Infrasound is low-frequency sound with frequencies below 20 hertz. It is created in nature, but also by technical facilities. Infrasound can spread many kilometers and penetrate the walls of structures.
The natural sources of infrasound include, for example, earthquakes and sea surf. Technical sources are - to name just a few - combined heat and power plants, airplanes and also wind turbines. In recent years, doctors and scientists have increasingly dealt with infrasound from wind turbines. Given the transition to renewable resources there has been an expansion of wind power which means an increase in turbines erected.
People who live near wind turbines often complain of sleep disorders, dizziness, headaches, and difficulty concentrating. Those who complain are frequently dismissed as crazy and are often left with no other choice but to move from the area. It is commonly held that sound frequencies below 20 hertz are not audible and therefore cause no health damage.
Perception below the hearing threshold
But is that correct? Professor Christian-Friedrich Vahl, Director of the Clinic for Cardiac, Thoracic and Vascular Surgery of the Johannes Gutenberg University Mainz likens the situation to the arguments by early radiologists who experimented with X-rays, and who claimed that because the waves could not be seen, they were harmless. It was only much later that researchers understood that X-rays could cause cancer.
Medical and scientific evidence is increasing that not only some animals, but also humans are able to perceive infrasound below the hearing threshold. This is not surprising actually, because "infrasound is an energy," explains Prof. Vahl, "And every energy has physical effects, whether you hear it or not." For two years, Vahl and his team have been addressing the question of how infrasound affects the power of the heart muscle. They have already completed two series of experiments investigating the acute effects of infrasound on human cardiac muscle, and the results are available: "In both series of tests, a clear reduction in cardiac muscle strength has been observed with infrasound signals," says the cardiac surgeon. Something that you do not consciously perceive can still make you sick, or at least have an effect.
In 2007, the Robert Koch Institute pointed out the potential danger of infrasound. Likewise, the "Feasibility Study" of 2014 of the Federal Environmental Agency found, "that negative effects of infrasound in the frequency range below ten hertz are not excluded even at sound pressure levels below the threshold of hearing".
Effects on the brain
Studies by scientists at the University Medical Center Hamburg-Eppendorf (UKE) also show effects of infrasound on the brain. They found that infrasound presented below the individual threshold of hearing activates certain regions of the brain. Interestingly, these were regions that are involved in the processing of stress and conflict. It’s not clear why but Professor Simone Kühn of the UKE has a hypothesis: "We have speculated that if you hear something consciously and know there is something, you can perhaps better hide it. [...] But with things that are so semi-perceptible, you may not have the directive to say, that's what I'm ignoring now. "Unconsciously perceived things may put you into a state of stress.” A follow-up study by the UKE is now looking into the question of whether the sleep quality and performance of volunteers changes after four weeks of nocturnal infrasound.
Worldwide attempts by the military to use infrasound as a non-lethal weapon are another indication that this low-frequency noise can have a negative effect on humans. Experts estimate that between ten and thirty percent of the population can feel the symptoms of infrasound.
Different measuring methods
Nevertheless, to this day there is no standard for the frequency range below 20 hertz, which would represent the noise level of wind turbines. On the contrary, authorities now enforce a measurement standard that partially filters out the infrasound emissions of wind turbines. Frequencies below 8 hertz are completely ignored. By averaging (third-octave analysis) so-called "tonal peaks" are largely smoothed out, which means that certain high levels are not visible in the result.
The German Federal Institute for Geosciences and Natural Resources (BGR) already showed in 2004 how the emissions from wind turbines in the infrasound sector really look and how far they can travel. The BGR is responsible for ensuring compliance with the Comprehensive Nuclear Test Ban Treaty (CTBT). For this purpose, the Federal Institute operates several measuring stations, of which two stations register infrasound. To avoid disturbing the measurement, the BGR determined the distance the wind turbine measuring instruments must have and concluded: "As a rule, a distance of about 20 kilometers between the station and the wind farm should be maintained to ensure an undisturbed registration and detection of transient acoustic signals." This is a distance from which the neighbors of wind farms could only dream.
Translation from German to English provided using Google Translate