van den Berg. The beat is getting stronger 2008.pdf

It is not clear whether the constant and relatively rapid repetition of wind turbine sound beats will have more or less effect on sleep quality, compared to vehicle or airplane passages. Pedersen and Persson Waye found that at dwellings where the (outdoor) sound level due to wind turbines exceeded 35 dB(A), 16% of 128 respondents reported sleep disturbance by this sound, of whom all but two slept with a window open in summer [22].

6. DISCUSSION AND CONCLUSION Atmospheric stability has a significant effect on wind turbine sound, especially for modern, tall turbines. First, it is related to a change in wind profile causing strong, higher altitude, winds while at the same time wind close to the ground may become relatively weak. High sound immission levels may thus occur at low ambient sound levels, a fact that has not been recognised in noise assessments where a neutral or unstable atmosphere is usually implied. As a result, wind turbine sound that is masked by ambient wind-related sound in daytime, may not be masked at night time. This has been dealt with elsewhere [2]. 32

volume 4 number 4

noise notes

Noise Notes 4-4

30/03/06

8:22 am

Page 33

the beat is getting stronger Secondly, the change in wind profile causes a change in angle of attack on the turbine blades. This increases the thickness (infra) sound level as well as the level of trailing edge (TE) sound, especially when a blade passes the tower. TE sound is modulated at the blade passing frequency, but it is a high frequency sound, well audible and indeed the most dominant component of wind turbine noise. The periodic increase in sound level when the blade passes the turbine tower, blade swish, is a well known phenomenon. Less well known is the fact that increasing atmospheric stability creates greater changes in the angle of attack over the rotor plane that add up with the change near the tower. This results in a thicker turbulent TE boundary layer, in turn causing a higher swish level and a shift to somewhat lower frequencies. It can be shown theoretically that for a modern, tall wind turbine in flat, open land the angle of attack at the blade tip passing the tower changes by approx. 2° in daytime, but this value increases by 2° when the atmosphere becomes very stable. The calculated rise in sound level during swish then increases from 1–2 dB to 4–6 dB. This value is confirmed by measurements at single turbines in the Rhede wind farm where maximum sound levels rise 4 to 6 dB above minimum sound levels within short periods of time.

Thirdly, atmospheric stability involves a decrease in large scale turbulence. Large fluctuations in wind speed (at the scale of a turbine) vanish, and the coherence in wind speed over distances as great as or larger than the size of an entire wind farm increases. As a result turbines in the farm are exposed to a more constant wind and rotate at a more similar speed with less fluctuations. Because of the nearsynchronicity, blade swishes may arrive simultaneously for a period of time and increase swish level. The phase difference between turbines determines where this amplification occurs: whether the swish pulses will coincide at a location depends on this phase difference and the propagation time of the sound. In an area where two or more turbines are comparably loud the place where this amplification occurs will sweep over the area with a velocity determined by the difference in rotational frequency. The magnitude of this effect thus depends on stability, but also on the number of wind turbines and the distances to the observer. This effect is in contrast to what was expected, as it seemed reasonable to suppose that turbines would behave independently and thus the blade swish pulses from several turbines would arrive at random, resulting in an even more constant level than from one turbine.

Also, within a wind farm the effect may not be noticed, since comparable positions in relation to two or more turbines are less easily realised at close distances. Sound level differences LAmax-LAmin (corresponding to swish pulse heights) within 5 minute periods over long measurement periods near the Rhede wind farm show that level changes of approximately 5 dB occur for an appreciable amount of time and may less often be as high as 8 or 9 dB. This level difference did not decrease with distance, but even increased 1 dB when distance to the wind farm rose from 400 m to 1500 m. The added 3-5 dB, relative to a single turbine, is in agreement with simultaneously arriving pulses from two or three approximately equally loud turbines. The increase in blade swish level creates a new percept, fluctuating sound, that is absent or weak in neutral or unstable atmospheric conditions. Blade passing frequency is now an important parameter as a modulation frequency (not as an infrasound frequency). Human perception is most sensitive to modulation frequencies close to 4 Hz of sound with a frequency of approx. 1 kHz. The hypothesis that fluctuations are important is supported by descriptions given by naïve listeners as well as residents: turbines sound like ‘lapping’, ‘swishing’, ‘clapping’, ‘beating’ or ‘like the surf ’. It is not clear to what degree this fluctuating noise notes

