This report summarizes extensive research by staff of the Solar Energy Research Institute and its subcontractors conducted to establish the or1g1n and possible amelioration of acoustic disturbances associated with the operation of the DOE/NASA MOD-1 wind turbine installed near Boone, North Carolina.Results have shown that the most probable source of this acoustic annoyance was the transient, 'unsteady aerodynamic lift imparted to the turbine blades as they passed through the lee wakes of the large, cylindrical tower supports.Nearby residents were annoyed by the low-frequency, acoustic impulses propagated into the structures in which the complainants lived.The situation was aggravated further by a complex sound propagation process controlled by terrain and atmospheric focusing.Several techniques for reducing the abrupt, unsteady blade load transients were researched and are discussed.

The detailed analysis of a series of acoustic measurements taken near several large wind turbines (100 kWand above) IntroductionUntil the fall of 1979, noise from large wind turbines had not been a major concern. The situation changed however as the 2 MW, MOD-1 turbine installed near Boone, North Carolina began to undergo a series of operational tests which resulted in a number of sporadic and totally unexpected noise complaints from a few residents living within 3 km of the installation. Since that time, a considerable effort has been undertaken by a number of organizations who have studied the phenomena to find out the exact nature of the noise responsible for annoying the neighbors, its origin and production mechanism, its propagation path, and what could be done to eliminate or at least reduce it to below perceptible levels. Some of the results of these studies have been reported previously [1].To date, acoustically-related annoyance from large wind turbines has been confined to a dozen families living within 3 km of the MOD-1. There have been no documented complaints of noise the author is aware of with any of the four MOD-OA turbines currently operating, and two surveys of the MOD-2 turbine have failed to find a tendency for impulsive noise generation similar to the MOD-1 in the measurements taken so far [3,4]. Some impulsive noise has been detected in a recent survey of the 17-m Darrieus/VAWT [5]. The situation in Boone, however, has been severe enough to warrant a close examination of the details of the MOD-1 experience. The causal factors responsible for the noise had to be identified; this information would then be used to develop a methodology to assess the annoyance potential of other wind turbine designs by measuring their acoustic radiation with reference to the MOD-1 data. Figure 1 summarizes the acoustic pressure spectrum associated with large wind turbines and indicates the dominate noise sources as a function of frequency. Not all wind turbines will exhibit the features of the spectrum shown. The ultimate cause of aerodynamically generated sound is the unsteady loading of the blades. The degree of this unsteadiness, for the most part, is responsible for the distribution of acoustic energy across the spectrum of Fig. 1. Characteristics of Large Wind Turbine NoiseConventional classifications of rotor noise include rotational, broadband or vortex, and impulse noise. Rotational noise is characterized by the large number of discrete frequency bands which are harmonically related to the blade passage frequency. The amplitude of these bands is determined by the sum of the steady load, which is a function of the commanded level of operation of the machine, and the unsteady loading at any moment arising from such sources as inflow turbulence and upstream wakes. Broadband or vortex noise results from the slightly viscous interaction of the unsteady lift and the blade boundary layer and is responsible for such mechanisms as flow separation and tip-and trailing' edge vortex shedding. Broadband noise, wh...

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