1 Danish Wind Industry Association Danish Wind Turbine Owners' Association Low Frequency Noise from Wind Turbines: Do the Danish Regulations Have Any Impact? An analysis of noise measurements. January 2014 Published : 9 January 2014 Project : Prepared : Bo Søndergaard Checked : Peter Henningsen Acoustica Akustik Støj Vibrationer Dusager 12 Tlf Web CVR-nr Aarhus N Direkte tlf Danmark Mobiltlf File Report Low frequency noise from wind turbines.docx
2 Page 1 FOREWORD At the wind turbine noise conference in Denver august 2013 Bo Søndergaard from Grontmij s acoustics department, Acoustica, presented results from an investigation of old and new measurements of noise from wind turbines . As this investigation was made without any funding with the purpose to present most recent findings at the conference no formal reporting was made at that time. Danish Wind Industry Association and Danish Wind Turbine Owners' Association have asked Grontmij if it was possible to publish a documentation of the findings from the presentation in Denver. This report is a more detailed documentation of the presentation from Denver. The reporting is paid for by the Danish Wind Industry Association and Danish Wind Turbine Owners' Association, but the results are from the independent investigation made by Grontmij at their own expense. The acoustics department at Grontmij in Denmark, Acoustica, is an independent consultancy with a long history for working for industrial clients, environmental authorities, organisations and private clients. As a guarantee for independency and quality Acoustica holds an accreditation from the Danish Accreditation, DANAK for testing in the field of noise and acoustics. Grontmij A/S Bo Søndergaard Acoustica
3 Page 2 SUMMARY For several years the concern that low frequency noise from wind turbines could cause serious annoyance for neighbours to wind turbines has been a dominant factor in the public debate in Denmark. From the beginning of 2012 the Danish regulations were revised and a criteria for the amount of in-door low frequency noise was introduced. Results from new and old measurements of noise from mainly Danish produced wind turbines are analysed and compared with the Danish regulations for noise and low frequency noise. In most cases low frequency noise is not the decisive parameter for Danish wind farms to comply with the Danish regulations. The new data in the analysis are from existing wind turbine projects and series produced wind turbines excluding dedicated prototypes. The purpose of the analysis is to investigate whether the new Danish regulation on low frequency noise have had any impact on the emitted low frequency noise and the low frequency noise at the neighbours or not. The wind farm examples do not give a clear answer to that. However it indicates that the situation has not changed and that the amount of low frequency noise at the residents is the same as for wind farms with smaller and/or older wind turbines. The analysis of the sound power levels and sound power spectra gives more information. Analysis of the sound power spectra shows that after 2010, where it was known that low frequency regulations were likely to be introduced, the relative amount of noise in the frequency range from 100 to 400 Hz is reduced. This includes the important part of the low frequency range from 100 Hz to 160 Hz. Whether this is because of the Danish regulation is not possible to say, but it is likely, that the regulation has increased the focus on this in the design phase. From the measurements it can be seen, that the low frequency tones, which were a significant part of low frequency noise in , , ,  and  are reduced for series produced wind turbines. In general the analysis shows that the development of low frequency noise with size do not follow the conclusions from the analyses in ,  and . The analysis show that on average the amount of low frequency noise is the same for large and small wind turbines relative to the total noise level and that the amount of low frequency noise for new large wind turbines is less than for old large wind turbines relative to the total noise level. The analysis is based on a larger number of measurement reports than previous analyses and experiences from post construction documentation. The results may change with inclusion of new data, but the conclusions are in line with the general conclusions in  and , where the results were influenced by prototype wind turbines. There is a large variation in sound power levels and sound power spectra within each group of wind turbines used in the analysis and it is important to check the details for each wind farm project already in the planning phase.
