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White Noise Market

http://recordingsofnature.wordpress.com/2012/04/30/vind-turbine-sound-field-recording/

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 meter high wind turbine

 



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.

http://recordingsofnature.files.wordpress.com/2012/04/file0895_hi_lo_17dbeq_16b.wav

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.

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 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…