The sounds of high winds

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pulses may occur in phase increasing pulse strength further. At some distance from the wind farm this sound characteristic, described as thumping or beating, can be very pronounced though in the wind farm, close to a turbine, we never heard this impulsiveness. Indeed, close to a turbine it seems that most sound is coming from the downgoing blade, not when it passes the tower. One has to be careful in estimating blade position, as an observer at, say, 100 m from the foot of the tower is 140 m from a 100 m hub and therefore hears the sound from a blade approximately half a second after it was produced, in which time a blade may have rotated over some 30°. At the Berlin WindTurbineNoise conference Oerlemans [2005] explained this phenomenon: when the blade comes down and heads towards the observer, the observer is at an angle to the blade where most sound is radiated (see remark on directivity just below equation B.5 in Appendix B). On top of that the high tip velocity (70 m/s) causes a Doppler amplification. Both effects increase the sound level for our observer. However, this observation cannot be used for a distant turbine as in that case the observer sees the rotor sideways. Then the change due to the directivity of the sound is small, and also the Doppler effect is nil as the change in the velocity component towards the observer is negligible. II.3 Early warnings of noisy wind turbines? One may wonder why the strong effect of the nightly wind profile or the thumping was not noticed before. In the 1998 publication IEC 16400 only the neutral logarithmic wind profile is used [IEC, 1998]. As recent as 2002 it was stated that wind turbine sound is not impulsive [Kerkers et al 2002], which was concluded from assumed, not from measured sound level variations. There have been some warnings, though. In 1998 Rudolphi concluded from measurements that wind velocity at 10 m height is not a good measure for the sound level: at night the (58 m hub height) turbine sound level was 5 dB higher than expected [Rudolphi 1998]. This conclusion was not followed by more thorough investigation. Since several years residential groups in the Netherlands and abroad complained about 19
annoying turbine sound at distances where they are not even expected to be able to hear the sound. Recently Pedersen et al [2003, 2004] found that annoyance was relatively high at calculated maximum sound immission levels below 40 dB(A) where one would not expect strong annoyance. As wind turbines become taller, the discrepancy between real and expected levels grows and as more tall wind turbines are constructed complaints may become more widespread. In the Netherlands residents near the German border were the first Dutch to be acquainted with turbines of 100 m hub heights. It may be that earlier discrepancies between real and projected sound immission were not sufficient to evoke strong community reactions and that only recently turbines have become so tall that the discrepancy now is intolerable. There are other reasons that early warnings perhaps did not make much impression. One is that sound emission measurements are usually done in daytime. It is hard to imagine the sound would be very different at night time, so (almost) no one did. Until some years ago, I myself could not imagine how people could hear wind turbines 2 km away when at 300 to 400 m distance the (calculated) immission level was, for a given wind velocity, already equal to the ambient background sound level (L 95 ). But it proved I had not listened in a relevant period: an atmospherically stable night. What is probably also a reason is the rather common attitude that ‘there are always people complaining’. Complaints are a normal feature, not as such a reason to re-investigate. Indeed Dutch noise policy is not to prevent any noise annoyance, but to limit it to acceptable proportions. Added to this is a rather general conviction of Dutch authorities and consultants that routine noise assessment in compliance with legal standards must yield correct results. If measurements are performed it is to check actual emission levels –usually in normal working hours, so in daytime. It is quite unusual to compare the calculated sound immission from a wind turbine (farm) with measured immission levels (so unusual that it is likely that we were the first to do so). 20
Page 1 and 2: The sounds of high winds the effect
Page 3 and 4: Promotores: prof dr ir H. Duifhuis
Page 5 and 6: Contents I WIND POWER, SOCIETY, THI
Page 7 and 8: VII THINKING OF SOLUTIONS: measures
Page 10 and 11: I WIND POWER, SOCIETY, THIS BOOK: a
Page 12 and 13: esidents) noticed the discrepancy b
Page 14 and 15: that even though wind turbines did
Page 16 and 17: legitimate problems [Wolsink 1990].
Page 18 and 19: Thinking that this could perhaps be
Page 20 and 21: gas must keep flowing” . 1 I do n
Page 22 and 23: sound and the noise that wind produ
Page 24: combined with the corresponding sec
Page 27: calculated one and whether sound ch
Page 31 and 32: was 57.9 dB, with a maximum deviati
Page 33 and 34: not correct at night, although the
Page 36 and 37: III BASIC FACTS: wind power and the
Page 38 and 39: 0.1. In a stable atmosphere vertica
Page 40 and 41: With regard to wind power some atte
Page 42 and 43: III.4 Main sources of wind turbine
Page 44 and 45: High frequency: trailing edge (TE)
Page 46: So, what's the sound like...? (....