4 Page 3 SUMMARY IN DANISH I adskillige år har bekymringen om, at lavfrekvent støj fra vindmøller kunne give anledning til væsentlig gene i omgivelserne, været en dominerende del af den offentlige debat i Danmark. Ved starten af 2012 blev der indført nye regler for støj fra vindmøller og grænseværdier for lavfrekvent støj blev indført. Resultater fra nye og gamle målinger af støj fra primært dansk producerede vindmøller er blevet analyseret og sammenlignet med de danske regler for støj og lavfrekvent støj. I de fleste tilfælde er det ikke den lavfrekvente del af støjen, der er afgørende for om et projekt kan overholde de danske regler. Data der indgår i sammenligningen er fra eksisterende vindmølleprojekter og serie producerede vindmøller. Dedikerede prototyper er udeladt fra analyserne, da de ikke vurderes at være repræsentative for de opstillede vindmøller. Formålet med analysen er at undersøge, om de nye regler for lavfrekvent støj har haft indflydelse på den udsendte lavfrekvente støj fra vindmøllerne og den lavfrekvente støj ved naboerne. Analyserne, baseret på vindmølleparker, giver ikke noget entydigt svar, men det ser ud til at forholdene ikke er ændret væsentligt og at andelen af lavfrekvent støj er af samme størrelsesorden som for vindmølleparker med mindre og/eller ældre vindmøller. Analysen af lydeffektniveauer og lydeffektspektre giver mere detaljeret information. Analysen af lydeffektspektre viser, at efter 2010, hvor det var kendt at der ville komme grænser for lavfrekvent støj, er den relative andel af støj i frekvensområdet fra 100 Hz 400 Hz reduceret. Dette omfatter den væsentlige del af det lavfrekvente frekvensområde fra 100 Hz til 160 Hz. Hvorvidt dette skyldes indførelse de nye grænseværdier, er det ikke muligt at sige, men det er sandsynligt, at reglerne har øget fokus på lavfrekvent støj i designfasen. Det er Grontmij s erfaring, at de lavfrekvente toner, som var en væsentlig del af den lavfrekvente støj i , , ,  og , er reduceret for de seriefremstillede vindmøller. Overordnet viser analysen, at udviklingen af lavfrekvent støj med størrelsen af vindmøllerne ikke følger forudsigelserne fra analyserne i ,  og , men at små og store vindmøller i gennemsnit har samme andel af lavfrekvent støj relativt til den samlede støj og at nye store vindmøller har en mindre andel af lavfrekvent støj end ældre store vindmøller relativt til den samlede støj. Analyserne er baseret på et større antal målerapporter end tidligere analyser, samt erfaringer med eftervisning af forholdene efter opstilling af vindmølleparker. Resultaterne kan ændre sig med inkludering af yderligere data, men konklusionerne er i overensstemmelse med de overordnede konklusioner in  and , hvor resultaterne var væsentligt påvirket af resultater fra prototype vindmøller. Der er stor variation i lydeffektniveauerne og lydeffektspektrene i de enkelte grupper af vindmøller og det er derfor vigtigt at vurdere detaljerne for de enkelte vindmølleprojekter allerede i planlægningsfasen.
