We help ideas meet the real world Project Report EFP-06 project Low Frequency Noise from Large Wind Turbines Summary and Conclusions on Measurements and Methods Client: Danish Energy Authority Page 1 of 43 19 December 2008 DELTA Danish Electronics, Light & Acoustics Venlighedsvej 4 2970 Hørsholm Danmark Tlf. (+45) 72 19 40 00 Fax (+45) 72 19 40 01 www.delta.dk
Page 2 of 43 Title Low Frequency Noise from Large Wind Turbines. Summary and Conclusions on Measurements and Methods Journal no. Project no. Our ref. A401929-01 BSG-KDM/asp Client Danish Energy Authority Amaliegade 44 1256 Copenhagen K Client ref. Contract no.: 033001/33032-0081 Preface The work presented in this report is part of the EFP-06 project called Low Frequency Noise from Large Wind Turbines Quantification of the Noise and Assessment of the Annoyance. The project is funded by the Danish Energy Authority under contract number 033001/33032-008. Supplementary funding to the project is given by Vestas Wind Systems A/S, Siemens Wind Power A/S, Vattenfall AB Vindkraft, DONG Energy, E.ON Vind Sverige AB. The project has been carried out in cooperation between DELTA, RISØ DTU, DONG Energy and Aalborg University. Remarks Report replaces report AV 140/08 of 30 April 2008. The revision is described below. None of the revisions have any influence on the conclusions stated in the report. - Page 36 line 4: 2 db replaced by 2.5 db - Page 36 Table 2 values for >2000: 37.3 31.3 17.3 replaced by 38.2 32.0 17.9 - Page 36 Table 3 values for >2000: 44.0 38.0 24.4 replaced by 44.0 37.8 23.7 - Page 36 line 6 from bottom: 3.7 db replaced by 3.4 db - Page 43: Reference 6 updated DELTA, 19 December 2008 Bo Søndergaaard Acoustics Kaj Dam Madsen Acoustics
Page 3 of 43 Contents Preface...2 Remarks...2 0. Summary...5 0.1 Resumé (in Danish)...7 1. Introduction...9 2. Measurement methods and prediction methods...10 2.1 Sound power measurements...11 2.2 Noise propagation model...13 2.3 Sound insulation measurements...16 2.4 Conclusions...18 3. Audibility of low frequency noise from wind turbines...19 3.1 Assessment of audibility...19 3.2 Assessment of masking...21 3.3 Literature study...22 3.4 Conclusions on literature study...23 4. Masking of low frequency noise from wind turbines...24 4.1 Background noise measurements...24 5. Noise from large wind turbines...26 5.1 Sound power measurements...26 5.2 Noise monitoring...29 5.3 Mechanism for generation of low frequency noise...32 6. Consequences for single wind turbines and wind farms...33 7. Conclusions...37 Infrasound...37 Changes of noise characteristics with increasing size of wind turbines:...38 Consequences for wind farms and indoor noise levels at neighbours...38 Annoyance from wind turbine noise...38 Methods for measurement and prediction of low frequency noise from wind turbines...39 7.1 Konklusion (in Danish)...40 Infralyd...40 Ændring i støjens karakteristika med øget vindmøllestørrelse...41
Page 4 of 43 Konsekvenser for vindmølleparker og det indendørs støjniveau ved naboer...41 Genevirkning af vindmøllestøj...41 Metoder til måling og prædiktering af lavfrekvent støj fra vindmøller...41 8. References...43
Page 5 of 43 0. Summary The project on Low Frequency Noise from Large Wind Turbines comprises a series of investigations in the effort to give increased knowledge on low frequency noise from wind turbines. The present report gives an overview of the part of the project dealing with objective measures such as noise generation, noise measurements and results, noise propagation, audibility and masking. From these investigations is suggested a consistent method on how to predict low frequency noise levels at neighbour positions indoors and outdoors. Furthermore a method is proposed on evaluating the audibility and masking of the low frequency noise. Annoyance of low frequency wind turbine noise will be discussed in another part of the project carried out by Section of Acoustics at Aalborg University and DELTA. The project was initiated because of a growing anxiety in the public that large wind turbines emit significantly more low frequency noise than smaller wind turbines already present. In this project large wind turbines are defined as wind turbines with nominal power above 2 MW. As no existing investigations could confirm or deny this, a series of investigations were designed to provide the necessary background information. To ensure continuity in the study of noise from wind turbines and applicability of the results, existing measurement standards and methods have been used and extended into the low frequency region when possible. When selecting wind turbines for measurements in the project the turbines at the RISØ DTU test site at Høvsøre were pointed out. These wind turbines represent the largest wind turbines in Denmark at the time of the investigation. The wind turbines included in this project are prototypes and the results should not be generalized without consideration. The investigation shows that there is a minor increase in the low frequency noise of the order of 2 3 db relatively and that this increase is mainly due to tones from gear etc. As audible tones in the noise from wind turbines are considered unacceptable in most countries there is a strong incentive to reduce the level of the tones for the manufacturers. With the necessary tone reduction the increase will be less than 1 db. The blade passing the tower has been mentioned as a specific source of low frequency noise. A theoretical study of this phenomenon has been conducted by RISØ DTU. The result is that this blade/tower interaction generates noise only at the lowest frequencies with a decreasing level with increasing frequency. The study confirms the findings from the measurements on large wind turbines and a literature study, that infrasound is negligible for this type of wind turbines.