volume 4 number 4

33

Noise Notes 4-4

30/03/06

8:22 am

Page 34

the beat is getting stronger character determines the relatively high annoyance caused by wind turbine sound and to a deterioration of sleep quality. Further research is necessary into the perception and annoyance of wind turbine sound, with correct assumptions on the level and character of the sound. Also the sound exposure level of fluctuations in the sound in the bedroom must be investigated to be able to assess the effects on sleep quality. It is obvious that in wind turbine sound measurements atmospheric stability must be taken into account. When the impulsive character of the sound is assessed, this should be carried out in relation to a stable atmosphere, as that is the relevant condition for impulsiveness. Also sound immission should be assessed for stable conditions in all cases where night time is the critical noise period. Wind speed at low heights is not a sufficient indicator for wind turbine performance. Specifically, when ambient sound level is considered as a masker for wind turbine sound, neither sounds should be related to wind speed at reference height via a (possibly implicit) neutral wind profile.

In stable conditions wind induced sound on a microphone is not as loud as is usually thought (creating a high background level lowering the ‘signal to noise ratio’), as in these conditions hub height wind speeds are accompanied by relatively low microphone height wind speeds. So, wind turbine sound measurements are easier when performed in a stable atmosphere, which agrees well with the night being the sensitive period for noise immission.

7. REFERENCES 1. European Wind Energy Association (EWEA), European Commission’s Directorate General for Transport and Energy (DG TREN): “Wind Energy - The Facts: An Analysis of Wind Energy in the EU25” (2004).

2. G.P. van den Berg: “Effects of the Wind Profile at Night on Wind Turbine Sound”, J. Sound Vib. 277 (4-5), 955-970 (2004).

3. M.V. Lowson: “A New Prediction Model for Wind Turbine Noise”, Wind Engineering 18 (2), 51-62 (1995).

4. F.W. Grosveld: “Prediction of Broadband Noise from Horizontal Axis Wind Turbines”, J. Propulsion 1 (4), 292-299 (1985).

5. H.H.

Hubbard

and

K.P.

Shepherd:

“Aeroacoustics

of

Large

Wind

Turbines”,

J. Acoust. Soc. Am 89 (6), 2495-2508 (2004).

6. S. Wagner, R. Bareiss and G. Guidati: “Wind Turbine Noise”, Springer, Berlin, 1996.

7. E.L. Petersen, N.G. Mortensen, L. Landberg, J. Hùjstrup and H.P. Frank: “Wind Power Meteorology: Part I: Climate and Turbulence”, Wind Energy 1 2-22 (1998).

8. The Working Group on Noise from Wind Turbines: “The Assessment & Rating of Noise from Wind Farms” (ETSU-R-97), Department of Trade and Industry, United Kingdom (1996).

9. J.R. Garrat: “The Atmospheric Boundary Layer”, Cambridge University Press, 1992.

10. C.L. Archer and M.Z. Jacobson: “Spatial and Temporal Distributions of U.S. Winds and Wind Power at 80 m Dervoide from Measurements”, J. Geophys. Res. 108 (D9), (2003).

34

volume 4 number 4

noise notes

Noise Notes 4-4

30/03/06

8:22 am

Page 35

the beat is getting stronger 11. S. Baidya Roy, S.W. Pacala and R.L. Walko: “Can Large Wind Farms Affect Local Meteorology?”, Journal of Geophysical Research, Vol. 109, D19101 (2004).

12. A. Smedman, U. Högström and H. Bergström: “Low Level Jets – A Decisive Factor for Off-Shore Wind Energy Siting in the Baltic Sea”, Wind Engineering 20 (3), 137-147 (1996).