5 Page 4 TABLE OF CONTENTS PAGE FOREWORD 1 SUMMARY 2 SUMMARY IN DANISH 3 1 INTRODUCTION 5 2 INVESTIGATIONS Noise from wind farms Noise emission from wind turbines. 7 3 OTHER CONSIDERATIONS Type of regulation Noise as a function of distance 16 4 CONCLUSIONS 17 5 REFERENCES 18
6 Page 5 1 INTRODUCTION Around low frequency noise from wind turbines became a major concern in the public in Denmark. There was a worry that larger wind turbines would emit substantially more low frequency noise than the wind turbines people knew already. As this had consequences for the development of on-shore wind energy the Danish energy authorities initiated an investigation into the topic. The results were reported in  and  by DELTA and an alternative interpretation were reported by the acoustics department at the University of Aalborg ,  and . In December 2011 the Danish regulations on wind turbine noise were revised and noise criteria was introduced for the in-door low frequency noise. Details on the regulations and the background can be seen in . Sørensen presented some of the consequences for the planning process in . In short the Danish regulation introduces a set of noise criteria as well as a method for assessing the criteria through measurements and predictions. The noise limits are described in the Statutory order 1284 of 15 December 2011 from the Danish ministry of the environment  as follows: The total noise impact from wind turbines may not exceed the following limit values: 1) At the most noise-exposed point in outdoor living area no more than 15 metres from dwellings in open countryside: (a) 44 db(a) at a wind speed of 8 m/s. (b) 42 db(a) at a wind speed of 6 m/s. 2) At the most noise-exposed point in areas with noise-sensitive land use: (a) 39 db(a) at a wind speed of 8 m/s. (b) 37 db(a) at a wind speed of 6 m/s. The total low-frequency noise from wind turbines may not exceed 20 db at a wind speed of 8 and 6 m/s indoors in dwellings in open countryside or indoors in areas with noise sensitive land use respectively. 2 INVESTIGATIONS The question now is; do the new regulations have any impact on low frequency noise from wind turbines and how can it be checked. As the neighbours are the main objective for the new regulations, a set of wind farm projects, where Grontmij has been involved in the post construction documentation of noise were reviewed. Another parameter which has been used as an indicator is the sound power level and specially the sound power spectra of the wind turbines. An analysis based on Grontmij s database on wind turbines has been made to see if there is a development. The database consists of the original data from  except for the specific prototype wind turbines. Measurement reports from the archives of Acoustica ranging back to 1988 are included and supplemented with new measurements from 2010 to 2013.
7 Page Noise from wind farms. As an independent consultancy Grontmij is often involved in post construction documentation of noise from wind farms. In Denmark this includes determination of the sound power level of individual wind turbines and prediction of the noise in the surroundings. In Figure 1 the noise contours, representing the noise limits, around a wind farm is shown. The blue line is 39 db(a), the red line is 44 db(a) and the pink line is 20 db(a) indoor low frequency noise, all at 8 m/s. Typically for the projects Grontmij has seen, the pink curve for low frequency noise is enclosed by the other curves indicating that low frequency noise is not the decisive parameter on whether the noise requirements are met or not. Figure 1. Typical noise contours for a Danish wind farm at 8 m/s. The blue line is 39 db(a), the red line is 44 db(a) and the pink line is 20 db(a) indoor low frequency noise. In Table 1 results from noise predictions for 6 wind farms according to the Danish regulations are shown. The first 3 projects are planned and erected before the regulations on low frequency noise were introduced. The last 3 were planned with the new regulations as a design criteria. Project no. 3 is actually a much older project with new measurements made in 2013, allowing for a comparison with the new criteria. The numbers in the parenthesis show the margin to the noise criteria. For the normal noise the margin is typically up to 2 db, while it is in the range of 3 to 7 db for low frequency noise. This again indicates that it is the normal noise which is the decisive parameter and not the low frequency noise. From these limited data it is difficult to conclude on the difference between older and newer projects, but it looks like low frequency noise from new and old projects is of the same order at the neighbours.
