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While watching a Mil Mi-17 helicopter takeoff I experienced strange thing. The helicopter produces a lot of noise even when just starting the engines.

But shortly before taking off the high-pitched sound of the jet engines changed somehow (I guess changing the tilt of the rotor blades) and I felt some sort o pulsing "sound" which I felt rather with my belly than with my ears. Really unpleasant experience.

Could it be infrasound?

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Yes. Each rotor blade produces a pressure wave. Here's an example from a Chinook:

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At a rotor speed of 4 rotations per second, that's 12 pressure spikes per second, or a frequency of 12 Hz. At frequencies that low, you can hear the individual waves rather than a coherent tone (try this Youtube video on a good set of headphones).

At high levels you tend to feel more than hear this frequency: you can feel the pressure variations on your skin, your eardrums and in your lungs, and you could get resonances in your body.

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