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iBoaterer[_2_]

In article ,
says...

On Thu, 06 Sep 2012 13:09:24 -0400, Wayne.B
wrote:

On Thu, 06 Sep 2012 12:23:06 -0400, wrote:

The thing I noticed at the ones we saw in Canada and North Dakota was
more of a thump than a loud noise. You feel it more than hear it.

===

What causes the "thump" ?

The displaced air. It is a low frequency pressure wave. You really
feel it as much as hear it. I suppose a lot has to do with the pitch
of the blades because it seems to come and go. We stopped outside
Minot SD and watched the two they have there for longer than my wife
was happy about but it was really a nice day, cool with a nice breeze.
We walked around them a while and I shot some pictures. That was one
of the places that I saw one being feathered and restarted a few
times. It really looked like they were using it to balance the grid.

The air would be displaced at a constant, round pattern, though.