of the British Association for the Advancement of Science presented a responsible scientist said: "These results suggest that low frequency sounds can cause unusual human experiences, even if they can not consciously perceive infrasound. Some scientists claim that these same noises came to supposedly haunted places before and mediated in this way strange impressions which then attributed to the people ghosts -. , Our findings support those statements "
Detection and measurement
Strong sources of infrasound can often locate by lying in the audible harmonics. But less strong sources require special sensors: Conventional sound pressure microphones usually are not sufficient due to their lower cut-off frequency well into the infrasonic range into it, whereas usual pressure sensors are not sensitive enough for most applications.
The infrasound study the atmosphere and the oceans is a young field of research, which, in particular to measure themselves by the possibility of nuclear weapons testing and ship movements, has developed.
Slightest sound pressure level of remote sources not only require extreme sensitivity, but also measures to shield local sources.
Micro Barometer differ from barometers in that they are protected by an overflow from overload by meteorological fluctuations in air pressure. For this, they measure more rapid pressure changes from 0.01 to 0.1 Hz more sensitive. About star-shaped tubes designed several measuring points are combined to compensate by averaging disorders. This works over areas that are less than half the wavelength. Over larger areas, the signals can be combined by considering the direction of incidence dependent delay differences by electronic data processing, see phased array antenna.
IMS Monitoring Network
In the context of monitoring the Comprehensive Nuclear- Test-Ban Treaty ( CTBT), a global, internationally operated network of infrasound stations ( IMS) ensures that no nuclear explosions in the atmosphere, remains undiscovered underwater or in outer space.
The data generated by these stations open a new task and area of research that focuses on the detection, localization and identification of infrasound sources. To obtain accurate and reliable information about the source of the data, fundamental studies of infrasound signals and their propagation in the wind and weather ever-changing atmosphere are essential.
German IMS stations
Fixed systems:
For the Federal Republic, the Federal Institute for Geosciences and Natural Resources ( BGR) is responsible for the operation of two of these infrasound plants belonging to the international surveillance network:
The IS26 in the Bavarian Forest and IS27 in Antarctica.
The Bavarian Forest, near the border with Austria and the Czech Republic, was launched in October 1999, the first measuring station ( IS26 ) with a total of five stations in operation that meets all the technical specifications of an infrasound station of the global monitoring network. In the selection of the site was taken into account that in this area already, which consists of 25 individual stations seismic measuring system PS19 is, the part of the international seismic monitoring network.
Mobile systems:
In addition to the infrasound stations permanently installed are four mobile infrasound equipment available to perform at any location infrasound measurements. A first practical test these systems in May 2002 to usage at Blaubeuren, when it came to clarifying the relation between infrasound and the Hum phenomenon.
Natural sources of infrasound
Low-frequency waves, which arise, for example, earthquakes, volcanic eruptions, meteorite fall, extreme weather conditions ( thunder during thunderstorms ) or by high seas, can be in the air over long distances spread up to several thousand kilometers.
Infrasound events related to weather phenomena and waves Mikrobarome be called.
Wind generated infrasound, if it is gusty or swirled.
Hairdryer
The case of wind in the Alps known as the foehn, is a strong infrasound source in the range of 0.01 to 0.1 Hz Non conclusively determined are the possible effects on humans. See also: Föhnbeschwerden.
Artificial sources
In the summer of 2004, with the four mobile measurement equipment of the BGR the infrasonic emission of a 200 kW wind turbine studied The measurements led to the conclusion that the noise emissions from wind turbines above 600 kW power in the frequency range of 1 Hz at distances of over 10 km would be provable. In the case of large systems consisting of wind farms a minimum distance of 25 km was estimated for an undisturbed measurement mode. However, experts point out that for a single large plant after only 300 to 500 meters, the human perception threshold is exceeded, the several orders of magnitude below its part of hazardous sound levels.
Also industrial plants can produce low-frequency noise (permanent or when specific operations ). If swaying these long-wavelength sound waves as a standing wave in an enclosed space or when building components (such as a long-span floor ceiling ) resonate, this may be noticeable and occasionally cause problems.
Upper and underground explosions and rocket launches generate infrasound, which can be used over long distance for the detection and localization.
The sonic boom of aircraft also has an infrasonic component.
Large cities, especially in high density areas and develop an infrasound field, which spreads far and can be very intense standing waves locally. Many large cities have numerous high-rise buildings with facades of stone and glass. Almost all modern office buildings are equipped with powerful air-conditioning and ventilation systems. This can result in the summer months mean that develop in many areas of the city infrasonic fields in which to bring the individual air currents of the fan each other in a very low resonance. In particular, in the quiet hours of the night is to hear the low-frequency sound component of large cities over long distances, and the infrasound share carries far.
The organ pipes of a real, not just "acoustic" realized, 64 -foot register produce in the lowest octave ( Subsubkontraoktave ) sounds in the infrasonic range. A fully loaded 64- foot stops - so far two such registers are known worldwide - has the deepest tone, the Subsubkontra -C, a frequency of 8.2 Hz
There are infrasonic detectors for alarm systems, strengthen the infrasonic emissions burglar typical actions 10 to 1000 times and are intended to work alongside other sensors for increased security.