13. K. Smith, G. Randall, D. Malcolm, N. Kelly and B. Smith: “Evaluation of Wind Shear Patterns at Midwest Wind Energy Facilities” (preprint), National Renewable Energy Laboratory NREL/CP-50032492, USA (2002).

14. I.A. Pérez, M.A. García, M.L. Sánchez and B. de Torre: “Analysis and parameterisation of wind profiles in the low atmosphere”, Solar Energy, in press (2004).

15. A.A.M. Holtslag: “Estimates of Diabatic Wind Speed Profiles from Near-Surface weather observations”, Boundary-Layer Meteorology 29, 225-250 (1984).

16. IEC

International

Standard

61400-11,

Wind

Turbine

Generator

Systems—

Part 11: Acoustic Noise Measurement Techniques, IEC 61400-11:1998(E).

17. F.V. Hutcheson and T.F. Brooks: “Effects of Angle of Attack and Velocity on Trailing Edge Noise”, pros. 42nd Aerospace Sciences Meeting, Reno, January 2004.

18. T. Nanahara, M. Asara, T. Sato, K. Yamaguchi, M. Shibata and T. Maejima: “Smoothing Effects of Distributed Wind Turbines, Part 1”, Wind Energy 2004 (7), pp. 61-74.

19. T. Nanahara: personal communication, June 14, 2004.

20. B&K: “Microphone Handbook”, Brüel & Kjaer, Denmark, 1995.

21. E. Pedersen: “Noise annoyance from wind turbines - a review”, Swedish Environmental Protection Agency (2003).

22. E. Pedersen and K. Persson Waye: “Perception and Annoyance due to Wind Turbine Noise – a DoseResponse Relationship”, J. Acoust. Soc. Am 116 (6), 3460-3470 (2004).

23. WHO: “Guidelines for Community Noise”, World Health Organization (Geneva) and Institute of Environmental Epidemiology (Singapore), 2000.

24. K. Persson Waye and E. Öhström: Psycho-acoustic characters of relevance for annoyance of wind turbine noise - J. Sound Vib. 250(1), 65-73 (2002).

25. E. Pedersen, K. Persson Waye and L.R.M. Hallberg: “Living Close to Wind Turbines–A Qualitative Approach to a Deeper Understanding”, proc. Internoise2004, Prague (2004).

26. E. Zwicker and H. Fastl: “Psycho-Acoustics, Facts and Models”, Springer-Verlag, Berlin (1999).

27. A.P. van Ulden, J.G. van der Vliet en J. Wieringa: “Temperature and Wind Observations at Heights from 2m to 200m at Cabauw in 1973”, 1976, De Bilt.

28. J. Jakobsen: “Low Frequency Sound from Wind Turbines”, proc. Low Frequency Noise and Vibration, Maastricht (2004).

noise notes

volume 4 number 4

35

Noise Notes 4-4

30/03/06

8:22 am

Page 36

the beat is getting stronger 29. T.F. Brooks, D.S. Pope and M.A. Marcolini: “Airfoil Self-Noise and Prediction”, NASA reference publication 1218, USA (1989).

30. ISO: “Acoustics — Frequency-Weighting Characteristic for Infrasound Measurements” (ISO 7196), International Organization of Standardization (1995).

31. E. On: Wind Report 2004”, E.ON Netz GmbH, Bayreuth, Germany (2004).

32. ESB: “Impact of Wind Power Generation in Ireland on the Operation of Conventional Plant and the Economic Implications”, ESB National Grid, Ireland (2004).

33. Health Council of the Netherlands: “Effects of Noise on Sleep and Health”, Health Council of the Netherlands, publication no. 2004/14, The Hague (2004).

34. G.P. van den Berg: “Wind Turbines at Night: Acoustical Practice and Sound Research”, Proc. Euronoise 2003, Naples (2003).

