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Here is the link to the research supposedly claiming that wind turbines can be heard 3.5 kilometres away inside a house 16% to 22% of the time. The article further claims on occasions audible noise can be heard nearly nine kilometres away.

https://news.flinders.edu.au/blog/2019/06/19/wind-farm-noise-recorded-almost-9km-away/

The raw mathematics tells me that if a turbine is 46db at 300 metres (according to the people who make them - GE). Then it will be 25db at 3500 meters using an inverse square law and a log10.

25db is audible in theory, but then muffle that inside a house and include 30-35db of background noise, then consider the researcher claims to be talking about 'AM' low frequency noises detected. Would that be audible by any scientific methodology?

A response to the researcher where I posited that a lawn-mower would not be heard 3.5km away got this response:

  1. A lawnmower is operated at ground level and thus there will be many obstacles that block the noise as it travels away from the source.
  2. Wind turbines in Australia are often located on a ridgetop, whereas surrounding houses are located on the plains or in a valley. Also, the hub height of a wind turbine is at least 80 m or higher above the ground for modern industrial wind turbines. This means that the wind may be strong enough for wind turbines to operate at high power capacities whilst the wind at the height of the houses may be negligible. As a result, the wind farm noise can be clearly heard due to the large contrast between the wind farm noise and the natural background noise.
  3. The noise from wind turbines can travel further as the noise source is higher.
  4. Wind turbines are much larger than a lawnmower and the large surfaces of the blades and tower can radiate sound.

There is something unsatisfactory about this answer, and how I would put it is, that the answer makes use of the more ideal situation a wind turbine has to project sound by being in direct sighting, but does not negate the inverse square law, which seems to be the biggest issue.

In fact, point 4 I believe is misleading, as at 3.5km a wind turbine's shape becomes quite irrelevant as the angle its blades subtends is so small.

Point 3 seems to be an assertion that requires more explaining why.

So the question is, does the researcher's claims have any validity?

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  • $\begingroup$ physics.stackexchange.com/questions/87751/… Adding this link, as it does provide some information about attenuation and human sensitivity to different frequency ranges. It would seem that any frequency below 300hz is absorbed over 3.5km, so it comes down to whether these low frequencies are still audible. $\endgroup$ – Gordon Rouse Dec 19 '19 at 20:29
  • $\begingroup$ oops - I mean "above 300hz" $\endgroup$ – Gordon Rouse Dec 19 '19 at 20:47
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Outdoor sound propagation is a really complicated process. Temperature inversions can lead to "ducting", where sound becomes trapped at a single altitude and propagate for long distances with cylindrical spreading as opposed to the conventional spherical spreading. Other times the meteorological conditions can lead to sound focusing, becoming abnormally loud in a certain region. You can also get what are called "shadow zones", where you get no sound when you should be getting plenty of energy. Moreover, all of these phenomena vary with time and space, making it nearly impossible to accurately predict sound levels from any given source with any decent level of precision. Many of the best models are statistical in nature (see, e.g., https://acousticstoday.org/sound-propagation-in-the-atmospheric-boundary-layer-d-keith-wilson-chris-l-pettit-and-vladimir-e-ostashev/).

I am not claiming that the researcher you contacted was right or wrong. I am just saying that back-of-the-napkin calculations such as the inverse square law can be wildly wrong when considering long propagation distances in complicated environments.

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  • $\begingroup$ Can you then comment on the specific condition where the wind turbine is in a layer of wind sitting above the level of a temperature inversion? My sketching of this situation suggests that the sound is more trapped in the warmer upper layer, and the houses in the cold valley (where there is no wind to obscure the turbine noise) are actually getting less sound energy? $\endgroup$ – Gordon Rouse Dec 21 '19 at 0:58
  • $\begingroup$ Sound tends to bend in the direction of lower sound speeds, which are associated with colder air. Thus, without going into details and ignoring the wind for the moment, the sound from the turbine that would likely get trapped in the cooler air below. Thus, I would expect more energy to arrive at the houses in the cold valley. The effect of wind is complicated, but I think you can probably just think of it as making the system having greater variance in general. If you are looking at peak incidents, I would think the wind would just make exceptional cases worse. $\endgroup$ – Michael M Dec 23 '19 at 15:09
  • $\begingroup$ Yes, I am corrected - I need to look at Snells law and the geometry closer to see what I get in the simple case of a discontinuity. The energy density is related to the deviation of the rays in such a model.(not the differential angle). $\endgroup$ – Gordon Rouse Dec 24 '19 at 21:55

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