Page 6 of 43 The results of the project lead to the conclusion that the new generation of wind turbines does not result in more low frequency noise at neighbours at the minimum distance defined from Danish rules, than smaller wind turbines already in operation.
Page 7 of 43 0.1 Resumé (in Danish) I projektet Lavfrekvent Støj fra Store Vindmøller er der gennemført en række undersøgelser med det formål at skabe en forøget viden om lavfrekvent støj fra vindmøller. Denne rapport giver et overblik over den del af projektet, der omhandler objektive begreber som støjgenerering, støjmålinger og resultater, hørbarhed og maskering. På baggrund af disse undersøgelser er der foreslået en metode til prædiktering af det lavfrekvente støjniveau udendørs og indendørs ved naboer til vindmøller. Derudover er der foreslået en metode til vurdering af hørbarhed og maskering af lavfrekvent støj. Genevirkning af lavfrekvent vindmøllestøj vil blive undersøgt i en anden del af projektet, som udføres af Afdeling for Akustik på Aalborg Universitet og DELTA. Projektet blev iværksat på grund af en stigende bekymring i befolkningen for, at store vindmøller udstråler væsentlig mere lavfrekvent støj til omgivelserne end de kendte mindre vindmøller. I dette projekt defineres store vindmøller som vindmøller med en nominel effekt over 2 MW. Ingen eksisterende undersøgelser kunne be- eller afkræfte dette og en række aktiviteter blev derfor beskrevet i projektet med henblik på at fremskaffe den nødvendige viden og dokumentation på området. For at sikre kontinuitet i behandlingen af støj fra vindmøller samt anvendelighed af resultaterne, er der taget udgangspunkt i de eksisterende standarder og metoder, som allerede anvendes og de er udvidet at gælde i det lavfrekvente område, så vidt det har været muligt muligt. I forbindelse med udvælgelse af de vindmøller, der indgår i projektets måleprogram blev vindmøllerne på RISØ DTU s test område ved Høvsøre udvalgt. Disse vindmøller repræsenterer de største vindmøller i Danmark på tidspunktet for undersøgelserne. Vindmøllerne i projektets måleprogram er prototypemøller, og resultaterne fra målingerne bør ikke generaliseres uden forbehold. Undersøgelserne har vist, at der er en mindre stigning i den lavfrekvente støj af størrelsesordenen 2 3 db relativt og at denne stigning hovedsageligt skyldes fra toner fra gear o.l. Da hørbare toner i støjen opfattes som uacceptable i mange lande, er der en stor tilskyndelse for vindmølle producenterne til at reducere tonernes niveau. Med den nødvendige reduktion af tonerne vil stigningen være mindre end 1 db. Støj fra vingernes passage af tårnet er ofte blevet nævnt som en særlig kilde til lavfrekvent støj. RISØ DTU har gennemført et teoretisk studie af dette fænomen og konkluderet, at tårn/vinge vekselvirkningen kun generer støj ved de allerlaveste frekvenser og at støjniveauet aftager med stigende frekvens. Studiet har bekræftet, hvad der også er set ud fra målingerne på de store møller samt fra det gennemførte litteraturstudie, at infralyd er uden betydning for denne type af vindmøller.
Page 8 of 43 Resultaterne fra projektet viser at, der i minimumsafstanden til vindmøller, som den er defineret i de danske regler, ikke kommer et øget bidrag af lavfrekvent støj fra den nye generation af store vindmøller i forhold til de mindre vindmøller, som allerede findes.
Page 9 of 43 1. Introduction A major goal of the project is to make it possible to predict and assess low frequency noise at neighbours to wind turbines or wind farms based on noise measurements close to the wind turbine. The reason for this is that measurements at the neighbours will have a stronger influence from background noise than measurements close to the wind turbine, making it difficult to make valid decisions from the measurements. Other aspects like noise generation and the development of low frequency noise from small to large wind turbines are investigated as well, giving an indication on future development. A study of the most referenced literature on low frequency noise from wind turbines has been made leading to a suggestion for evaluating audibility and masking to be able to compare and qualify the data in the literature. Annoyance of low frequency wind turbine noise will be treated in another part of the project by Acoustics at Aalborg University and DELTA. A few results on annoyance are commented on in this report on the basis of measurements made at neighbours to wind turbines. The project deals with noise from large wind turbines. As most wind turbines with a nominal power up to 2 MW are considered as commercially available turbines, large wind turbines in this project is defined as turbines with a nominal power above 2 MW. It can be difficult to distinguish between infrasound, low frequency noise and what is referred to as normal noise. In Figure 1 the 3 frequency ranges are illustrated. The ranges overlap with each other meaning there is no distinct physical reason for these definitions of frequency ranges. The hearing threshold is shown in Figure 1 as well. The hearing threshold is the level at which the noise is just audible at that frequency. The hearing threshold changes almost 80 db in the low frequency range.
