This review is focussed on large-scale, horizontal-axis upwind turbines. Vertical-axis turbines are not considered here as they are not sufficiently efficient to be deployed in the commercial generation of electricity. Recent developments in horizontal-axis wind turbine noise research are summarised and topics that are pertinent to the problem, but are yet to be investigated, are explored and suggestions for future research are offered. The major portion of recent and current research on wind turbine noise generation, propagation and its effects on people and animals is being undertaken by groups in Europe, UK, USA, Japan, Australia and New Zealand. Considerable progress has been made in understanding wind turbine noise generation and propagation as well as the effect of wind farm noise on people, birds and animals. However, much remains to be done to answer many of the questions for which answers are still uncertain. In addition to community concerns about the effect of wind farm noise on people and how best to regulate wind farm noise and check installed wind farms for compliance, there is considerable interest from turbine manufacturers in developing quieter rotors, with the intention of allowing wind farm installations to be closer to populated areas. The purpose of this paper is to summarise recent and current wind farm noise research work and the research questions that remain to be addressed or are in the process of being addressed. Topics that are the subject of on-going research are discussed briefly and references to recent and current work are included
Scholarly research on wind turbine noise has been on-going since the early 1980s, with much of the early work undertaken by the United States National Aeronautics and Space Administration (NASA) on horizontal-axis wind turbines, with the rotor downwind of the support tower (“downwind turbine”). The location of the tower upwind of the rotor resulted in very turbulent flow being incident on the turbine blades which, in turn, resulted in the generation of thumping sounds as the blades passed close to a leg of the tower that generated the flow disturbance. The thumping noise disturbed nearby residents and caused cause rattling of dishes and annoyance for a number of residents living within 3 km of a single turbine . Some residents reported feeling the sound more than hearing it, which resulted in a sensation of uneasiness and personal disturbance.
In modern turbines, the rotor location has been changed to upwind of the support tower, as less thumping noise is generated in this configuration, resulting in this type of turbine being used in all modern wind farms that generate electricity for commercial use. Thus, this review is focussed on large-scale, horizontal-axis wind turbines.
Wind farm noise research can be divided into a number of distinct categories: turbine noise generation, turbine designs to minimise noise generation, noise propagation to surrounding communities, effects of noise on surrounding communities (including fauna) and regulation (including compliance checking).
Turbine noise research includes work on understanding noise generation mechanisms, control of these mechanisms to reduce overall noise levels, as well as calculation and rank ordering of the sound power output of various wind turbine noise sources. Research also includes work on quantifying problems such as tonality and amplitude modulation; measurement of turbine noise emission, such as directional characteristics; and quantifying the effect of topography and meteorological conditions on the sound power emission. Understanding of noise generation mechanisms is fundamental to the development of quieter blade and turbine designs that do not significantly reduce overall performance.
Noise propagation from turbines to surrounding communities includes work on the development of better propagation models that can provide more accurate predictions of noise levels at near and distant communities. Of particular interest is the calculation of worst case noise levels as well as the range of noise levels that will be experienced at any location as a function of weather conditions and time of day as well as the expected duration of particular levels over the longer term. Whenever predicted noise levels are provided by developers, it is important that they are accompanied with uncertainty estimates and this is an area of research requiring more effort. The more accurate prediction of noise propagation of off-shore wind farms to nearby on-shore communities is also of interest. As most disturbance caused by wind farms is at night after residents have retired to bed. For this reason, there is considerable interest in translating outdoor predicted noise levels to indoor predicted levels for various housing constructions with and without open windows. In addition to developing better noise prediction models, it is also important that measurements of environmental noise before and after a wind farm is constructed are undertaken and that ambient noise from other noise sources are properly taken into account when estimates of the contribution of wind farm noise to the overall noise level are made. This is an active area of research at the moment, with a number of procedures currently under investigation. As most noise measurements are undertaken with microphones exposed to a significant wind level, the development of measurement systems that are insensitive to wind noise is a research area that attracts a significant level of interest. As many community complaints are centred around low-frequency noise and the possibility of the presence of infrasound, it is particularly important that any measurements of ILFN (infrasound and low-frequency noise) are well isolated from the effects of wind disturbance.