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<br />A typical maximum value of (V/rn) is 20/120 so the maximum value <br />of the second term is about 0.085. This amounts to about 0.3 dB <br />difference in noise level between cut-in and cut-out windspeeds and <br />can be neglected in comparison with other terms. However, as V <br />increases, the geometrical factor A may change, increasing or <br />decreasing according to the specifics of individual blade designs. <br /> <br />Thus for blade noise from the horzontal-axis machine we may <br />write <br /> <br />2 6 <br />W 0: r ( rn) . <br /> <br />Since (rn) has a typical limiting value, the noise power of the <br />blades as a whole increases with the square of their length, as <br />also does the power output of the machine. Therefore, the blade <br />noise power per kilowatt of machine capacity is invariant with <br />machine scale size but depends sensitively on blade tip speed. <br />The value of the geometrical factor remains to be determined <br />experimentally for each machine, pitch setting, and windspeed. <br />Thus, a. scale-up in power rating from the 100 kW of the Plum Brook <br />machine to 3000 kW, the largest currently projected, implies an <br />increase in blade noise of about 15 dB per machine. <br /> <br />To the blade noise one must add t.he noise power of the supporting <br />structure which, being of dipole origin, increases with the sixth <br />power of the windspeed. When the two components, blade noise clnd <br />tower noise, are combined, it is obvious that the blade noise <br />will be more prominent at the cut.-in windspeed. Since tower <br />noise is a familiar element from experience with radio towers, <br />etc., and is known to be no problem, we may conclude that the <br />largest projected wind machine operating at cut-in speed presents <br />the worst case with respect to noise prominence. The next <br />question is, how loud will this be in comparison with the back9round <br />noise level? <br /> <br />Which brings us to the subject of noise emitted by adjacent <br />sources. In the worst case, this means the noise generated by <br />the wind in passing over the ground near the hearer. This noise, <br />like that of the blade tip, varies with the sixth power of the <br />windspeed and depends on the type of ground cover. Under typical <br />conditions of attenuation, the noise generated near the hearer <br />dominates that coming from more distant parts of the ground <br />area. <br /> <br />Under these conditions, the machine noise will be most predominantly <br />louder than the background noise when the machine is operating <br />at its cut-in speed and when the hearer is close to the machine. <br />Personal observations referred to in the literature have related <br /> <br />8 <br />