Page 10 of 43 120 Lp [db] 100 80 60 40 Hearing Threshold DS/ISO 226:2004 and Watanabe & Møller Infrasound Low frequency Sound Normal Sound 20 0 1 20 10 160 20 20000 1 10 100 1000 10000 100000 Frequency [Hz] -20 Figure 1 Different frequency ranges and the Hearing Threshold 2. Measurement methods and prediction methods To be able to make reliable assessment of noise from wind turbines in the environment it is necessary to have reliable and applicable methods for determining the emitted noise, for noise propagation and for determining the noise insulation of houses. This is true in general and also for low frequency noise. In Figure 2 is illustrated a typical situation of sound propagation from a wind turbine to an indoor neighbour. This includes the strength of the wind turbine sound source, the sound attenuation caused by the propagation over distance with influence of the type of landscape, the weather and the sound attenuation due to sound reduction of the building. All put together giving the indoor noise level experienced by the neighbour to the wind turbine. The 3 steps illustrated in Figure 2 are Sound power measurements, noise propagation and sound insulation. Each of these will be discussed in the following paragraphs.
Page 11 of 43 Step 2: Noise propagation Step 3: Sound Insulation Step 1: Sound Power Measurement Figure 2 The 3 steps in noise prediction. 2.1 Sound power measurements Reference [1] Søndergaard, Bo and Ryom, Carsten Low Frequency Noise from Large Wind Turbines - Sound Power measurement method. AV 135/08. DELTA April 2008 At the moment there is a widely accepted measurement method described in the measurement standard IEC 61400-11:2002 edition 2.1. Since 1998, where the first version of the standard was published, this measurement method has been preferred around the world when it comes to comparison of noise emission from different wind turbines and giving input to noise predictions. The result of this method is a wind speed dependent apparent sound power level of an equivalent point source located at the rotor centre. Information on the horizontal directivity of the equivalent point source can be determined as well. The tonality of the noise is analyzed and reported too. The setup for the measurement is shown in Figure 3. The measurement distance d = hub height + half a rotor diameter.
Page 12 of 43 Figure 3 Measurement setup. The microphone is put on a board on the ground mounted with a half standard wind screen. The used ground board is shown in Figure 4 and Figure 5 and has a diameter of at least 1 m. The wind speed is measured through the produced power and a calibrated power curve. An anemometer is placed in front of the wind turbine making it possible to measure the wind speed when the wind turbine is stopped for background noise measurements. The distance b to the anemometer is between 2 and 4 rotor diameters. The measurement position on a ground board serves 2 purposes: It keeps the microphone out of the wind and reduces the wind noise in the equipment It reduces the ground reflections to a simple + 6 db correction at all frequencies due to pressure doubling The wind induced noise in the measurement equipment is considered the most significant problem when extending the measurement standard to frequencies below 50 Hz. Experiences from a JOULE project led to designing a secondary wind screen as shown in Figure 5. Measurements in the project indicate an improved signal to noise ration at frequencies below 50 Hz, indicating a reduction in the wind induced noise in the measurement equipment. The insertion loss of the wind screen is measured and corrected for.
Page 13 of 43 Figure 4 (Left) Standard measurement setup with ground board Figure 5 (Right) Measurement setup with DELTA H wind screen 2.2 Noise propagation model Reference [2] Plovsing, Birger Low Frequency Noise from Large Wind Turbines Selection of a Propagation Model. AV 1096/08. DELTA April 2008 Existing propagation models considered in the project can be divided into three groups:
Page 14 of 43 Empirical models o ISO 9613-2: Attenuation of sound during propagation outdoors Part 2: General method of calculation o Nordic general prediction model for noise from industrial plants Wave equation models Ray models o Parabolic Equation method (PE) o Boundary Element Method (BEM) o Fast Field Program (FFP) o Linearized Euler (EU) o Numerical ray tracing models o Semi-analytical ray models Empirical models Most calculations of noise from wind turbines are made according to empirical models. The two models under consideration have common characteristics and share, among other things, equations for ground attenuation. The advantage of both methods is that they are available in commercial software. However, the weather condition in the model has been fixed to moderate downwind, and the model has been developed for moderate propagation distances and with the source and receiver close to the ground. A general weakness of empirical models is that they cannot be expected to produce reliable results outside the range of model variables where measurements have been available. It is well known that the two methods do not produce good results for high sources such as wind turbines. Wave equation models Wave equation models are numerical models based on the wave equation. Calculations may be performed in the frequency or time domain although frequency domain models are most often used. The only model relevant to consider is the Parabolic Equation method (PE). However, PE will, due to the calculation time and lack of commercial software, only be of relevance in special cases where strong weather effects at long distances have to be predicted accurately.