8 Page 7 Project Wind Turbines Neighbour with highest noise level Distance [m] L pa (6 m/s) [db] L pa,lf (6 m/s) [db] L pa (8 m/s) [db] L pa,lf (8 m/s) [db] 1 6 V90-3MW ,9 (2,1) 14,8 (5,2) 42,6 (1,4) 17,3 (2,7) 2 6 SWT 2, ,1 (0,9) 14,4 (5,6) 43,3 (0,7) 16,8 (3,2) kw WTG ,2 (0,8) 12,7 (7,3) 42,7 (1,3) 14,4 (5,6) 4 5 SWT ,6 (-0,6) 13,7 (6,3) 43,5 (0,5) 16,1 (3,9) 5 3 V112-3MW ,3 (0,7) 11,0 (9,0) 37,9 (1,1) 12,7 (7,3) 6 3 SWT ,4 (0,6) 12,5 (7,5) 42,1 (1,9) 14,9 (5,1) Noise criteria 42/ /39 20 Table 1. Main results from typical wind farm projects in Denmark. The numbers in parenthesis show the margin to the noise criteria. For project 4 at 6 m/s the noise is above the noise criteria. It is allowed for owners and part owners to exceed the noise criteria at their own residence. Note that the neighbour with the highest noise level is not always the neighbour with the shortest distance. For some of the projects the nearest neighbour are at a shorter distance with lower noise levels. 2.2 Noise emission from wind turbines. Much of the discussion so far has been based on sound power levels of the wind turbines and whether the wind turbines generate more low frequency noise when the size increases. For this review an analysis based on 213 measurement reports is made. All the original data from the project in  are included, except for the 4 prototype wind turbines. Data are supplemented with older measurement reports from the archives at Grontmij, where 1/3-octave band data were available, and new measurement reports. The reports represent wind turbines ranging from a few kw to the newest version of MW turbines and give the sound power spectrum at 8 m/s at 10 m height. The analysis is based on the same principles as in  to  but on a much better statistical basis with 213 measurement reports, with only 65 measurement reports in the previous analyses e.g. . As the new data also includes older types of stall regulated wind turbines with a nominal power between 2000 kw and 3000 kw sorting of data is a little different. The data are sorted into 4 groups: 200 kw, ]200 kw 1000 kw], ] kw], >2000 kw, >2000 kw old types and >2000 kw new types. This makes it possible to evaluate new large wind turbines compared to wind turbines designed and erected before the investigation in  and  was published. For comparison with previous investigations the group ]200 kw 2000 kw] is included The spectra are normalised to L WA of the individual wind turbine before averaging. A comparison of the averaged spectra for each class can be seen in Figure 2 and the number of spectra in each class can be seen in Table 2.
9 Page 8 Figure 2. Normalized apparent sound power levels in one- third-octave bands. Mean of groups of wind turbines. Error bars show ± 95 % confidence interval around the mean in every 1/3- octaveband for the group > 2000 kw. Group Number of spectra Number of spectra at 10 Hz 200 kw 16 5 ]200 kw 1000 kw] ]1000 kw 2000 kw] 27 1 > 2000 kw ]200 kw 2000 kw] > 2000 kw new > 2000 kw old 28 4 Table 2 Number of spectra in each group. As not all measurements are covering the entire frequency range the count of spectra including the 10 Hz 1/3-octave band is shown as well. All spectra are measured down to at least 50 Hz and for the new large turbines down to at least 16 Hz. The spectra representing the different groups do not deviate much in the low frequency region from 10 Hz to 160 Hz, except for the group of wind turbines below 200 kw. These turbines are relatively rare these days and due to the early design are dominated by machinery noise and tonal noise. Small wind turbines are represented by the group ] kw] which is consistent with the previous analyses in  to . In Figure 3 a comparison of the group ] kw] and the group > 2000 kw wind turbines is shown. The differences in the low frequency range are small, which can be seen in more detail in Figure 4, where the spectrum for the group > 2000 kw is subtracted from the spectrum of group ] kw]. It is clear that the group of turbines above 2000 kw deviates very little from the group ] kw] wind turbine spectrum and positive deviations are less than 1 db. In the low frequency range the difference is significant at 10 Hz, 80 Hz, 160 Hz, and 200 Hz. The statistics for all 1/3- octave bands are given in Table 3 from a Students t-test comparison of the 2 groups.
10 Page 9 Figure 3, Normalized apparent sound power levels in one- third-octave bands. Mean of two groups of turbines: ]200 kw kw] and > kw Figure 4 Normalized apparent sound power levels in one- third-octave bands. Spectrum of the group > 2000 kw wind turbines relative to the spectrum for the group ] kw] wind turbines. A positive value means that the level of group > 2000 kw at that frequency is higher than the corresponding value for group ] kw]. Error bars show 95% confidence interval around the mean in every 1/3-octaveband for the group > 2000 kw wind turbines.
