Amplitude Modulation (AM) describes periodic fluctuations in the level of wind turbine noise which is related to the rotational speed of the turbine blades. This characteristic is often observed by a listener as a ‘swishing’ noise and is a normal characteristic of modern wind turbine operation.
However, in some cases periods of increased ‘swishing’ or ‘thumping‘ are reported which can lead to nuisance or disturbance, often described as excessive or enhanced AM.
In 2016 the Institute of Acoustic’s Amplitude Modulation Working Group (IOA AMWG) established a method by which to measure and quantify the magnitude of AM. The ‘Reference Method’ reliably identifies the presence of AM within a sample of data and derives a metric by which to best describe the degree of AM present.
There is currently no Government supported basis by which to define the threshold at which AM becomes excessive or enhanced. However, a penalty scheme originally proposed by RenewableUK in 2013 and a subsequent proposed planning condition published in the IOA Bulletin Nov/Dec 2017 presents a provisionally agreed method by which penalties are applied to varying levels of AM magnitude.
Blue Sky Acoustic’s technical team use specialist in-house software to perform automated AM analysis in accordance with the IOA AMWG’s Reference Method. Following analysis of data samples, the magnitude of AM across a range of operational wind speeds is assessed based on the proposed planning condition detailed in the IOA Bulletin Article Nov/Dec 2017.