Page 15 of 43 Ray models In numerical ray tracing models a ray path is constructed numerically by making small steps along the ray path in such a way that the elevation angle of the ray satisfies Snell s law. A ray tracing algorithm is an iterative computational algorithm which calculates many ray paths and selects those that arrive at the receiver. In a semi-analytical ray model an algorithm is applied which directly calculates the ray without the iteration necessary for numerical ray tracing. However, a solution for direct calculation of the ray is only available for an atmosphere with a linear vertical sound speed profile leading to circular rays. Therefore, more realistic sound speed profiles have to be approximated by linear profiles. It could be expected that numerical ray tracing models would be more accurate than semi-analytical ray models as the correct ray path is calculated avoiding the approximation of the real sound speed profile by a linear profile. However, in practice the use of simple numerical ray tracing does not seem to increase the calculation accuracy significantly compared to a well adjusted semi-analytical ray model. Therefore, taking into account the considerable reduction in calculation time by the semianalytical ray models, these models are often preferred for engineering purposes The Nord2000 model and the European Harmonoise model The Nord2000 model and the European Harmonoise/Imagine model are both semianalytical ray models, although the former is using circular rays while the latter is based on the analogy of curving the ground instead. However, the curving of the ground is based on circles assuming linear sound speed profiles as well. It is expected that the accuracy of the two methods are almost the same, also in the low frequency range. At present, the Harmonoise method is not fully documented and has not been thoroughly tested in practical cases. The Nord2000 method has matured for five years, since the method was completed, and has been adjusted based on practical experience with the model during the five years. A software implementation is available at DELTA. The Nord2000 method has been validated by a large number of measurements or other kind of reference results. In an Energinet.dk project PSO F&U project nr. 2007-1-2389 Nord2000 was tested for wind turbine noise propagation. Preliminary results from this project are promising. The general recommendation is to use Nord2000 method for prediction of outdoor sound propagation from wind turbines. The method is fast, compared to the wave equation models and software implementations are available.
Page 16 of 43 2.3 Sound insulation measurements Reference [3] Hoffmeyer, Dan and Søndergaard, Bo Low Frequency Noise from Large Wind Turbines Measurements of Sound Insulation of Facades. AV 1097/08. DELTA April 2008 As low frequency noise is often most prominent indoors it is necessary to know the sound insulation of the neighbouring houses. It is not possible to give data for all types of houses so the project has focused on how to measure the sound insulation. A measurement method based on EN ISO 140-5:1998, supplemented by findings from Acoustics at Aalborg University (AAU) on how to select microphone positions was used. Microphone positions, according to EN ISO 140-5:1998 and the Danish Environmental Protection Agency (DEPA) recommendations, were used as well. The measurement positions suggested by AAU give most reproducible results. It is, however, not clear at this moment whether the results underestimate the insertion loss for practical uses. The specified method for measuring the outdoor/indoor level difference for a building façade at low frequencies is based on the use of a large loudspeaker outside the house and several microphones inside the house. A microphone mounted on the façade under consideration represents the outside noise level. The level differences are averaged on energybasis. There is no correction for the indoor room-acoustic environment. Using this procedure to determine the insertion loss of a house at frequencies below 200 Hz requires considerable signal power and a low background noise level. Especially at 8 Hz and 10 Hz it is difficult to obtain a sufficient signal to noise ratio, and correlation techniques like MLS could be considered. However, the energy content of wind turbine noise at these frequencies is limited and the insertion loss could be set to zero without serious consequences. An example of one of the four 3D positions suggested by AAU is shown in Figure 6.
Page 17 of 43 Figure 6 Example of a 3D position with the microphone placed in the three-dimensional corner where two walls and the ceiling meet A comparison of the insertion loss determined with the 3 types of measurement positions for one of the 5 houses included in the project are shown in Figure 7. At frequencies below 50 Hz the 3 methods are in good agreement. At frequencies above 63 Hz the 3D corner positions give higher noise levels inside resulting in lower noise insulation. The ISO and DEPA (LFM in the chart) methods both include positions in the room away from room boundaries. The noise measurements in the corners give results 4-5 db above the room average. It might be reasonable to correct measurements to the room average for frequencies above 63 Hz. If this is done, the results of the three measurement methods will be close. However, a correction of the method is still subject to discussion.
Page 18 of 43 30.0 25.0 Level difference [db] 20.0 15.0 10.0 5.0 average 3D average LFM average ISO 140-5 0.0-5.0 8 10 12.5 16 20 25 31.5 40 50 63 80 100 125 160 200 Frequency [Hz] Figure 7 Location: Værløse. Measurement results for the small-sized room. Outdoor/indoor level differences in db per one-third octave measured with the specified method (3D), with the low frequency method, and with the ISO method. The variation among the results from the different houses and rooms houses is wide. The standard deviation calculated for the level differences of the 10 rooms is between 2.1 to 7.8 for the frequencies measured. In the present work, it has not been possible to conclude on the connection between building and window types, sizes and the measured level differences at low frequencies. If the results from the project are used for specific situations, the average value of the results should be used, or a large uncertainty stated. 2.4 Conclusions When predicting the noise according to the principles described in the previous sections, the results are dependent on the input data like sound power, noise insulation and the details of the transmission path. In the sound power measurement method IEC 61400-11:2002 a guideline is given on how to assess the uncertainty of the results. The uncertainty of L WA is of the order of 2 db. The uncertainty in the individual 1/3-octave bands is higher, specifically at the lower frequencies. For the noise insulation data given in this investigation, the standard deviation on the average of all houses can be as high as 7.8 db with the lowest value of 2.1 db.
