Wind turbine swoosh and thump are quiet and rare respectively

Wind turbines make a little extra noise when the blades pass the tower. Technically, it’s called amplitude modulation.

What does amplitude modulation mean?

Well, it’s just one of the two ways in which wave forms change. A standard wave is regular, with peaks the same distance apart (frequency) and the same height (amplitude). If you change the height of the peak, that is modulating the amplitude. And if you change the distance between the peaks, that’s frequency modulation. That’s all that AM and FM on radios mean at heart, that they are changing the amplitude or the frequency of the radio waves to encode information.

amfm

When amplitude modulation is applied to sound waves, it means that the noise intensity of the peak and trough change; they get louder or quieter.  The bigger the distance between peak and trough, the bigger the difference in sound.

Thinking about it for a second will tell you that wind turbine noise features amplitude modulation as a primary characteristic. The quiet swoosh of a blade passing the mast is a result of amplitude modulation. It’s a little louder at that point in its rotation. Typically, this is referred to in the literature as Normal Amplitude Modulation. It’s most obvious to people to the sides of the wind turbine rather than upwind or downwind.

There is another form of amplitude modulation that’s been relatively poorly explained. A small subset of wind farms occasionally experience a sound more like a thump than like a swoosh. Historically, records in the UK where this condition was most commonly complained about found that four of 133 wind farms occasionally experienced this condition, and that the noise itself wasn’t louder, it just had different characteristics that made it slightly more noticeable to some people. Literature refers to it as Other Amplitude Modulation or Enhanced Amplitude Modulation, which is only slightly less generic than amplitude modulation. It’s a bit lower in frequency and is typically only significant in impact downwind of the turbine, which is important has general turbine noise is greatest downwind. And not all downwind sites receive the noise in the same way, with significant variance even for the same site under superficially similar conditions.

It’s become such a recurrent feature in complaints, however, that further study was required. People put forward papers at conferences claiming it was a common feature. They advanced all sorts of theories. They claimed that it was much louder.

What does the research say?

Well, by far the most comprehensive study was published in December 2013 by RenewableUK. They funded Hoare Lea Acoustics, an independent and accredited organization of long standing with 27 acousticians across 13 offices, to assess the historical studies, assess the historical acoustics records, perform acoustical modelling, perform additional acoustic testing and provide an analysis and synthesis of the results. This was broken down into seven work packages covering explanations, research, measurement, dose-response relationships, acoustic records and the final report.

It’s worth quoting extensively from their summary:

  • the effect has only been reported on a limited number of wind farm installations;
  • even on those sites where the effect has been positively identified to occur, it is intermittent;
  • the effect may occur for just a few rotations of the blade, or it may persist for periods of several minutes or hours;
  • OAM is not a common feature of all wind farms and, even for those wind turbines and wind farms where its occurrence has been reported, it is an intermittent and atypical feature.

The team was able to determine what was causing the noise characteristics: loss of laminar flow of air over the blade, otherwise known as stalling. This was found to occur in a variety of conditions including wind shear at the top of the blade rotation causing differential wind speeds, and upwind obstructions causing significant turbulence.

Diagrammatic view of turbine blade stall, with delaminating eddies above the blade. This is understood to be the source of the ‘thump’ of Other Amplitude Modulation.

As is pointed out, this stalling of the blade at one point in the rotation, typically at the top, did not continually occur, but sometimes only lasted for a few rotations.

So, there’s a fairly solid understanding of the cause and some of the conditions under which it can occur. However, this amplitude modulation is usually hard to pick out from other noises with any degree of reliability because it just isn’t that loud. They say it would be possible, but just isn’t worth it because the amount of noise just isn’t big enough to be material.

Their assessment of annoyance amplitude modulation studies showed that it just isn’t that big a factor in people getting annoyed.

Outside of the field of wind energy, it’s quite possible that this effort would have been abandoned years ago.

Where does this leave us?

Other Amplitude Modulation occurs on perhaps 3% of wind farms, it occurs only occasionally, it usually only occurs for brief periods of time, is mostly only audible to people downwind of the wind turbines and studies show that most people don’t find it annoying.

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