LIST OF SYMBOLS SYMBOL: DEFINITION [UNIT] α: angle of attack [radian or degree] δi∗: displacement thickness of turbulent boundary layer [m] ν: kinematic viscosity of air [m2 s-1] ρ: correlation coefficient (here: between (1/3) octave band level and LA) Ω: turbine rotor angular velocity [rad s-1] a: correction factor for boundary layer thickness (value: 2 – 4) c: velocity of sound in air [m s-1] C: blade chord length [m] Dh: directivity function [-] f: frequency [Hz] fmod: modulation frequency [Hz] fpeak,TE: peak frequency of trailing edge sound [Hz] fpeak,if: peak frequency of in-flow turbulence sound [Hz] fB: blade passing frequency [Hz] fi: α-dependent factor for boundary layer thickness [-] Fbb: fluctuation strength [vacil] h: height [m] H: turbine height [m] href: reference height for wind speed (and direction) [m] k: integer number (of harmonic frequency) K1: constant (128.5 dB) Kα: α dependent increase in trailing edge sound level [dB] M: Mach number (at radius R: M = ΩR/c) [-] ∆L: increase in sound level [dB] LA: broad band sound level [dB(A)] LA5: 5-percentile of broad band sound levels over a time period [dB(A)] LA95: 95-percentile of broad band sound levels over a time period [dB(A)] m: stability exponent [-] mf: modulation factor [-] 36

volume 4 number 4

noise notes

Noise Notes 4-4

30/03/06

8:22 am

Page 37

the beat is getting stronger N: number of blades [-] r: distance [m] R: rotor radius = blade length [m] ∆R: increment in R [m] RX: range between maximum and minimum sound levels (X= bb or f) [dB] RX,90: range between 5- and 95-percentile of sound levels (X= bb or f) [dB] Re: chord based Reynolds number (Re = ΩRC/ν) [-] vh: wind speed at height h [m s-1] vref: wind speed at reference height [m s-1] vxx: wind speed at height xx m [m s-1] Spi: 1/3 octave band weighing function for TE sound [dB] SPLi: sound pressure level [dB] St: Strouhal number [-]

SUBSCRIPTS: A: A-weighted bb: broad band f: at frequency of (1 /3) octave band i: component of TE sound (i = p, s, α) if: in-flow p: pressure side s: suction side TE: trailing edge

APPENDIX I DOMINANT SOURCES OF WIND TURBINE SOUND With modern wind turbines there are three important mechanisms that produce sound. These will be reviewed here up to a detail that is relevant to this paper. A. Infrasound: thickness sound. When a blade moves through the air, the air on the forward edge is pushed sideways, moving back again at the rear edge. For a periodically moving blade the air is periodically forced, leading to ‘thickness sound’. Usually this will not lead to a significant sound production as the movement is smooth and thus accelerations are relatively small. When a blade passes the turbine tower, it encounters wind influenced by the tower: the wind is slowed down, forced to move sideways around the tower, and causes a wake behind the tower. For a downwind rotor (i.e. the wind passes the tower first, then the rotor) this wake causes a significant change in blade loading. The change in wind velocity near the tower means that the angle of attack of the air on a blade changes and lift and drag on the blade change more or less abruptly. This change in mechanical load increases the sound power level at the rate of the blade passing frequency, fB. For modern turbines fB = NΩ/(2π) typically has a value of approximately 1 Hz. As the movement is not purely sinuoidal, there are harmonics with frequencies kfB, where k is an integer.

Harmonics may occur up to 30 Hz, so thickness sound coincides with the infrasound region (0–30 Hz). Measured levels at 92 m from the two-bladed 2 MW WTS-4 turbine showed that measured sound pressure levels of the individual blade harmonics were less than 75 dB, and well predicted by calculations of wind-blade interaction near the turbine tower [5, 6]. The envelope of the harmonics peaks at the fifth harmonic (k = 5 with noise notes