Page 19 of 43 In the noise propagation the most important parameters are the wind direction and the wind speed. For frequencies below 200 Hz and for high sources, the noise propagation is simpler than in most situations. Uncertainties in the individual 1/3-octave bands are expected to be of the order 5 db. Predicted in-door levels of the L pa,lf must be expected to have an uncertainty of approximately 5 db. If specific data for a house type is available the uncertainty is reduced. 3. Audibility of low frequency noise from wind turbines Reference [4] Pedersen, Torben Holm Low Frequency Noise from Large Wind Turbines A procedure for evaluation of the audibility for low frequency sound and a literature study. AV 1098/08. DELTA April 2008 Large amounts of literature on low frequency noise from wind turbines are available when a search on the internet is made. Not all of this literature has the necessary documentation to make results comparable or even understandable. Conclusions on the audibility of the low frequency noise in the literature are often made from psycho-acoustic inadequate procedures and methods. In order to qualify the conclusions of the audibility of low frequency sounds, a procedure for calculating the audibility of low frequency sound (and infrasound) has been defined, as a first part of this literature study. Furthermore reasonable demands to the documentation of data in literature are specified. 3.1 Assessment of audibility The audibility of low frequency noise is important because the difference between audible and annoying is small. This means that when the audibility of a sound is established an estimate on whether it is annoying or not is achieved. Another important parameter in hearing is the critical bands. The basis for the detection and loudness perception of a sound, as well as the bases for the masking of one sound with another, is the critical bands. Below 500 Hz, the critical bands are approximately 100 Hz wide. The hearing threshold (HT) is applied as a weighting of the measured spectrum independent of the frequency resolution and the resulting spectrum is energy added for each critical band to get the total HT-weighted level per critical band. The low frequencies of the wind turbine noise will be regarded to be below the hearing threshold, if the critical band levels found from the HT-weighted wind turbine spectra is less than 0 db.
Page 20 of 43 The method is illustrated in Figure 8 and Figure 9. It can be seen that the method is relatively insensitive to the analysis bandwidth except for the 1/1-octave band analysis. 70 Sound Pressure Level, db/banwidth 60 50 40 30 20 10 0-10 10 100 1000 10000 Frequency, Hz octave 1/3 octave 1/6 octave 1/24 octave FFT, Res. 1.3 Hz Hearing threshold Figure 8 Spectra with different resolution of the noise from a 1,3 MW wind turbine, referred to a distance of 280 m (measured at 70 m). The abscissa is the level per effective analysis bandwidth. The hearing threshold is also shown at the graph. The A-weighted sound pressure level of the noise is 33 db.
Page 21 of 43 30,0 Sound pressure level per critical band, db 25,0 20,0 15,0 10,0 5,0 Octave 1/3 octave 1/6 octave 1/24 octave FFT, Res. 1.3 Hz 0,0 0-100 100-200 200-300 300-400 400-500 Critical band, Hz Figure 9 The critical band levels computed from HT weighted frequency analyses of the noise from the 1.3 MW reference wind turbine shown in Figure 8. The different curves are the result of calculations based on analyses with different analysis bandwidths 3.2 Assessment of masking In the preceding section, a method was defined for comparing the wind turbine noise in quiet surroundings with the hearing threshold. In practice there is always some background noise, at least from wind in vegetation and buildings, so the wind turbine noise may not be audible due to masking, even if the HT-weighted critical band levels are above the hearing threshold. Assuming that no tones are present in the noise, the wind turbine noise is broadband and varying as is the wind-induced noise from vegetation or buildings. In this case the following principles can be used: 1. In simple cases the spectre of the wind turbine noise and the background noise can be compared directly and independent of any frequency weighting (the spectra shall be measured or referred to the same frequency weighting and analysis bandwidth (1/3 octave bands or less):
Page 22 of 43 a. The wind turbine noise will be masked if the levels in all analysis bands of the wind turbine noise is more than 2 db below the levels of the background noise b. If the levels of all analysis bands of the total noise is less than 2 db above the background noise the wind turbine will be masked. If this is not the case, the wind turbine noise may be masked even if the levels of some analysis bands of the wind turbine exceed the levels of the background noise. In this case the following rule apply: The wind turbine noise is masked if the levels of the critical bands of the A-weighted wind turbine noise are more than 2 db below the levels of the critical bands of the A-weighted background noise. The energy sum of the levels of the two lowest critical bands of the A-weighted wind turbine noise equals in practice L pa, LF. 3.3 Literature study Based on the principles in 3.1 and 3.2, it is possible to compare and evaluate data from different sources. Ideally, the comparisons and conclusions of a literature study should be based on reliable and well documented information suitable for comparison with other studies eventually by conversions to other situations. With regard to spectra or other results from measurements of wind turbine noise the following information should ideally be available (The key information is marked with an asterisk): 1. Measured sound pressure level/spectrum or sound power level/spectrum* 2. Frequency weighting* 3. Background noise level/spectrum* 4. Indoor or outdoor measurements* 5. Analysis bandwidth of spectrum* 6. Measurement distance* 7. Wind speed 8. Measurement direction (upwind, downwind...) 9. Microphone position (on a plate or free field at a specified height) 10. Type of windscreen 11. Height of turbine
