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<br />and the number of blades. It is refracted in the same manner as <br />higher-pitched sounds and will therefore have limited range <br />except under extraordinary weather conditions. The natural wind <br />also generates infrasound especially at places like ridgelines <br />where the terrain causes it to be exceptionally turbulent. Such <br />infrasound is of widespread prevalence in some locations downwind <br />from windy mountain ridges, such as Boulder, Colorado, and is not <br />known to have any environmental impact. Special devices are <br />required to detect it. Infrasound from windfarms is therefore <br />expected to have no environmental significance. <br /> <br />Finally, it should be noted that the intensity of impact in are~as <br />adjacent to windfarms will be proportional to the density of <br />population in those areas. Because windfarm sites are <br />characteristically barren and windswept, they are not preferred <br />sites for habitations, and population will be extremely sparse. <br /> <br />d. Television and Radio. - A study of broadcase interference by <br />the Radiation Laboratory of the University of Michigan (Senior, <br />et al., 1977) found limited interference with the video channel <br />of television reception that was worst on the highest frequencies; <br />that is to say, the upper UHF channels. No interference with FM . <br />reception or with the audio channel of television signals was <br />observed. <br /> <br />The television interference arose from specular reflection from <br />the metal wind-machine blades acting singly. If the television <br />station were imagined as the sun and the wind-machine blades as <br />polished reflectors, the interference would correspond to flashes <br />of reflected sunlight seen at the receiving antenna. For any <br />given azimuthal position of the wind machine and pitch setting of <br />the blades, the interference would affect a given rather narrow <br />sector of directions from the wind machine. The interference <br />pattern resulting from all pOSSible azimuths and blade pitch <br />settings is a cardioid pointing toward the television transmitter <br />plus a narrow lobe pointing away from it. It was calculated that <br />in the worst case interference might extend a few miles from the <br />wind machine. It was considered that interference would be <br />objectionable where the momentarily reflected signal was a tenth <br />as strong as the directly received signal. <br /> <br />The calculations were made on the assumption that the wind-machine <br />blades would be metal. The use of nonmetallic materials for the <br />blades or restricted use of metal in a nonspecular mode, would <br />greatly reduce the potential for television interference. <br /> <br />Several mitigating measures are possible. The transmitting antenna <br />may be sited at a distance from the windfarm, it may be raised high <br /> <br />10 <br />