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July 28, 2010 John M. Newman Missoula Office of Planning and Grants 435 Ryman Street Missoula MT 59802 Mr. Newman: Once again, I appreciate being able to provide some comments regarding the Wind Energy Conversion Systems (“WECS”) revision to the Missoula City Zoning Ordinance. This is in addition to my July 2, 2010 letter that outlined several concerns with the original proposed WECS ordinance. In this prior letter, I did not comment on Paragraph D.3 which endeavors to address potential concerns about vibrations produced by WECS. The term “vibration” refers to energy propagating through solids while “sound” is energy propagating through fluids (gases or liquids). Vibration is measured by instruments known as accelerometers that measure the acceleration of the surface of interest, usually in units of meters per second per second (m/s2). A WECS may experience vibrations due to unbalanced or misaligned blades, loose bolts (on footings, tower, flanges), wind turbulence at system resonant frequencies, or by component damage. Such vibrations are an important consideration to the mechanical and operational integrity of the WECS but such vibrations would rapidly and completely attenuate within a few feet of the structure and would become imperceptible to humans. Perhaps the rationale for introducing Paragraph D.3 into the ordinance was in response to old concerns that developed around a 1977 NASA experimental turbine in Boone, NC. The turbine was a “downwind” machine the generated low frequency pulses whenever the blades cycled through the wind shadow formed by the support tower. The pulses were inaudible but they induced resonance in structures that caused windows and other objects to vibrate audibly. Modern turbine designs have eliminated this effect. For the above reasons, I would therefore recommend removing Paragraph D.3 as it is not a valid concern. Sincerely, Brian Kerns Alternative Energy Technologies The University of Montana 1121 East Broadway Street, Suite 110 Missoula MT 59802 (406) 532-3228