In addition to the overall noise level, periodic variations in the loudness of wind turbine noise, known as amplitude modulation (AM), also significantly contribute to the annoyance experienced by residents living near wind farms. Due to the high dependence of AM on meteorological conditions and the type of wind turbines, the level and duration of AM are hard to predict. These characteristics have an important impact on the annoyance response of residents. The level of annoyance is expected to depend on the AM depth, the number of AM occurrences and the AM continuity. The aim of this paper is to investigate AM characteristics in the vicinity of two wind farms in South Australia. It has been found that to successfully quantify tonal AM based on the "Reference Method" proposed by the UK Institute of Acoustics, removing the A-weighting, changing the range of band-pass filter frequency and reducing the prominence ratio are also necessary. AM density at night-time is much higher than at day time (25% versus 15%). However, there is not significant difference between AM depth at night-time and day time. Furthermore, AM is more likely to occur when the wind turbines are operating significantly below their maximum rated power.
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