Indoor simulation of amplitude modulated wind turbine noise

Abstract

Wind energy is the world's fastest-growing renewable energy source; as a result, the number of people exposed to wind farm noise is increasing. Because of its broadband amplitude-modulated characteristic, wind turbine noise (WTN) is more annoying than noise produced by other common community/industrial sources. As higher frequencies are attenuated by air absorption and building transmission, the noise from modern large wind farms is mainly below 1000 and 500 Hz for outdoor and indoor conditions, respectively. Many WTN complaints relate to indoor, nighttime conditions when background noise levels are lower. As recently reported, indoor noise has the potential to cause sleeping disorders. Studies on human response to amplitude modulated WTN have been mainly focused on the outdoors, where a large amount of measured data exists. This is not the case for indoors, where it is much harder to gather data. Hence, there is a need to understand the transmission of WTN into dwellings and to develop indoor annoyance metrics. In this article, we investigate the transmission of WTN into residential-type structures. Using an outdoor WTN recording and structures with different properties/ configurations, we made a series of computer simulations for indoor noise predictions and assessed the results employing several widely used metrics for WTN, for example, spectral content, modulation depth and overall levels. In general, the indoor noise levels are higher, and the average modulation depth is similar to those of outdoor recordings. In addition, there is a significant change in the spectral shape. These results could potentially explain indoor WTN annoyance. 507predicted from propagation models based on ray tracing that simulates the ground and air effects. 7 In addition, the effect of resonant conditions in modern large-scale wind turbines has recently been studied by Tibaldi et al., 8 who reported that aeroelastic modes might be excited, depending upon the wind turbine operational external excitations.Broadband aerodynamic blade noise is responsible for the amplitude modulation (AM) observed mainly in large turbines, that is, a pulsating broadband sound. 2,3,9 This AM, commonly referred to as 'swishing', 'whooshing' or 'pulsating' noise, is currently considered to be the main cause of annoyance for residents living near wind farms. 10,11 Recently, infrasound from wind turbines has also received significant attention. 11,12 However, the generation of this noise component is not well understood and cannot easily be quantified, since a limited number of conclusive measurements are available.Turbines are not typically as loud as other sources of industrial or transportation noise at a similar distance. The AM characteristic is what makes WTN more annoying than other types of noises at the same sound pressure level (SPL). Very quiet rural communities that lack the masking of nighttime traffic noise are particularly affected. AM noise is perceived as more annoying than a constant amplitude sound containing the same spectrum and averag...

Development of an Advanced Fluid-Structure-Acoustics Framework for Predicting and Controlling the Noise Emission from a Wind Turbine under Wind Shear and Yaw

“…It should be noted that the low-frequency noise is important in far-field as it can propagate in a long distance with very small air absorption. To check the AM noise, the modulation depth for Case 4 is calculated according to the method proposed in [7] and shown in Table 3. From the table, it is seen that the modulation depth is slightly reduced.…”

“…There are two types of AM noise: the one is created by changing the blade noise directivity to receiver [4] and the other is caused by non-uniform inflow and operation, for example the operation in wind shear and yaw. There are three methods (Japanese F-S method [5], UK method [6] and min-max method [7]) to characterize the modulation depth. Recently, Barlas et al [8] studied the amplitude modulation caused by a wind turbine wake using an advanced Computational Fluid Dynamics (CFD) (EllipSys3D/AL) and a sound propagation model based on solving parabolic wave equation and found that the modulation depth can reach 4-5 dB even with the same strength of noise source.…”