Wind turbine noise, an independent assessment | Rand Acoustics
Wind turbines larger than one megawatt of rated power have become an unexpected surprise for many nearby residents by being much louder than expected. The sounds produced by blades, gearing, and generator are significantly louder and more noticeable as wind turbine size increases. Long blades create a distinctive aerodynamic sound as air shears off the trailing edge and tip. The sound character varies from a “whoosh” at low wind speeds to “a jet plane that never lands” at moderate and higher wind speeds. Blade-induced air vortices spinning off the tip may produce an audible “thump” as each blade sweeps past the mast. Thumping can become more pronounced at distance, described as “sneakers in a dryer,” when sounds from multiple turbines arrive at a listener’s position simultaneously.
Other professionals have developed thresholds, or criteria, for sound level to protect public health that may be applied to planning for wind turbine permitting. Recommendations from Hayes McKenzie Partnership in 2006 limited maximum wind turbine sound levels at residences to 38 dBA and no more than 33 dBA when “beating noises” are audible when the turbines spin.
Dan Driscoll presented his analysis in 2009 (Environmental Stakeholder Roundtable on Wind Power, June 16, 2009) with a Composite Noise Rating analysis of 33 dBA to reduce rural community response to the level of “sporadic complaints.”
Michael Nissenbaum issued his findings in 2010 from his medical study at Mars Hill, recommending a 7000-foot setback for public health. The World Health Organization published sound level thresholds of sleep disturbance and adverse health effects from peer-reviewed medical studies (Night Noise Guidelines for Europe, October 2009).
Our next column will compare our sound level versus distance data with these medical, health, and community response criteria and show what distances are necessary to protect public health.
Currently there is no effective, reliable noise mitigation for wind turbines of this size other than shutdown.
Therefore, at this time it appears appropriate that proposed wind turbine sites should position wind turbines at least one mile away from residential properties and further for sites with more than one wind turbine. Smaller wind turbines (under one megawatt power rating) produce less noise than those currently being marketed and installed for grid power in Maine; these may be an option when distance is an issue.