volume 4 number 4

37

Noise Notes 4-4

30/03/06

8:22 am

Page 38

the beat is getting stronger fB = 1 Hz), indicating a typical pulse time of (5 Hz)-l = 0.2 s which is 20% of the time between consecutive blade passages. The WST-4 is a downwind turbine with an 80 m tubular tower, where the wind velocity deficit was estimated to be 40% of the free wind velocity [5]. For modern, upwind rotors the velocity deficit in front of the tower is smaller. As a consequence blade–tower wake interaction is weaker than for downwind turbines. From data collected by Jakobsen it appears that the infrasound level at 100 m from an upwind turbine is typically 70 dB(G) or lower, near downwind turbines 10 to 30 dB higher, where 95 dB(G) corresponds to the average infrasound hearing threshold [28]. Infrasound from (upwind) wind turbines thus does not appear to be so loud that it is directly perceptible. B. Low frequencies: in-flow turbulent sound. Because of atmospheric turbulence there is a random movement of air superimposed on the average wind speed. The contribution of atmospheric turbulence to wind turbine sound is named ‘in-flow turbulence sound’ and is broad band sound stretching over a wide frequency range. For turbulent eddies larger in size than the blade this may be interpreted as a change in the direction and/or velocity of the incoming flow, equivalent to a deviation of the optimal angle of attack.

This leads to the same phenomena as in A, but changes will be random (not periodic) and less abrupt. For turbulent eddies the size of the chord length and less, effects are local and do not occur coherently over the blade. When the blade cuts through the eddies, the movement normal to the wind surface is reduced or stopped, given rise to high accelerations and thus sound. In-flow turbulence sound has a maximum level in the 1/3 octave band with frequency: fpeak, if = (St 0.7R Ω) / (H – 0.7R )

(A1)

where Strouhal number St is 16.6 [4, 6]. Most sound is produced at the high velocity, outer parts of the blades. For a modern, tall, three-bladed wind turbine with hub height H = 100 m, blade length R = 35 m and angular velocity Ω = 2πfB/3 = 2 rad s-1 (20 rpm), fpeak,if = 11 Hz which is in the infrasound region. Measured fall-off from fpeak,if is initially approx. 3 dB per octave, increasing to 12 dB per octave at frequencies in the audible region up to a few hundreds of hertz [4, 6]. C. High frequencies: trailing edge sound. Several flow phenomena at the blade itself or in the turbulent wake behind a blade cause high frequency sound (‘airfoil self-noise’). Most important for modern turbines is the sound from the turbulent boundary layer at the rear of the blade surface where the boundary layer is thickest and turbulence strength highest. Trailing edge sound has a maximum level in the 1/3 octave band with frequency fpeak, TE = 0.02 Ω R /(δ* M 0.6 )

(A2)

where Mach number M is based on airfoil velocity. The displacement thickness of the turbulent layer is:

δ * = a 0.37 C Re −0.2 /8

(A3)

for a zero angle of attack. Re is the chord based Reynolds number [29]. The experimental factor a accounts for the empirical observation that the boundary layer is a factor 2 to 4 thicker than predicted by theory [3, 6]. For air of 10 °C and 38

volume 4 number 4

noise notes

Noise Notes 4-4

30/03/06

8:22 am

Page 39

the beat is getting stronger atmospheric pressure, a typical chord length C = 1 m, and other properties as given above (section B), fpeak,TE = 1700/a Hz. With a = 2 to 4, fpeak,TE is 450 – 900 Hz. The spectrum (see Spi below) is symmetrical around fpeak,TE and decreases with 3 dB for the first octave, 11 dB for the next, the contribution from further octave bands is negligible [29]. According to Brooks et al. [29] trailing edge sound level can be decomposed in components SPLp and SPLs due to the pressure and suction side turbulent boundary layers with a zero angle of attack of the incoming flow, and a component SPLa that accounts for a non-zero angle of attack α. For an edge length ∆R each of the three components of the immission sound level at distance r can be written as [29]:

(

)

SPL i = 10 log δ i * M 5 ∆R D h / r 2 + Sp i + K1 – 3 + K i

(A4)

and total trailing edge immission sound level as:

