Can people feel frequencies

10.07.2015 16:52

Can you hear "inaudible" sound?

Dipl.-Journ. Erika Schow Press and public relations
Physikalisch-Technische Bundesanstalt (PTB)

PTB has examined the border areas of hearing (infra and ultrasound) in an international cooperation project

Are wind turbines harmful to people? Some believe that, others dismiss - emotions quickly boil. In order to bring more objectivity into the discussion, an international team of experts turned to the basics of hearing at the lower limit of the hearing frequency range (infrasound), but also at the upper limit (ultrasound). The project, which is part of the European Metrology Research Program, was coordinated by the Physikalisch-Technische Bundesanstalt (PTB). On the PTB side, not only acousticians were involved, but also experts in the fields of biomagnetism (MEG) and functional magnetic resonance imaging (fMRI). Their result: humans hear deeper tones than previously known. And the mechanisms of perception are more diverse than previously assumed. A broad field opens up here, in which psychology must not be disregarded. And in any case, there is still more research to be done.

If a wind turbine is to be built in front of their own property, then many proponents of the energy transition will become enemies of wind power. Fears are spreading that the infrasound generated by the rotors and the air flow could make you sick. Some residents of such a facility actually notice sleep disorders, drop in performance and other complaints, while others notice nothing. Infrasound, these are very low tones below the hearing threshold of around 16 Hertz. This means that they are inaudible and far too weak to trigger health complaints, according to the wind energy industry and authorities.

"Both scare tactics and general dissuasion do not lead here," Christian Koch is certain. “Instead, we have to find out more about what happens when sound is perceived at the limit of hearing.” The PTB acoustician is the head of the international project, in which measurement technology experts from several metrology institutes and scientists from the Max Planck Institute for Human Development in Berlin and the Ear Institute at UCL (University College London) studied the basics of hearing "inaudible" sound for three years. Such very low (infrasound below about 16 hertz) or very high sound (ultrasound above about 16,000 hertz) occurs in many areas of everyday life: infrasound occurs not only in wind turbines, but sometimes also when a truck is at the house thunders past or when a homeowner installs an electricity generator in the basement. Ultrasound comes, for example, from the commercially available ultrasonic cleaning baths, which you can use to thoroughly clean your glasses. Or from a "marten fright": a device that uses very high tones to make the marten lose their taste for car cables. A special variant of such devices for expelling young people is under international discussion from an ethical point of view. Adults want to calm down with very high tones that can only be heard by children and adolescents.

"In all of these areas, there are sometimes very high volumes involved," says Christian Koch. An audible loud sound can damage the hearing - and pull on the nerves. But what about "inaudible" sounds? And what does a person really hear? To find out, an infrasound source was constructed in the project that generates tones without any overtones. That was not trivial because tones almost always come along with their associated overtones, but the researchers didn't need high tones here. Test subjects were asked about their subjective hearing perception. These qualitative and quantitative statements were compared with imaging methods, namely magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). The results: People hear lower tones than previously assumed, namely from 8 Hertz; this is at least a whole octave lower than the lowest tone of the previously assumed lower audible frequency range. Because up to this frequency an excitation of the primary auditory cortex could be detected. All those concerned expressly stated that they had heard something, although there was not always a tonal perception. It has also been observed that regions of the brain that play a role in emotions respond. “That means that people then perceive rather vaguely that something is there and that that could also mean a danger,” says Christian Koch.

Many questions are still open. “Basically we are only at the beginning. Further research is urgently needed, ”emphasizes Koch. The application for a follow-up project is already running. The researchers want to specifically examine those people who feel annoyed by "inaudible" sound. After all, not everyone feels the same way; For some, a wind turbine next to their house leaves them completely cold. And then the effects must also be taken into account that some people get sick out of fear of an objectively non-existent danger. Therefore, if possible, psychologists should also be part of the team.

The scientists also see a great need for research in the other extreme, ultrasound. Although the measuring devices used are among the most accurate in the world, the researchers were unable to measure whether and what a person hears above the previously assumed upper hearing threshold. But since it is also true for these high notes that a very loud tone can damage the hearing, more research needs to be done here.

The results of the international research project could lead to the introduction of uniform - and binding - protective provisions across Europe for these border areas of hearing. They are missing so far.
es / ptb

Contact Person:
Dr. Christian Koch, Department 1.6 Sound, phone: (0531) 592-1600, email: [email protected]

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