Detection and Evaluation of Amplitude Modulation From a Wind Energy Development, Ireland

Abstract

Amplitude modulation (AM) is a characteristic of wind turbine noise that has only been recognised as an issue in recent years. It is a characteristic related to aerodynamic noise and descriptions of it include "swishing", "whooshing" or less frequently a "thumping" sound. Due to increased awareness among exposed communities AM presents a potentially serious obstacle to future wind farm developments. This work reports on the application of a recently developed calculation method for AM in a practical setting. Correlations will be drawn with subjective reports of AM by nearby residents keeping a noise diary. The suitability of the method and its ability to quantitatively confirm subjective reports of AM will be assessed. A study is presented here whereby subjectively recorded occurrences of AM by residents living near a wind energy development in Ireland are correlated to calculated levels over a twenty day period. In order to detect and calculate AM a method published by the Amplitude Modulation Working Group of the Institute of Acoustics, referred to as the Reference Method, is applied. A subjective assessment of the sound recordings to confirm the presence of AM is discussed, including estimating the expected frequency range that AM occurs. The results of the Reference Method calculation are presented for periods with and without a subjective report of AM by the residents. Consideration is given to the criteria and thresholds for valid AM ratings within the Reference Method especially where intermittent periods of AM are identified. The Reference Method is shown to be highly suitable as a quantitative measure of AM which correlates well with subjective reports. Caution must be taken when using the method as valid periods of AM may be overlooked due to the rigorous detection thresholds set by the method.