Demystifying the sound of wind farms

windfarm149

Debates about wind farms frequently revolve around the issue of sound. Yet this debate often occurs in a vacuum, seemingly forgetting the fact that to be alive is to be immersed in sound. 

From the moment we wake we are exposed to a wide range of sounds–from birds letting us know the day has begun; to the trains, trams and trucks moving around metropolitan Melbourne; and to agricultural equipment chugging away in regional areas.

Wind turbines DO produce sound. That isn’t in dispute. Where the contestation lies is whether the sound is annoying. As someone who has been in close proximity turbines at several wind farms I can attest to the sound being unremarkable (surprisingly quiet, in fact).

In the following post, Danish blogger Kristian Larsen demystifies the sound produced by an operating wind turbine. It was originally posted at his blog Recordings of Nature:

This post describes a field sound recording of a wind turbine, 29-4-2012. There are MANY in Denmark, but only few in the Copenhagen region for some reason. The noise, especially the very deep and far reaching rumbling sound is currently heavily debated. It appears to be a challenge to really get hold of how those wind turbines actually sound and the precise characteristics will always depend on types and sizes of wind turbines. It is a bit complicated.

Using my usual dummy head recording setup, with QTC40′s, pressure equalizers and an extra layer of wind protection, I have tried to make a clean and realistic recording of a wind turbine.

850kW - 80 meters wind turnbine
Listen to a short sound sample of the wind turbine noise at low and higher wind speeds – waw 4Mb

This short clip summarizes the wind turbine noise at low and higher wind speeds. Here in uncompressed wav format to preserve the full original quality and full as-recorded low frequency contents.

The recordings were made at a local wind turbine, a relatively new 80 meters high, 850 kW, Vestas V52 turbine. It was an early Sunday morning, 8 am, to minimize the noise from a nearby motorway. It felt  kind of windy, estimated 5-6 m/s, but  fluctuating. The sound from the wind turbine varied a lot according to the wind speed and the position/distance. Directly underneath it was surprisingly silent. But at a distance of 50 meters in front of the tower there was a spot where the noise appeared most clear and direct, so this was chosen for the  recordings.

Download  mp3- 11 minutes of vind turbine noise.

The video above is a 10 minutes take. Notice how  the fluctuating wind alters the rotation speed and the sound level. In the video and mp3 version, a bass roll off has been applied to reduce the deep rumble and to make the sound more “realistic” in headphones.

Looking more into the technical details, the sound appears to be composed of a very deep rumble, some semi-deep gear sounds, and higher pitch turbulent noises from the fast rotor blades. As the specs of the Earthworks, QTC40′s microphones says +/- 1 dB from 4Hz to 40 kHz, I was hoping to pickup the full audio spectrum from the turbine, maybe even deep infra-sounds. A major challenge here, was the wind which has a habit of generating plenty of similar deep rumbling. To reduce this problem, I was applied a double layer of nylon stocking material as wind protection and often manually shielding the setup with my jacket. I wonder how the official noise measurements are carried out overcoming the task of recording very low frequencies at high wind speeds.

Wind turbine noise spectral view. First part represents low winds (~4m/s) and last part at higher winds (~7m/s)

Taking a closer look at the results, the figures up and below give a comparison of the frequency spectrum, when comparing the sequences with lowest and highest wind speed, estimated 4 and 7 m/s. This gives an unscientific indication of the direct contribution from the wind turbine to the overall noise in competition to the motorway and other wind noises. Apparently, increasing the wind speed gives an amplification of about 6-10dB right evenly over the range 10-10000 Hz. The low-frequency content is substantial. Of course, this is would hardly be visible in conventional A-weighted noise estimates.

wind turbine noise spectrum low and high winds
Wind turbine noise spectra. Green is low winds (~4m/s) and red high winds (~7m/s)

Speaking about the very low-frequency or subsonic noise, as the word says it can hardly be heard.  To be able to reach those frequencies the sample below has been pitched up by a factor of 4.  That means 10 Hz transforms to 40 Hz, and 20 Hz -> 80 Hz, etc. Dont know how useful this technique is, but it sure add a strong effect to the recordings…

ORIGINAL  high/ low winds  – 4 MB wav file

4x PITCHED UP  – 1 MB wav file

More images from the recording setup:

Recording setup appx 50m from the wind turbine
Recording setup appx 50m from the wind turbine
Dummy head setup with double layer wind protection 50 meters from wind turbine.
The wind turbine on a distance

So, what is your experience with wind turbine noise? Outdoor or indoor. Can you recommend other audio recordings illustrating this? You are welcome use the comment section below.

2 thoughts on “Demystifying the sound of wind farms

  1. Listening to turbine sound thru computer speakers I can still hear local coal mines 10 – 15 kms away The comparison is FELT strongly: the coal mine noise, emitted (along with other pollution) from destruction, is a negative influence and I feel sorrow, anger and despair. The Wind turbines sound ‘nice’ in the knowledge of their renewableness and give a positive feelings of hope and even joy. There are variations that the mines don’t have (except when they’re blasting) and so it’s like music. I’m happy.

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