Infrasound is sound with a frequency below the human hearing surface , i.e. below 16 Hz . Infrasound occurs everywhere in the natural environment, but is also generated artificially, for example in transport or by technical devices.
Some animals such as elephants , giraffes and blue whales (infrasound waves have a particularly long range in water) can perceive sound in a part of this frequency spectrum and probably also use these sounds for communication . In particular, infrasound waves of very low frequency propagate well over large distances.
Physical and psychological effect
Even if people can hardly hear infrasound without aids, it can be heard at high sound pressures. The perception threshold increases with decreasing frequency from about 90 dB at 10 Hz to over 120 dB at 1 Hz. Because of the different position of the hearing threshold in different people, a low tone that is inaudible to some can appear annoying to other people. In addition, the low-frequency vibrations (shocks) in particular can be felt at high sound pressures.
The fact that infrasound causes indefinite fear in people is reported again and again and is proven in the following section.
- Infrasonic - The 17 Hz infrasound experiment
On May 31, 2003, a group of British scientists led by Richard Wiseman carried out a mass experiment in which they exposed 700 people to music. This was with a 17 Hz - sine wave enriched by 90 dB and by a subwoofer generated with a Langhubmembran. This corresponds to around 10,000 times the sound intensity in the vicinity of a wind turbine and is also well above the human perceptibility threshold, which is 77 dB at this frequency. The loud music reduced the perceptibility, although many participants could still see the infrasound. The subwoofer was placed in a seven-meter-long plastic tube of the type used in sewer construction so that it divided the total length of the tube in a ratio of 1: 2. The experimental concert (titled Infrasonic ), performed in the Purcell Room concert hall in London , consisted of two performances of four pieces each. Two of the pieces of music were underlaid with the described 17 Hz tone. To make the test results independent of the pieces of music, the 17 Hz tone in the second performance was placed just below the two pieces that were free of it in the first performance. Participants were not told which of the pieces contained the clay. When the note was played, a significant number of respondents (22%) reported anxiety, discomfort, extreme sadness, irritability combined with nausea or fear, a "cold run down the back" and pressure on the chest. When these results of the British Science Association were presented, said one of the scientists responsible, "These results suggest that sounds can trigger low frequency in people unusual experiences even perceive when they are not aware of infrasound assets."
Detection and measurement
Infrasound with frequencies above about 5 to 10 Hz can be visualized at levels> 20 dB with normal measurement technology. Strong sources of infrasound with very low frequencies can often be localized through harmonics in the audible range . Less powerful sources, however, require special sensors: due to their lower limit frequency, conventional microphones do not reach into the infrasound range for frequencies <5 Hz, while conventional pressure sensors are too insensitive for most applications or do not react sufficiently quickly (5 Hz requires a resolution of less than 0.1 seconds). As a rule, the range from 8 Hz to around 40 Hz is considered.
Infrasound investigations were also developed, for example, to register nuclear weapon tests (see IMS surveillance network). The lowest sound pressure levels from distant sources not only require appropriate sensitivity, but also measures to shield local sources.
Microbarometers are special devices for examinations in the earth's atmosphere . They differ from barometers in that they are protected by an overflow opening against overload caused by meteorological fluctuations in air pressure. To do this, they measure faster pressure changes from 0.01 to 0.1 Hz all the more sensitively. Several measuring points are combined using star-shaped hoses in order to compensate for disturbances by averaging. This works over areas that are smaller than half the wavelength. Signals can be combined over larger areas by taking into account transit time differences depending on the direction of incidence using electronic data processing, see phased array antenna .
IMS monitoring network
As part of the monitoring of the Nuclear Test Ban Treaty (CTBT), a worldwide, internationally operated network of stations ( IMS ) is to ensure that no nuclear explosions go undetected underground, under water, in the earth's atmosphere or in space . This system should also include 60 stations for measuring infrasound. The data obtained with these stations open up a new field of activity and research with a focus on the detection, localization and identification of infrasound sources.
For the Federal Republic of Germany, the Federal Institute for Geosciences and Natural Resources (BGR) is responsible for the operation of two of these infrasound measuring systems that belong to the international monitoring network: the IS26 in the Bavarian Forest and IS27 in the Antarctic .
In October 1999, in the Bavarian Forest, near the border with Austria and the Czech Republic, the first measuring system (IS26) with a total of five stations went into operation, which fulfills all the technical specifications of an infrasound station in the worldwide monitoring network. When selecting the location, it was taken into account that the PS19 seismic measuring system, which consists of 25 individual stations and is part of the international seismic control network, is already located in this area.
In addition to the permanently installed infrasound stations, four mobile infrasound measuring systems are available so that infrasound measurements can be carried out at any location. These systems passed their first practical test in May 2002 when they were used at Blaubeuren when it came to clarifying the connection between infrasound and the humming phenomenon .
Natural infrasound sources
Low-frequency waves, which arise, for example, from earthquakes , volcanic eruptions , meteorite falls , polar lights or high swell, can spread in the air over great distances of up to several thousand kilometers.
Infrasound events in connection with weather phenomena and swell are called microbaromas .
Wind generates infrasound when it is gusty or turbulent.
The falling wind in the Alps, known as the foehn , is an infrasound source in the range from 0.01 to 0.1 Hz. There is no evidence of the effects of the infrasound on humans.
The thunder of thunderstorms can be accompanied by infrasound waves. A special feature are sprites in connection with nocturnal summer thunderstorms: infrasound was detected in more than 70 percent of the cases.
