Observations placeholder

Infrasound  - The Hidden Annoyance of Industrial Wind Turbines  - PROFESSOR CLAUDE RENARD; NAVAL COLLEGE & MILITARY SCHOOL OF THE FLEET (FRANCE)

This article is an updated summary of a lecture given by the author in 1997, entitled "Infrasound: Quiet, Pernicious Pollution." At that time, it was given in response to concerns arising from 

• the marketing in Sweden of a non-lethal infrasound weapon designed for riot control
•  the recognition of “Sick Building Syndrome” (SBS) caused by infrasound emitted by air conditioning systems
•  and the increase in the number of wind turbine installations in Brittany.

The rural areas of this region have a high population density, and the nuisances caused by infrasound would be as great or even greater than that of the visual pollution or radio interference preventing television reception!

In the weeks that followed, several points of information came to light, revealing that, in the first Airbus 340 planes, the setting of the pressurisation was such that it caused infrasound that affected the passengers. It was also disclosed that a "Euralille" high-rise block in Lille (France) had been evacuated due to vibrations on the 5th floor. Reports revealed that 644 agents of the new "Archet" hospital in Nice (France) had suffered from nausea and headaches. Some had even had to be admitted to the hospital. In 2005, there were accounts of similar health problems at the "Nord" hospital in Marseille.

This article has now been published in response to some good news: The (French) Académie de Médecine has recommended to the (French) government that the construction of wind turbines exceeding 2.5 MW at less than 1500 m from dwellings should from now be suspended.

This is good news, but not very good news. The writer is concerned that this venerable institution has only taken into account the ”annoyance” caused by audible noise (hissing of the blades, the noise from the gearings in the multiplier), and not the annoyance caused by infrasound. In view of this omission, the aim of this article is to inform the public about these inaudible but harmful noises.

..............Some infrasound is produced by turbulent flow of air on obstacles. This causes the formation of Von Karman swirling paths (called Von Karman vortices), which are made up of a series of eddies swirling alternately in one direction and then the other.

They emit both audible and inaudible sound, which is either jet sounds for which the frequency N (in Hz) is given by the Krüger and Marsherer formula: N = (0.055), V/E, where V is the wind speed (in m/s) and E the distance (in m) between the two obstacles limiting the jet, or trail sounds on an obstacle having a thickness or diameter E, for which the emission frequency is given by the Strouhal and Krüger formula: N = (0.2), V/E.

In the latter case, the eddies are alternately emitted by one edge and then the other of this long obstacle. Depending on the speed of the wind, these phenomena can become audible and cause the whistling emitted by windows that are badly closed, or by electric wires or cables.

The infrasound produced by wind turbines (the tower and the rotor blades) falls under this category. M.L. Legerton's team (Inter-Noise 96) showed that, at 100 m from a wind turbine, the infrasound had peaks of 1.4 Pa emitted every 0.65 sec. as the rotor blades passed the wind turbine tower.