11 Page 10 1/3-octave band [Hz] t d.f. single sided p 10 2,3301 7,8 0, ,5 0,4998 7,6 0, , ,1 0, , ,3 0, , ,1 0, ,5 0, ,5 0, , ,3 0, , ,0 0, , ,1 0, , ,2 0, , ,2 0, , ,5 0, , ,1 0, , ,5 0, , ,8 0, , ,2 0, , ,8 0, , ,9 0, , ,0 0, , ,2 0, , ,3 0, , ,0 0, , ,4 0, , ,2 0, , ,3 0, , ,8 0, , ,5 0, , ,5 0, , ,2 0, , ,0 0, , ,5 0,0165 Table 3. Parameters from Student s t-test comparison of group ]200 kw 2000 kw] and group >2000 kw. Figure 5 Normalized apparent sound power levels in one- third-octave bands. Mean of two groups of turbines: ]200 kw kw] and > 2000kW. The group of turbines ]200 kw kw] are shifted one third of an octave down in frequency as suggested by Møller in  and . The comparison shows that the development of low frequency noise with size is not as strong as assumed by Møller.
12 Page 11 It has been suggested by Møller in  and  that the development in low frequency noise with size corresponds to a shift by one 1/3-octave in the spectrum. In Figure 5 a comparison is made where the spectrum for the group of wind turbines ]200 kw kw] wind turbines is shifted one third of an octave down. It is obvious that the original presentation in Figure 3 gives a better match between the 2 groups and the assumption can be rejected as can the consequences of this assumption when extrapolated to 5 MW and 10 MW wind turbines. In order to investigate the results from Figure 3 further a comparison between new (after the regulation) and old (before the regulation) wind turbines from group >2000 kw is made in Figure 6. The definition of new and old is a little diffuse, but measurement reports from before 2010 are considered to be old and after 2010 as new. Around 2010 it was obvious to the industry that regulations on low frequency noise would be introduced and possible measures would be introduced. There is a clear difference in the frequency range from 100 Hz to 500 Hz where the relative amount of noise is lower for the new wind turbines than for the old wind turbines in this group. The differences are significant as can be seen in Table 4.This suggests that there is a development towards less low frequency noise, possibly because tonality in this frequency range is an area of focus for the developers. Also aerodynamic and aero acoustic optimisation of the blades tends to shift the aero acoustically generated noise towards higher frequencies. Figure 6 Normalized apparent sound power levels in one- third-octave bands. Comparison of new and old wind turbines > 2000 kw. Error bars show 95% confidence interval around the mean in every 1/3-octaveband for new wind turbines above 2000 kw
13 Page 12 1/3-octave band [Hz] t d.f. single sided p 10 0,3862 0,4-12,5 0,1210 5,2 0, ,6830 7,0 0, , ,8 0, , ,1 0, ,5 0, ,4 0, , ,3 0, , ,3 0, , ,0 0, , ,9 0, , ,1 0, , ,8 0, , ,9 0, , ,8 <0, , ,8 <0, , ,6 0, , ,4 0, , ,6 0, , ,6 <0, , ,7 0, , ,3 <0, , ,9 0, , ,8 0, , ,2 0, , ,1 0, , ,6 0, , ,4 0, , ,2 0, , ,0 0, , ,7 0, , ,7 0,0003 Table 4 Parameters from Student s t-test comparison of old and new wind turbines in the group >2000 kw. The difference is significant at frequencies 125 Hz to 400 Hz and again at 630 Hz to 1250 Hz and 8kHz and 10kHz. There are not enough measurements for old wind turbines at 10 Hz to give a result. The apparent sound power level from the measurement reports is shown as a function of nominal power in Figure 7 both as the total level, L WA and the low frequency part of the level, calculated as the sum of all 1/3-octave bands from 10 Hz to 160 Hz, L WA,LF. The figure shows that the low frequency part of the level increases at a higher rate than the total level. From a regression analysis based on wind turbines with a nominal power above 200 kw the rate is 0,43 db for each doubling of the nominal power in MW. This is a modest development compared to previous analyses of this and suggests that low frequency noise emission can be reduced. The slopes of the lines are not statistically significant different from each other [90 % confidence interval 7.2 to 10.5 for L WA and 8.6 to 11.3 for L WA,LF ]. If the tendency seen in section 2.2 continues and more new wind turbines are included in the analysis, the difference is likely to be reduced even more.