Page 49 and 50: Figure IV.2: turbines (dots W1….W
Page 51 and 52: L Wj , assumed identical for all k
Page 53 and 54: IV.1. As explained above, the wind
Page 55 and 56: sound level in the measurement inte
Page 57 and 58: Leq,5 min in dB(A) 50 45 40 35 30 A
Page 59 and 60: The same lines as in figure IV.5B,
Page 61 and 62: at hub height, not to a variation i
Page 63 and 64: IV.10.1 Measured and calculated imm
Page 65 and 66: wind farm to the measurement locati
Page 67 and 68: spherical divergence, that is: prop
Page 69 and 70: Thus, the logarithmic wind profile,
Page 71 and 72: There are now three factors influen
Page 73 and 74: direction may change significantly.
Page 75 and 76: V.2 Measurement results V.2.1 Locat
Page 77 and 78: P48 microphone. The sound was then
Page 79 and 80:
1/3 octave band Lp in dB 1/3 octave
Page 81 and 82:
1 1 0 1 0 0 1 0 0 0 1 0 0 0 0 A B 8
Page 83 and 84:
same magnitude as close to the turb
Page 85 and 86:
In figure V.5 the equivalent 1/3 oc
Page 87 and 88:
Also plotted in figure V.7 is the v
Page 89 and 90:
V.3 Perception of wind turbine soun
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kHz. For wind turbines we found tha
Page 93 and 94:
V.4 Conclusion Atmospheric stabilit
Page 96 and 97:
VI STRONG WINDS BLOW UPON TALL TURB
Page 98 and 99:
(hub height) wind velocity is 4 m/s
Page 100 and 101:
For the hourly progress of wind vel
Page 102 and 103:
In figure VI.5 the frequency distri
Page 104 and 105:
when V 80 was over 4 m/s. Such a de
Page 106 and 107:
dunes on the North Sea coast, Vliss
Page 108 and 109:
when based on extrapolated wind vel
Page 110 and 111:
The differences between actual and
Page 112 and 113:
hours. This corresponds to an avara
Page 114 and 115:
VII THINKING OF SOLUTIONS: measures
Page 116 and 117:
stability. The turbine thus operate
Page 118 and 119:
sound level was measured close to a
Page 120 and 121:
This type of control can also be ac
Page 122 and 123:
consecutive periods of 15 minutes i
Page 124 and 125:
increase of blade pitch with tilt (
Page 126 and 127:
elatively quiet areas as it control
Page 128 and 129:
VIII RUMBLING WIND: wind induced so
Page 130 and 131:
Finally, in this overview, Boersma
Page 132 and 133:
The friction created by wind shear
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the eddy relative to the screen sur
Page 136 and 137:
(L at,1/3 ~ -26.7·logf), but press
Page 138 and 139:
VIII.3 Comparison with experimental
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the small size of the unscreened mi
Page 142 and 143:
A B reduced pressure level Lred (dB
Page 144 and 145:
microphone, in a 9 cm foam cylinder
Page 146 and 147:
plotted is the calculated screening
Page 148 and 149:
VIII.5 Applications As microphone w
Page 150 and 151:
IX GENERAL CONCLUSIONS The research
Page 152 and 153:
modulation frequencies close to 4 H
Page 154 and 155:
Results from various onshore, relat
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limits, but nighttime immission lev
Page 158 and 159:
X EPILOGUE This is the end of my to
Page 160 and 161:
“….. about 80 per cent of the p
Page 162 and 163:
ACKNOWLEDGMENTS I want to express m
Page 164 and 165:
SUMMARY Ch. III Ch. II Ch. I This s
Page 166 and 167:
and as a result layers of air are l
Page 168 and 169:
500 kW. However, based on the real,
Page 170 and 171:
well known for an unobstructed wind
Page 172 and 173:
SAMENVATTING Bobby vraagt: 'Hoort u
Page 174 and 175:
Vaak wordt aangenomen dat er een va
Page 176 and 177:
gebleken dat de menselijke gevoelig
Page 178 and 179:
Bij een windpark kunnen de fluctuat
Page 180 and 181:
REFERENCES Archer C.L. and Jacobson
Page 182 and 183:
Kerkers A.J. (1999): “Windpark Rh
Page 184 and 185:
Petersen E.L., Mortensen N.G., Land
Page 186 and 187:
Van Ulden, A.P. and Wieringa J. (19
Page 188 and 189:
APPENDICES 179
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dB(G): unit of level after G-weight
Page 192 and 193:
u f : rms longitudinal component of
Page 194 and 195:
downwind turbine with an 80 m tubul
Page 196 and 197:
and total trailing edge immission s
Page 198 and 199:
So for a modern turbine at high spe
Page 200 and 201:
45 wind E (100 gr) 20 Leq (A) Leq,5
Page 202 and 203:
Wiens brood men eet, ….. Een plei
Page 204 and 205:
Vochtproblemen in woningen Venuslaa
Page 206 and 207:
Geluidsbelasting van een windturbin
Page 208 and 209:
Metingen van laagfrequent geluid in
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The sounds of high winds

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