Q1. You mention the NASA wind turbine research of the 1980s. Is that relevant to the type of wind turbines used today?
Research into very large (multi-megawatt) wind turbines began at NASA in 1975. Much of this work was undertaken by very competent aero-acousticians, drawing on experience gained in the context of propeller and jet-engine development, and which has successfully resulted in substantial improvements in aero-engine noise. They identified at an early stage why the existing “downwind rotor” turbines were so noisy, and in 1979 commenced theoretical and practical evaluation of the first very large “upwind-rotor” turbine, the 2.5 MW “MOD-2”. In this context, in 1981 they confirmed the predicted reduced noise characteristics, while also investigating the adverse power generation and noise effects associated with close spacings between wind-turbines. They subsequently identified additional circumstances under which the low-frequency and infrasound generation of such upwind-rotor turbines could be compromised, and performed important studies on the human perception of low-frequency noise and infrasound. The latter investigations initially concentrated on the noise characteristics of the earlier downwind-rotor turbines, but the underlying physics governing hearing perception relate also to the upwind-rotor configuration.
Over the intervening 25-35 years, the basic physics of aerodynamic noise generation has not changed, the adverse effects of unduly close-spaced wind-turbine interaction remain the same, and the characteristics of human hearing have not changed. These aspects all continue to have immediate relevance to modern wind-turbine installations, yet this research has often been dismissed as old-fashioned and irrelevant by the wind-development community.
Q2. How do wind turbines produce infrasound and is this hazardous to humans if they cannot hear it?
The infrasound is generated by the aerodynamic lift forces on the blades, which are necessary to provide the driving torque to rotate the blades and generate electrical power. Newton’s law requires that there are corresponding forces of reaction on the air passing over the blades. Although these forces may be comparatively “steady”, the constantly changing position of the blades means that the resultant force pattern acting on the surrounding air is also changing so that, inevitably, infrasound is generated. Additional factors, such as the difference between blades encountering slow moving air at the bottom of their cycle, and faster moving air higher up, causes further changes to the blade-lift and force pattern, which can result in higher intensity and more impulsive infrasound.
It has been considered that the levels of infrasound generated by wind turbines are too low to cause adverse health effects, but such opinions have often relied on a mistaken interpretation of the “threshold of hearing”. There have been numerous instances over the last 40 years where people have reported adverse effects at sound pressure levels which are too low for people to “hear” it, while recent research is starting to identify possible mechanisms by which this process may take place.
Q3. There seems to be a somewhat semantic argument regarding “sensitization” and “annoyance” related to wind turbine exposure. Can you explain what “sensitization” is?
This question is perhaps most easily answered from my own experience. From 1979 to 1981, I worked directly on the low-frequency noise and infrasound from a ground-based gas-turbine compressor installation in a rural area, which was causing complaints and sleep-disturbance for residents up to 1 mile away. When I initially started working at the site, I did not consider the noise to be excessive and in conditions of a brisk breeze it was barely perceptible even at 100 yards. After two years, however, I found that if I drove up to the site having just traveled on a noisy motorway,while still in the motor car with the engine running I could “sense” directly whenever the gas-turbines were operating. This was not an immediately audible effect, nor was it in any way associated with “annoyance”, but this “sensing” of whether or not the turbines were operating nevertheless prove to be unerringly accurate. This was a process of enhanced perception which had developed during my time spent on site, and which became increasingly apparent as time progressed.
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