Reception and effects of infrasound in humans - the changes matter
Even though the detection of infrasound effects on humans is only at the beginning of growing attention, framework conditions can already be identified. Crucially, the sound-sensitive receptors of humans and mammals adapt to an extremely wide range of sound pressures. For hearing, it is known that perception in the cochlea and brain can adapt to sound pressures between 0 dB (2x10-5 Pa, hearing threshold at 1 kHz) and 120 dB (20 Pa, human pain threshold), i.e., up to a million times the lower threshold. In this extremely wide pressure range, comparatively tiny pressure differences are perceived as auditory sound - after adjustment to a given base level. It should be remembered that the approximate sound pressure of a conversation at room volume (about 60 dB or 0.02 Pa) is about 5 million times lower than the average air pressure at sea level (1 bar, 101 325 Pa). For infrasound, analogous data are missing so far, but there are indications that similar ratios apply there.
Our vestibular system registers steps and movements by otolithic sensor cells in the inner ear. These detect minute pressure differences in the frequency range of infrasound by means of the inertia of CaCO3 crystals (references in Roos, 2019). In doing so, they work largely independently of the current air pressure, so they are just as sensitive when hiking in the Himalayas as when climbing in the Elbe Sandstone Mountains.
Residents of wind turbines have repeatedly documented that negative symptoms disappear when the turbine is turned off (e.g., Kaula 2019). Background sound pressures, which occur as noise without distinct pulses and are measurable, e.g., when the turbine is at rest, are apparently not perceived as disturbing, although they can assume considerable differences depending on wind strength, e.g., by a factor of a hundred (NCE 2015). Together with other observations (e.g., Palmer 2017), this indicates that the steep pressure changes of infrasound pulses are the real health problem, rather than the absolute value of the sound pressure. The lack of correlation between a sound spectrum without infrasound pulses and the documented complaints of residents, as happened in the Finnish study mentioned above, also supports this assumption, albeit indirectly.
