Amplitude Modulation Noise from Wind Turbines
In 2015 the Department of Energy and Climate Change (DECC) teamed up with a group of industry professionals and the UK Institute of Acoustics (IoA) to produce a comprehensive review of Amplitude Modulation (AM) noise emitted by wind turbines. AM noise from wind turbines adds an additional sound characteristic to the normal wind turbine noise and is known to cause increased annoyance to nearby communities and receptors. Currently there is no legislation in place to enforce AM considerations on the onshore wind sector, however there is expected to be a revision of the current standards to account for AM.
Amplitude modulation is the term given to a fluctuation in the amplitude of a signal, often referred to as the signal strength or loudness when in the context of noise. AM does have common uses such as in the AM radio when the amplitude of the wave carries music or human speech to the end receiver. An example of what an AM signal looks like can be seen in the figure at the bottom of this text.
The amplitude modulation working group established by the IoA defines AM from wind turbine noise as “periodic fluctuations in the level of audible noise from a wind turbine (or wind turbines), the frequency of the fluctuations being related to the blade passing frequency of the turbine rotor(s).”
Measuring the AM component of a signal can be difficult as the modulation frequency can be relatively small – in the region of 1 Hz. In order to capture the AM component in a sound pressure signal, the acoustic measurement has to be recorded at a suitably high recording resolution. The IoA working group recommends a signal resolution of 10 Hz is used for the measurement and that each sample should have the LAeq and octave band spectrum recorded. Equipment with a high resolution and sufficient recording capacity is therefore necessary to accurately capture the relevant data.
After the data has been measured, data processing and analysis are required to extract the relevant information and evaluate the AM component of the signal. The data processing involves de-trending the time series data and converting the data into the frequency domain to identify the fundamental AM frequency. Once this has been completed, the time-series data is then reconstructed with only the AM component of the signal, which can then be compared to the original signal to determine the strength or depth of AM.
The team at Xi have been working on wind turbines since 2005 and have the required experience, measurement equipment and post-processing capability to measure and quantify the AM component of a wind turbine.