And now we really would like to concentrate what is infra sound. And it's really a horrible imagination that there is something a noise that is there, but we cannot hear or not feel. And nevertheless, it should harm ourself.

But before we did, unfortunately, that is coming from wind turbines. But before we discuss that, what is infra noise physically seen? Infra noise is just normal sound that is defined with a frequency below 20 hertz.

So infra sound is air pressure fluctuations with the frequency below 20 hertz. And it has wavelength between 17 meters and 340 meters. And there are different sources that emit infra sound. For example, there are natural sources like the wind or the sound of the sea.

But there are also technical sources, for example, the traffic or a washing machine, a refrigerator, a heating and also wind turbines. So it's not only wind turbines that emit those strange infrasound. There are many different sources.

And what we can say about the perception threshold of the human beings, it increases with decreasing frequency. So in this graph, I want to explain this a bit more in detail. In this graph, you can see the sound level in decibel. It is plotted against the frequency in hertz.

And the human listening area is shown in blue. So at these sound levels and frequencies, humans can hear. And the areas in brown are so-called low frequency noises and everything to the left of the red line is infrasound.

The lower the frequency, the higher the sound pressure level has to be so that humans can still perceive the noise. And the human ear can only hear the deep tones of infrasound at very high sound pressure levels. And in the infrasound range, there is a smooth transition from hearing to feeling.

So the perceptibility independent of hearing is 23 decibel at approximately 100 hertz and approximately 120 decibel at 3 hertz. So sound levels above these values are then perceived in the area of feeling.

And the threshold of awareness, it varies a bit from person to person. Okay, I'm really glad that it's not only wind turbines that emit this infrasound. But when we come back to the wind turbines, which part of a wind turbine is really emitting now this infrasound from where does it come from?

Yeah, the infrasound is due to turbulences and the rotor blades passing the tower. And in these cases, there is a pulse shaped signal that is the infrasound. And in case the wind turbines operate with almost constant circumferential speed, there's

a repetition of these pulse signals at a regular interval. And if we look at these signals in the spectral view that we have shown before, we can see the so called blade passing harmonics. And these are the frequencies that are the integral multiplies of the basic harmonics

of the wind turbines. So I want to show this here. In this graphic, you can now see an infrasound measurement from the LUBW on a wind turbine. And it was measured at the distance of 150 meters from a two megawatt system.

And on the y axis, you can see the narrowband sound pressure level. And on the x axis, the frequency of the individual noises. So the background noise is shown in green. And this was measured when the plant was at a standstill.

And in purple, you can see the overall noise when the system was in operation. So the infrasound of the system can therefore be measured. And at the lowest frequencies where the black arrows point, peaks can be seen in the overall noise. And these peaks are the blade passing harmonics that I have explained before of this system.

And you can also see that the sound pressure level is highest at frequencies between one and five hertz. And these are the frequencies that dominate the infrasound from the wind turbines. So in this graphic, you can see a measurement on the same system again, but this time at

a distance of 700 meters. So in the graph, the narrowband sound pressure level is again plotted against the frequency. And the noise when the system is switched off in green and the noise when the system

is running is shown in orange. And you can see that the two graphs are almost on top of each other. So that means that at a distance of 700 meters, the infrasound of the wind turbine is lost in the background noise of the traffic, industry and wind.

And it is also clear that the wing harmonics which could still be measured at a distance of 150 meters, they cannot longer be detected in this distance of the wind turbine. Sorry for interrupting Johanna, but I still have problems with this logarithmic decibel scale because we humans are not used to elevate this logarithmic scales.

And I just did another graph and went back even it's not used to display it like that. But displaying on the vertical axis is just a physical pressure in Pascal. And when I do that, what we see here in those two lines is the infrasound noise level when

the wind turbines are not in operation. So it's the background noise in the 150 meters distance and the 700 meters distance like you have shown. And when I add then the other two curves when the turbine is in operation, then we see that

those lines there is a difference, but it is very close. And when we only show it on a linear scale with the absolute pressure, it is really very close. And now we could say, yes, but why did you have chosen such a scale going up to 3 Pascal? That's the reason because we will see later other pressures that really go into that direction.

Yes, that's what I want to show. So in order to evaluate whether we can perceive the measured infrasound, let's take a look at this measurement here. So the third octave level is now plotted against the frequency of the infrasound measurements

at a distance of 150 meters, 300 meters and 700 meters from the wind turbine. And in addition, the human perception threshold is plotted in gray. So humans can neither hear nor feel any measured values below this.

And in the sound at frequencies below 20 hertz, so to the left of the red dashed line in the graph here. And in this area, all infrasound signals from the wind turbines are well below the perception threshold. So that means that the infrasound cannot be perceived by humans.

And at frequencies above approximately 40 hertz, the noises are then above the perception threshold. But however, this is no longer infrasound and people can hear these noises from wind turbine. I interrupt you once more because I need to show once more my linear scale.

And I also added here but in my scale, it's in blue, this perception threshold. And now we see the perception threshold and we really humans can notice this infrasound. And once more, those lines from the infrasound coming from the wind turbines.

And there we see that big distance that is between this perception threshold. Yes, and for comparison, at this point, the infrasound from the wind turbines will be compared with other sources that are emitting infrasound. So in the two graphics, you can see the third octave levels again.

And the infrasound area of the wind turbines measured by the LUBW is shown in orange at a distance of approximately 300 meters from these wind turbines. So for comparison, the infrasound was measured in the interior of a car with the windows

closed in the graphic on the left, and also the infrasound on the road that 2000 cars pass per hour. And you can see that both the sound pressure level in the car and the level on the street are significantly higher than that of the wind turbines. And the graph on the right shows the sound pressure level measured by the wind in a meadow

at a wind speed of 6 meters per second and at a speed of 10 meters per second. And the infrasound from this wind alone is of a similar order of magnitude as the infrasound from the measured wind turbines.

And here I would like to introduce another infrasound emitter, namely a trampoline that many families have in the garden. So like a loudspeaker, a trampoline has a membrane that moves.

And the measurement I'm showing was made by Dr. Stefan Holsoy from the University of Bayreuth. He has published a lot of educational work on the subject of infrasound from wind turbines on his home page. And that's why we would like to take this opportunity to thank him for his valuable

input on the subject of infrasound and wind turbines. And in any case, he measured the sound pressure level at different distances that occurs when you stand on such a trampoline and just bob your feet without lifting them.

And the bobbing can be recognized by the peaks up to a distance of 10 meters from the center of the trampoline. So the levels right next to the trampoline are even between 80 and 100 decibel, thus higher than those of the wind turbines that we saw on the slides before.

I could not stop myself to also introduce this trampoline line into my slides with the wind turbines and the perception threshold. And we see this here in this green dotted line. And I have taken out of those many curves you displayed the one in a two meter distance

where maybe the grandmother is sitting with a cup of coffee, watching their children or grandchildren jumping on the trampoline. And there we see there is a big difference between the turbines, the trampoline and again towards the threshold of perception.

And with that, we stop discussing this here and in the next video, there we show you, we have done our own measurements also for infrasound and the results we have taken there we will present in the next video. Thank you very much.

Thank you.