(

SPL TE = 10 log Σ i 10 SPL i / 10

)

(A5) where the index i refers to the pressure side, suction side or angle of attack part (i = p, s, α). The directivity function Dh equals unity at the rear of the blade (θ = 180°) and falls off with sin2(θ/2). Because of the strong dependence on M (~ M5) trailing edge sound is dominated by sound produced at the high velocity parts: the blade tips. Spi gives the symmetrical spectral distribution of the trailing edge sound spectrum centered on fpeak,TE and is maximum (0 dB) at this centre frequency. The constant K1 - 3 = 125.5 dB applies when the chord based Reynolds number exceeds 8 × 105 and the pressure-side turbulent boundary displacement thickness δi∗> 1 mm, as is the case for modern tall turbines. Ki is non-zero only if i = α. For small non-zero angles of attack (α < 5°) the boundary layer thickness shrinks δ∗ with a factor fp = 10-0.042α at the pressure-side and grows with a factor fs = 100.068α at the suction-side; δα∗, = δs∗, so fα = fs. Kα has a large negative value for α = 0. For 1° < α < 5° and M = 0.2 it can be approximated by Kα = 3.6α - 12.1 ([29], formula 49 with Kα = K2-K1+3). With equation A4, equation A5 can be rewritten as:

(

SPL TE = 10 log δ * M 5 ∆R D h / r 2

(

)

+ K1 − 3 + 10 log Σ i 10 (

10 log ( fi ) + Spi + Ki ) / 10

)

(A6)

The last term in A6 is the a-dependent part. For the peak frequency 1/3 octave band level (Spi = 0) the last term in equation A6 is 3 dB for a = 0, and 4.4 dB at α = 2°, then increasing with approx. 1.7 dB per degree to 9.4 dB at α = 5°. The level increase relative to the level at α = 0 is given in table AI The swishing sound that one hears when a blade passes the tower is less than 3 dB (in daytime) [8]. It must correspond to a change in sound level of 1 dB to be heard at all. An increase of 1 dB corresponds to an increase in a from zero to a value of 1.7° (0.03 radians), an increase of 2 dB corresponds to 2.5° (0.04 radians). So we

Table AI. Increase of trailing edge sound level with angle of attack α A 1 2 3 4 SPLTF (α) – SPLTE(α=0) (dB) 0.4 1.4 2.9 4.6 noise notes

volume 4 number 4

5 6.4 39

Noise Notes 4-4

30/03/06

8:22 am

Page 40

the beat is getting stronger estimate the change in a at the tower passage as 2.1 ± 0.4°. Part of this is due to the lower wind velocity at the lower blade tip relative to the rotor average (0.8°, see section 3 of main text), the rest is due to the slowinf downb of the wind by the tower. For small angles the change of wind speed with angle of attack α at radius R is: dVwind/dα = ΩR (A7) So for a modern turbine (Ω R ≈ 70 m/s at tip at 20 rpm) the wind speed deficit where the blade tip passes the tower and α = 2.1° (0.037 radians) is 2.6 m/s. In a (rotor averaged) 14 m/s wind this is 20%. This deficit is due to the influence of the tower as well as the (daytime) wind profile.

noise notes

LIGHT UP, SOUND DOWN According to the All-India Federation of Fireworks Association, Diwali is going to be a lot quieter and more eco-friendly this year. All manufacturers are taking note of the edict of the National Sound Level Committee and ensuring that noise emitted by crackers would not exceed 125dB. What has made manufacturers so ready to fall in line is a realisation of the sharp change in the public’s taste: now the emphasis is on colour and aerial variety, rather than merely sound.

THE PRICE OF PEACE Staffordshire theme park, Alton Towers, has been fined £5000 after the ruling that it was guilty of noise nuisance under the 1990 Environmental Protection Act. However, the case, brought by Mr and Mrs Rope, who live just 100 yards from Alton Towers’ main entrance, has cost them an estimated £250,000.

40

volume 4 number 4

noise notes