I've been trying to decide what sort of mill to make, and noise level is significant for me.
At the rural site I have occasional very high winds, and neighbors within a few hundred feet. (The suburban site is your typical '50s ticky-tacky suburb spacing, though it has reliable lake-effect evening winds, so it would likely only have toys and prototypes.)
My impression is that high TSR HWAT blades produce a lot of noise at high windspeeds, especially if they start stalling out or overspeeding on their way to feathering in high winds. (It was the recent comparison to "sounding like a helicopter" in such situations that got me thinking about it.)
For HWATs I expect the noise mechanisms are:
- Vortex peeling from the whole blade during stall.
- Vortex peeling from blade tips.
- Vortex peeling from blade trailing edges if not properly streamlined.
- Low-frequency "thrumming" from the variation of the decelleration of the wind by the blades as they turn.
With VWATs I get the impression that a Savonius might be a lot quieter due to the low TSR, and that a Darrius might sound more like a HWAT since they have similar tip speeds (though the "egg-whisk" designs don't have the tip vortices).
But not yet having actually MADE and HOISTED any of these devices, and not having easy access to any active sites, I don't have a subjective feel for the noise levels involved.
I realize most of you folks probably haven't used noise meters on your mills, so your results won't be directly comparable. But a lot of you have built and hoisted a lot of mills, visited more of 'em. And I bet there are experts out there, too. I'd like to tap this knowlege base.
So how about it? What are your impressions of the noise of various designs and implementations of them? Especially of the various homebrew-blade HWAT designs that have been discussed here. And has anybody who'se built a Sandi Savonius got an impression of that?