Page 23 of 43 12. Number of turbines 13. Wind type (turbulent...) 14. Make and effect of wind turbine(s) The six key parameters are sufficient if we are satisfied with information of the type: There were some wind turbines; did they generate low frequency sound of any significance in the surroundings? If we want more specific information for comparison across references more that the key information s may be needed. Many references do not provide the key information. This makes comparisons or references to a common basis difficult and time consuming, and may only be made by combining information from other sources, or by use of general knowledge applied to the specific situations. 3.4 Conclusions on literature study There seems to be solid evidence and general agreement among researchers and technicians that wind turbines do not emit audible infrasound. The levels are far below the hearing threshold. Audible low frequency sound occurs both indoors and outdoors, but the levels in general are close to the hearing and/or masking threshold. Many other noise sources, e.g. road traffic, emit low frequency noise. For road traffic noise (in the vicinity of the roads) the low frequency noise levels are higher both indoors and outdoors. In general the noise in the critical band up to 100 Hz is below both thresholds, but the level in the 100-200 Hz critical band is audible for normal hearing persons, if no other sound than the natural background noise is masking the wind turbine noise. At a distance of 6 hub heights it seems that in average the noise levels from the turbines are close to the Danish outdoor noise limits, but in all cases the indoor limit for L pa, LF seems to be observed. The swishing sound from the blades is noted by a number of authors. It is found that for large modern turbines the most modulated range is the frequency band 350-700 Hz. In an earlier study from 1996, it was found that the smaller turbines, which were common at that time, had maximum modulation in the range around 1 khz. The swishing sound is actually more low frequent, but it is not in the low frequency range. Anyway, this effect may be called low frequency by some people in connection with the sound from the large turbines. Together with the slower rotation this is a noticeable change of the sound characteristic from wind turbines. Tones may occur from the turbines, but for well designed turbines they are usually not prominent. The large turbines may have tones of lower frequencies due to the lover rotational speed.
Page 24 of 43 4. Masking of low frequency noise from wind turbines The methods in section 2 describe how to predict the noise contribution from wind turbines in the environment based on sound power measurements. Results achieved from these methods are without background noise. Knowledge of the predicted level has to be supplied with information on whether the noise is actually perceived or whether it is masked by background noise, as described in section 3 In Denmark background noise from traffic, industry etc. is not accepted as masking background noise. This leaves only the natural background noise to be considered. As wind turbine noise is dependent on wind speed a series of measurements have been made of the wind induced background noise indoors and outdoors at undisturbed residences far from wind turbines, traffic etc. 4.1 Background noise measurements Reference [5] Søndergaard, Lars S. and Søndergaard, Bo Low Frequency Noise from Large Wind Turbines Background noise measurement and evaluation. AV 138/08. DELTA April 2008 The measurements have been made at 2 residences with different types of vegetation: One with primarily coniferous vegetation and one with primarily foliiferous vegetation. For the residence with foliiferous vegetation, measurements have been made for a summer and a winter situation. Indoors the measurement positions were selected as four 3D-positions, while the outdoor measurements were made in a single free field position. The wind speed and direction was measured at 10 m height in a position representing the free wind at the residence. All measurements were synchronized and averaged over 1 minute periods. A linear regression of the noise versus wind speed is assumed for the wind speed range considered from 4 10 m/s. To be able to present a background noise spectrum at any wind speed a linear regression is made in each 1/3-octave band. In Figure 10 predicted A-weighted spectra are shown. Indoors there is a difference between coniferous and foliiferous, but not between summer and winter. This could indicate that the building is more important than the vegetation at low frequencies. Outdoors there is a difference between foliiferous summer and winter, while summer is close to coniferous.
Page 25 of 43 40 Indoor spectrum at 6 m/s wind speed 40 Indoor spectrum at 10 m/s wind speed 20 20 L pa [db re 20 μpa] 0 20 40 60 80 Coniferous, summer Foliiferous, winter Foliiferous, summer 6.3 12.5 25 50 100 200 1/3 octave center frequency [Hz] L pa [db re 20 μpa] 0 20 40 60 80 Coniferous, summer Foliiferous, winter Foliiferous, summer 6.3 12.5 25 50 100 200 1/3 octave center frequency [Hz] 40 Outdoor spectrum at 6 m/s wind speed 40 Outdoor spectrum at 10 m/s wind speed 20 20 L pa [db re 20 μpa] 0 20 40 60 80 Coniferous, summer Foliiferous, winter Foliiferous, summer 6.3 12.5 25 50 100 200 1/3 octave center frequency [Hz] L pa [db re 20 μpa] 0 20 40 60 80 Coniferous, summer Foliiferous, winter Foliiferous, summer 6.3 12.5 25 50 100 200 1/3 octave center frequency [Hz] Figure 10 A-weighted spectra of indoor and outdoor background noise from vegetation at 6 and 10 m/s. The indoor values are the mean of measurements in four 3D corners. A summary of the results are shown in Table 1. The outdoor levels for the infrasound is of the same order as measured close to the wind turbines according to IEC 61400-11:2002, indicating that there is no significant amount of infrasound in the noise from wind turbines.
