> Something subtle is going on. Sometimes I can hear it. Sometimes I can't,
> Sometimes a microphone 'hears' it when my own ears can't.
Norman,
Turbulence and the "wake effect" is generally poorly understood.
> Even when I'm a further 3 miles downstream in my favorite patch of woodland,
> where I check out mics and recorders, I sometimes record a very prominent low
> frequency 'lift' in whichever channel is facing the direction of the
> turbines. If my ears were younger and sharper I might be able to define what
> I mean by 'lift' better. It's not quite a 'target' sound. It's more like
> there's a problem with my equipment.
The wake from a wind turbine travels for miles. The turbulence spirals off
from the turbine tips rather like smoke rings without the smoke. This is
minimised with aircraft wingtips which have been modified for a few years
now to increase efficiency but turbines don't have to worry much about
efficiency. Quite a lot of energy is involved in generating turbulence.
However, this turbulence is stable and is carried a few miles downstream
unless it meets a fixed object, when the energy is dispersed as noise.
The LF lift is probably low frequency noise generated when the turbulence
meets the trees or the ground. Do you have a recording of it?
> I'll be interested to keep an eye on what smarter audio scientist than me
> can come up with to explain such an odd thing.
There is very little published about turbine wake noise, especially
secondary wake noise generated at a distance from the turbines. You can find
all sorts of accounts of turbine noise which assume the noise comes from the
turbines themselves, but "induced noise", "secondary noise" or "modulation
noise" generated by wake turbulence meeting an object some distance from the
turbines is rarely discussed.
A wind turbine is collecting something like 2 to 4 MegaWatts of energy and
even the small proportion of this which is converted into turbulence is
significant. This energy can travel for miles, only slowly dispersing unless
it hits trees or other objects when the turbulence energy is released as
noise. This noise is generally described as "whooshing" or whistling, but it
also has low frequency components which can be picked up by a microphone.
Exactly the same thing happens when a steady wind blows through trees or
around buildings, but with a wind turbine, the turbulence occurs at the rate
of about once per second producing pulses of noise. A hollow structure like
a chimney can resonate at a low frequency which is hard to pin down with
your ears, but a directional mic helps.
The Danish Wind Industries Association has a load of technical info but this
is mathematical and concentrates on turbines interfering with each other:
http://www.windpower.org/en/search.html?q=wake+effect
in particular:
http://www.windpower.org/download/1573/01_Nygaard_-_DEWaM_%28DONG_Energy_Wak
e_Model%29.pdf
David Brinicombe
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