The study was carried out to investigate the causes of, and solutions to, the occurrence of an acoustic characteristic known as “Other Amplitude Modulation” (OAM).
The report explains the differences between “Normal Amplitude Modulation” (NAM), which is the common swishing sound made by turbine blades as they pass through the air, and OAM, which is an infrequent and uncommon sound which typically lasts only for a few minutes.
OAM is caused by sudden variations in the direction and speed of the wind. These variations mean that the wind hits different parts of the turbine blade at different speeds, causing it to stall momentarily. This stalling action produces a “whooshing” sound.
OAM is not louder than NAM; it is relatively quiet, typically no louder than 35 to 40 decibels at a distance of about 1 kilometre, but it is deeper in pitch. OAM is comparable to the sound of traffic noise heard at around 1 km from a single carriage A-road.
Maf Smith, Renewable-UK’s Deputy Chief Executive, said: “As a result of the in-depth research we’ve commissioned, we’ve identified the causes of OAM, and, most importantly, the industry has identified a way to deal with it effectively.
“On the limited and infrequent occasions when OAM occurs, we can address it by using software to adjust the way turbines operate, changing the angle of the blades.
“Beyond that, the industry has worked with members of the UK’s leading acoustics institute to develop a planning condition for local authorities to use, which we’re publishing today alongside this work. This states that if OAM occurs, it’s up to the wind industry to resolve it.”
“We’re proud to have commissioned this ground-breaking research as it pushes the boundaries of our knowledge of wind turbine acoustics considerably further forward. It’s a tangible example of the wind industry acting in a responsible manner, demonstrating that we’re continuing to be good neighbours to the communities who host wind farms in the UK”.