Page 26 of 43 Measurement Campaign L pa,lf [db re 20 µpa] L pg [db re 20 µpa] Wind speed [m/s] Site 1 Summer Outdoor: 16.5 39.6 Indoor: 7.4 27.8 Site 2 Winter Outdoor: 30.9 40.5 Indoor: 9.5 20.1 Site 2 Summer Outdoor: 25.4 44.1 Indoor: 7.1 27.0 Outdoor: 51.4 69.3 Indoor: 42.6 58.8 Outdoor: 60.6 69.7 Indoor: 39.1 48.2 Outdoor: 51.5 73.3 Indoor: 35.9 54.8 3-11 5-10 4-11 Table 1 Typical background noise levels of infrasound and low frequency noise from wind induced noise indoor and outdoor. The indoor values are the mean of measurements in four 3D corners 5. Noise from large wind turbines 5.1 Sound power measurements Reference [6] Søndergaard, Bo and Madsen, Kaj Dam Low Frequency Noise from Large Wind Turbines Results from sound power measurements. AV 136/08 Rev. 1. DELTA December 2008 Reference [7] Søndergaard, Bo Low Frequency Noise from Large Wind Turbines Results from previous sound power measurements. AV 137/08. DELTA April 2008 Measurements have been made on 4 prototype wind turbines, mainly situated at the RISØ DTU test site for large wind turbines at Høvsøre. The wind turbines at the site are the largest and newest types of wind turbines available in Denmark. These wind turbines are selected for the measurements, even though they are to be considered as prototypes. The wind turbines included in the project are: Vestas V100 3MW, Siemens S3.6 VS, and Siemens S2.3 VS (2 different turbines). All measurements are made and reported after the principles of IEC 61400-1:2002 by DELTA, with the deviations necessary for the work in this project. The reports are not published but are internal project reports, only made available for the project partners.
Page 27 of 43 As the noise from the wind turbines was low at the lowest frequencies the measurement results in the infrasound region up to 20 Hz was influenced by background noise. The measurement results are compared to results from previous measurements made by DELTA on smaller wind turbines. In order to decide whether a development of the low frequency noise has occurred all spectra were normalized to L WA =100 db re 1pW, making it possible to compare the spectral shape of the noise. In Figure 11 the spectra are grouped after produced power and then compared. Sound Power spectra Normalized to L WA = 0 [db re 1 pw] 0-10 10 12.5 16 20 25 31.5 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000-20 L WA,1/3-okt - L WA [db re 1pW] -30-40 -50-60 200 kw: 4 200-1000 kw: 23 1000-2000 kw: 10 > 2000 kw: 4 =<200 200-1000 1000-2000 =< 2 MW Project > 2MW 9 WT above > 2MW -70-80 1/3-octaveband centerfrequency [Hz] Figure 11 A-weighted sound power spectra grouped according to rated power and averaged. The 37 wind turbines from previous measurements are grouped as suggested. Included in the figure are 2 curves for wind turbines above 2 MW. The blue curve is the average of the four wind turbines from the project. The orange curve includes another 5 measurements made before the beginning of the project. These measurements only include data down to 25 Hz. The vertical bars represent plus/minus one standard deviation on the average of wind turbines up to 2 MW. The spectra in Figure 11 show that the spectral shape of the noise from wind turbines has not changed significantly over time even though the general level has. The curves representing the large wind turbines is approximately 2 3 db above the average of all the smaller wind turbines in the frequency range below 200 Hz. This is mainly due to the content of gear tones in the noise.
Page 28 of 43 In Figure 12 narrowband (fft) spectra are shown for the wind turbines in the project. It is obvious that the tones influence the A-weighted noise level at frequencies below 200 Hz. Two modes of operation are included for wind turbine 2. In mode 3 the tones are shifted slightly and the level of the tones are reduced. FFT analysis of noise from the wind turbines in the project 50 45 40 35 L pa [db re 20 upa] 30 25 20 Turbine 2 mode 0 Turbine 3 Turbine 1 Turbine 2 mode 3 Turbine 4 15 10 5 0 0 100 200 300 400 500 600 Frequency [Hz] Figure 12 A-weighted narrowband (fft) spectra for the wind turbines included in the project. The tones in the noise are present in 3 out of 4 wind turbines at this stage. The directivity of the noise is not particularly strong but the tones can be stronger at other directions than directly downwind. An important result from the measurements is that infrasound is negligible for this type of wind turbine. The tendency is that the new wind turbines through a frequency converter are able to vary the rotational speed of the rotor considerably. Thus there is an extra challenge when designing the wind turbines to avoid resonances in the structure at main frequencies of the drive train.
Page 29 of 43 5.2 Noise monitoring Reference [8] Søndergaard, Lars Sommer, Madsen, Kaj Dam and Ryom, Carsten Low Frequency Noise from Large Wind Turbines Noise monitoring af Høvsøre. AV 139/08. DELTA April 2008 This part of the project investigates if a relationship can be found between noise-related annoyance experienced by neighbours to large wind turbines, and meteorological parameters and specific noise characteristics. Noise monitoring was carried out at a neighbour house close to the Risø DTU Test Site Høvsøre as a free field measurement outside the house for two periods. The monitoring position can be seen in Figure 13. At the same time registrations were made of meteorological data and operation of the turbines at the Risø DTU Test Site Høvsøre. During the measurement campaign, the occupants at the neighbour house registered when they felt annoyed by the noise from the test site. Figure 13 Noise monitoring position at Neighbour to Høvsøre Test Site For periods with registered annoyance data analysis was made and a typical example of analysed data is shown in Figure 14. The figure shows time traces of the recordings made just before, during and after such a period of registered annoyance. Each data point represents an average of 10 minutes of recording. The vertical lines indicate the beginning and ending respectively of registered annoyance. It can be seen that just before the ending of annoyance the amplitude of the 40 Hz 1/3 octave noise level drops about 6 db. Apart from the 40 Hz 1/3 octave values the other noise parameters do not give any clear indication that can be coupled with the annoyance registration.