14 Sound Power Level [db re 1 pw] Page y = 8,854*log(x) + 74, y = 10,278*log(x) + 59,603 Regression LWA LWA LWA,LF Regression LWA,LF Nominal Power [kw] Figure 7 L WA and L WA,LF as a function of nominal power at 8 m/s. 3 OTHER CONSIDERATIONS 3.1 Type of regulation In this article as well as in other reviews and presentations the basis has been the sound power level at 8 m/s. This wind speed has been the reference wind speed during the period where noise from wind turbines has been measured. This is a good parameter for the new type of turbines where pitch-rpm control has taken over from stall and active stall regulation of wind turbines. Measurements where a wider wind speed range has been covered show that the pitch-rpm regulated wind turbines from group > 2000 kw, have the maximal noise emission around 8 m/s. Older wind turbines especially stall and active stall regulated wind turbines have the maximum noise emission at higher wind speeds. Sometimes the maximum may be 5 db or more higher than the value at 8 m/s. This can be seen in Figure 8, where the noise as a function of wind speed is shown for the 3 types of regulation. For comparison the curves are normalized to 0 db at 8 m/s. Experience from measurements show that the low frequency noise increases at approximately the same rate as the normal noise and older wind turbines may actually produce more low frequency noise at wind speeds above 8 m/s than the modern large pitch controlled wind turbines. It is possible that neighbours to wind farms have been exposed to more low frequency noise from the old turbines than they are from the present generation of turbines.
15 L WA re L WA.8 m/s [db] Page 14 Noise curves Modern Pitch Active stall Stall Wind speed [m/s] Figure 8. Noise curves for 3 types of wind turbines. The curves are normalised to 0 db at 8 m/s for comparison, but are from actual measurements and are examples only. In Figure 9 and Figure. 10 this is illustrated with a comparison of the spectra for a stall regulated wind turbine from group ]200 kw 1000 kw] and a pitch-rpm regulated wind turbine from group >2000 kw at 8 m/s and 10 m/s are shown. The wind turbines are selected to have comparable spectra and levels at 8 m/s. It is obvious that the noise and the low frequency noise has increased significantly for the stall regulated wind turbine compared to the pitch-rpm regulated wind turbine at 10 m/s. This is a direct consequence of the noise curves in Figure 8 and is a general trend.
16 Page 15 Figure 9. Spectra for a stall regulated wind turbine from group ]200 kw 1000 kw] and a pitch- RPM regulated wind turbine from the group > 2000 kw at 8 m/s. Figure. 10 Spectra for a stall regulated wind turbine from group ]200 kw 1000 kw] and a pitch- RPM regulated wind turbine from the group > 2000 kw at 10 m/s
17 Page Noise as a function of distance In Denmark the required distance from a wind turbine to nearest neighbour is at least 4 times the total height of the wind turbine. Low frequency noise is reduced less with the distance than higher frequencies. In Table 5 predicted values of noise and low frequency noise are presented for the groups ]200 kw to 2000 kw] and > 2000 kw wind turbines with different total heights. Mean spectra from each group are used in the predictions. The results are scaled to a noise level at the neighbour of 44 db(a) corresponding to the noise limit at 8 m/s. In the predictions the total height for group ]200 kw to 2000 kw] is 75 m (50 m hub height and 50 m rotor diameter) and 150 m for the rest of the turbines. Differences in in-door low frequency noise level are within 1.6 db. New wind turbines > 2000 kw are 0.6 db higher than small wind turbines. For wind farms the predicted levels can change with different layouts, but most often the noise is dominated by the nearest wind turbines and as the noise is reduced by 6 db from spherical spreading at all frequencies the contribution from wind turbines at larger distances will often be small. At least for typical Danish wind farms with 2 to 5 wind turbines. It is possible to design wind farm layouts that give different results, but it is chosen to use a simple example for clarity. The results in Table 5 are in line with the results from existing wind farms in Table 1. Wind Turbine ]200 kw 2000 kw] >2000 kw >2000 kw New >2000 kw Old Distance [m] L pa,lf [db re 20 µ Pa Table 5. Predicted in-door low frequency noise L pa,lf at 8 m/s for different groups of wind turbines assuming the noise level is 44 db(a).
