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.
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:
- 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.
- 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.
- The noise from wind turbines can travel further as the noise source is higher.
- 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?