Can we be absolutely sure that infrasound in the spectrogram is caused by wind turbines?
In the figure below there is shown side by side, three infra-sound spectrograms measured at various distances on the same day. Measuring distances from the nearest wind turbines are 1.5 km, 2 km and 14 km from left to right.
During the intervals as indicated by the arrows, the closest power plants have been shutted down (dyed green space). Also, after at 9, some shorter shut-downs can be seen.
Wind turbine shut-downs are clearly visible in spectrogram yet 14 km away.
This indicates that the spectral spikes in spectrograms (shown as vertical stripes) which disappear when the plants are stopped, are derived from the plants in question. In spectrograms there can be seen weaker infrasound signal that comes from further located wind power plant.