Wind turbines emit a broad spectrum of sound. The emitted power is a few watts, of which around 20 to 50 milliwatts are accounted for by the audible sound component. Infrasound created especially for wind turbines with stall Scheme ( "stable" and "active-stall"). Modern systems with pitch control also generate infrasound to a small extent ; this is no longer noticeable even at a short distance from the systems. This distance is significantly less than the distance that the TA Lärm stipulates in Germany between wind turbines and buildings. In the mid-1990s, manufacturers switched from stall control to pitch control, and since around 2009, almost exclusively pitch-controlled systems have been installed in Germany. Compared to other artificial sources such as cars or airplanes, wind turbines emit very little infrasound. In passenger cars, the infrasound levels measured in the interior at a speed of 130 km / h are several orders of magnitude higher than the values measured on wind turbines.
Wind turbines do not make a significant contribution to the occurrence of infrasound in the environment; the infrasound levels they generate are well below human perception thresholds. There is no scientific evidence that suggests that infrasound has harmful effects in this level range. The scientific consensus is that the weak infrasound emitted by wind turbines has no harmful effects on health. There is no scientifically reliable evidence for fears that are sometimes expressed that infrasound poses health risks. In the public and media debate, various clinical pictures such as "Wind Turbine Syndrome", "Vibro Acoustic Disease" or "Visceral Vibratory Vestibular Disturbance" are used, none of which have been scientifically or diagnostically recognized.
Symptoms of illness that are attributed to infrasound from wind turbines are considered to be “ communicated diseases ”, which, with a few exceptions, were only reported after 2008, when anti-wind power groups began to portray wind turbines as harmful. This year, the pediatrician Nina Pierpont postulated a “wind turbine syndrome” in a self- published book. T. was well received. In the scientific debate, this work and the hypothesis it contains are rejected because of serious methodological errors. The study is based on information from 38 residents of wind turbines that Pierpont recruited via the Internet. There were only 23 phone calls; the symptoms of 15 other people were reported by telephone only by third parties.
In the summer of 2004, the infrasound emissions of a 200 kW wind turbine were investigated using the four mobile measuring systems from BGR . The measurements led to the result that noise emissions from wind turbines (at that time) were detectable above 600 kW in the frequency range around 1 Hz at distances of over 10 km. Experts point out that the emissions of a single large system already fall below the human perception threshold after 300 to 500 meters, which in turn is several orders of magnitude below dangerous noise levels. The Bavarian State Office for the Environment published a paper on the subject in 2014.
The State Institute for the Environment, Measurements and Nature Conservation Baden-Württemberg (LUBW) carried out systematic measurements from 2013 to 2015 in a long-term project on common modern wind turbines with nominal outputs between 1.8 MW and 3.2 MW as well as other technical and natural infrasound sources. An interim report on this was published in February 2015. According to this, the infrasound is clearly below the perception threshold even in the vicinity of the systems at distances of 150 m to 300 m. When the systems were running, the infrasound level was 55 dB (G) to 80 dB (G), while the infrasound level when the systems were switched off was only 50 dB (G) to 75 dB (G) from natural sources. The frequency weighting in dB (G) denotes a weighted sound pressure level with a filter function G and is specified in ISO 7196 (1995). The filter function G carries out a frequency weighting in the spectral range from approximately 10 Hz to 25 Hz.
At a distance of 700 m, the infrasound level when the systems are switched on is only slightly higher than when the systems are switched off, since most of the infrasound is caused by the wind itself. The measurements also showed that the infrasound level dropped significantly at night, as important infrasound sources such as traffic decreased. The infrasound level caused by traffic in the residential area was 55 dB (G) to 80 dB (G) and thus at exactly the same level as the infrasound level of wind turbines, which were measured at a distance of 150 m to 300 m. The measurements also showed that wind turbines and other sound sources can be assessed in accordance with TA Lärm . If the basis for approval is complied with, no negative effects from noise emissions are to be expected from wind turbines.
Industry and Transport
Another important source of infrasound is traffic (see above). In the interior of cars in particular, high infrasound levels of 100 to 105 dB occur, which were the highest values in a long-term study carried out from 2013 to 2015 and which exceeded other infrasound sources by several orders of magnitude. It is interesting that people who are sensitive to infrasound from industrial plants do not feel that their health is harmed by the low-frequency engine noises. Presumably the temporal variation of the frequencies plays a considerable role with regard to an unpleasant sensation. If the frequencies vary, the signal is less disruptive than if the same frequency is transmitted all the time.
Industrial systems also generate (permanently or during certain processes) low-frequency noises. Vibrating machines, grinders, looms or air outlets with long pipes or connected ducts are proven sources of infrasound. If long-wave sound waves build up as a standing wave in a closed room or if building components (e.g. a wide-span ceiling) start to resonate , this can be very clearly perceptible and, with long-term exposure, cause health problems in a sensitive group of people.
The sonic boom of airplanes also has an infrasound component. In a lecture at A + A 2015, M. Vierdt gave a lecture on measurements of infrasound, where high infrasound components (over 100 dB) were measured in the cockpit.
The organ pipes of a real (not just “acoustically” realized) 64-foot register generate tones in the infrasound range in the lowest octave (sub-subcontractive octave). With a fully developed 64-foot register, the lowest note, the sub-subcontra-C, has a frequency of 8.2 Hz (details here ).
There are infrasound detectors for alarm systems that can detect infrasound emissions during typical burglary activities.
Infrasound sources in private households are e.g. B. Washing machine , refrigerator and oil and gas heaters . The highest infrasound levels occur in washing machines in the spin cycle, whereby the perception threshold is sometimes exceeded.
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