18 Page 17 4 CONCLUSIONS The purpose of this review is to investigate if the new Danish regulation on low frequency noise has had any impact on the emitted low frequency noise and the low frequency noise at the neighbours or not. The wind farm examples do not give a clear answer. It gives the impression, that the situation has not changed and the amount of low frequency noise at the residents is the same as for wind farms with smaller and/or older wind turbines. Looking at the sound power levels and sound power spectra gives more information. Analysis of the sound power spectra shows that after 2010 the relative amount of noise in the frequency range from 100 to 400 Hz is reduced significantly. This includes the important part of the low frequency range from 100 Hz to 160 Hz. Whether this is because of the Danish regulation is impossible to say, but it is likely that the regulations have increased the focus on this in the design phase. It is the experience of Grontmij, that the low frequency tones, which were a significant part of low frequency noise in , , ,  and  are reduced for series produced wind turbines. In general the analysis shows that the development of low frequency noise with size do not follow the conclusions from the analyses in ,  and . The analysis show that on average the amount of low frequency noise is the same for large and small wind turbines, relative to the total noise level and that the amount of low frequency noise for new large wind turbines is less than for old large wind turbines, relative to the total noise level. The analysis is based on a larger number of measurement reports than previous analyses and experiences from post construction documentation. The results can change if the dataset is increased further, but the conclusions are in line with the conclusions in  and , where the results were influenced by prototype wind turbines. There is a large variation in sound power levels and sound power spectra within each group of wind turbines used in the analysis and it is important to check the details for each wind farm project. It is also important to follow the development into the next generation of wind turbines where new technologies are likely to be introduced.
19 Page 18 5 REFERENCES  Søndergaard B. Low Frequency Noise from Wind Turbines: Do the Danish Regulations Have Any Impact?, 5th International Conference on Wind Turbine Noise Denver August 2013  Søndergaard B. Low Frequency Noise from Large Wind Turbines Summary and Conclusions form Measurements and Methods. AV 140/08. DELTA April 2008  Madsen K. D. Low Frequency Noise from Large Wind Turbines Final report. AV 1272/10. DELTA November 2010  Møller H. Low Frequency Noise from Large Wind Turbines, Acoustics University of Aalborg June 2010 (in Danish)  Møller H. Low Frequency Noise from Large Wind Turbines update 2011, Acoustics University of Aalborg May 2011 (in Danish)  Møller H. Low-frequency noise from large wind turbines J. Acoust. Soc. Am. 129 (6), June 2011  Statutory order 1284 of 15. December 2011 Noise from wind turbines from the Danish ministry of the environment (in Danish)  Jakobsen J. Danish regulation of low frequency noise from wind turbines. 15th International Meeting on Low Frequency Noise and Vibration and its Control Stratford upon Avon UK 22nd 24th May 2012  Sørensen T. Experiences with the New Danish Rules for the Calculation of Low Frequency Noise from Wind Turbines 15th International Meeting on Low Frequency Noise and Vibration and its Control Stratford upon Avon UK 22nd 24th May 2012
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