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Sound Characteristics of Multiple Wind Turbines

Dr Huub Bakker & Mr Bruce Rapley


This Paper illustrates the sound characteristics of multiple wind turbines and introduces the idea and of Heightened Noise Zones (HNZ) created when multiple wind turbines are in operation. The idea is presented to illustrate the complexity of sound from a wind farm. The sound character of a single turbine is presented in comparison.

Both the Manawatu and Makara wind farms are spread over a large land area within their respective locales. Analysis of the turbine layout in both locales shows wind turbines installed in straight lines and arcs. The potential effect of these formations at affected homes is to enhance sound emissions and propagation due to the additive effects of turbines operating approximately together. The effect is significant under adverse weather conditions (e.g. a south-east wind in the case of some homes in the Manawatu and north-west or southerly conditions at Makara) and not significant under different non-adverse weather conditions. (The effects of weather are discussed in another Paper.)

The effect of a single turbine is shown in Figure 1, where a single operating turbine is shown in a row of 5 turbines. The predicted sound level at the home 1240 metres distant is predicted as LAeq 30.9 dB(A). The measured sound level, under calm conditions at the home, was LAeq 30.1 dB(A) with the turbine in operation. (The assumptions are a sound power level of 116 dB(Lin) developed from a breeze of 9 m/s at the turbine, ISO 9613-2 propagation over flat terrain, an ambient temperature of 8 degrees Celsius and a relative humidity of 70% at the home.)

The effect of 5 turbines under similar conditions is illustrated in Figure 2. The sound levels are shown in sound level contours and a calculated level at the home (or noise-sensitive location).

These variations can be explained by the idea of Heightened Noise Zones.

Heightened Noise Zone (HNZ)

The Heightened Noise Zone (HNZ) is the combined effect of directional sound and vibrations (wave trains) from the towers, the phase between turbines’ blades and lensing in the air or ground. The interference between the noise (audible), or vibration, from different turbines creates very localised patches of heightened or lowered noise/vibration (Figures 6 and 7) in much the same way as ripples on a pond can combine to form large ripples (anti-node) or relatively still water (node)...


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