Page 30 of 43 In Figure 15 are shown frequency spectra taken from the same period of registered annoyance, and just after the period of registered annoyance. A clear difference can be seen in the low frequency range and especially at the 1/3 octave band 40 Hz a tonal content can be seen for the annoying period. START STOP 80 350 Sound Pressure Level [db re 20uPa] Wind Speed [m/s] 70 60 50 40 30 20 10 330 310 290 270 250 230 210 190 170 Wind Direction 10m [Degree] 0 19-09-2006 12:00 19-09-2006 13:12 19-09-2006 14:24 19-09-2006 15:36 19-09-2006 16:48 19-09-2006 18:00 Date and time 19-09-2006 19:12 19-09-2006 20:24 19-09-2006 21:36 40 Hz LAeq LpALF LpG Wind Speed 10m Wind direction 10m 150 19-09-2006 22:48 Figure 14 Time trace before, during and after a period with registered annoyance. Start and stop of registered annoyanceare marked in the plot. Wind direction and wind speed are plotted along with noise parameters.
Page 31 of 43 With and without registered annoyance 40 Sound Presseure Level [db(a) re 20uPa] 30 20 10 0-10 -20 4 Hz 5 Hz 6.3 Hz 8 Hz 10 Hz 12.5 Hz 16 Hz 20 Hz 25 Hz 31.5 Hz 40 Hz 50 Hz 63 Hz 80 Hz 100 Hz 125 Hz 160 Hz 200 Hz 250 Hz 315 Hz 400 Hz 500 Hz 630 Hz 800 Hz 1000 Hz 1250 Hz 1600 Hz 2000 Hz 2500 Hz 3150 Hz 4000 Hz 5000 Hz 6300 Hz 8000 Hz 10000 Hz 19-09-2006 18:00 19-09-2006 21:00-30 Frequency [Hz] Figure 15 Frequency spectra for periods with and without annoyance. 19-09-2006 18:00 is from the annoying period. From the registration of operating conditions for the wind turbines at the test site it can be seen that turbine number 2 was shut down exactly at the time where the 40 Hz noise is reduced by approximate 6 db at the monitoring position. For the whole period a rather constant wind speed at 10 meters height at the Høvsøre Test site of 10 m/s is seen. The wind direction changes from 250 degrees to 270 degrees at the end of the annoyance period. Stable meteorological conditions were seen for the analyzed period. A more thorough analysis is given of the meteorological parameters during the monitoring periods in [9] From the monitoring measurements the following was seen: During periods where the occupants registered annoyance a tonal noise in the 40 Hz 1/3 octave band was present in the measurements. Removal of the 40 Hz tone made the annoyance disappear even with other wind turbines still operating No coupling between other noise parameters and registered annoyance could be seen
Page 32 of 43 The 40 Hz noise could be coupled with the operation of one of the turbines at the test site. The specific prototype wind turbine in this test configuration was not pursued further by the manufacturer. For the listening test carried out in another part of this project, sound samples from this turbine should be chosen to represent an annoying sound sample. 5.3 Mechanism for generation of low frequency noise Reference [10] Helge Aagaard Madsen Low frequency noise from MW wind turbines mechanisms of generation and its modeling Risø-R-1637(EN). April 2008 In the present project the causes of low frequency noise for upwind rotors have been investigated. From previous work in the US and Sweden on 2 bladed downwind wind turbines of MW-size it has been found that the main source of low frequency noise is the unsteady forces on the rotor from wind shear and the interaction with the tower. A 3.6 MW turbine has been modeled with the above mentioned noise prediction model. Running the model on this turbine a number of important turbine design parameters with influence on the low frequency noise have been identified as well as other parameters, not linked to the turbine design. Of important parameters can be mentioned: rotor rotational speed blade/tower clearance rotor configuration - upwind/downwind unsteadiness/turbulence inflow Furthermore, the directivity characteristics of low frequency noise have been computed as well as noise reduction as function of distance from the turbine. In general, low levels of low frequency noise have been computed for the upwind rotor in standard configuration. When in-flow turbulence was included good agreement with noise measurements was found. The study confirms the findings from the measurements that infrasound is negligible for this type of wind turbine.
Page 33 of 43 Figure 16 Comparison of measured and predicted sound pressure levels. When in-flow turbulence is included (in the present case 10% turbulence intensity) there is a good agreement between measured and predicted values. The bump around 20 Hz could be a discrete frequency from the drive-train. Below 10 Hz the background noise dominates the measurements as was also seen in the measurement reports of the individual measurement campaigns. Thus, it seems that in the present case the blade/tower interaction only contributes to the low frequency noise level for frequencies below 10-15Hz, and for higher frequencies it is the interaction of the blade with the non-uniformities in the in-flow such as turbulence that generates the low frequency noise. 6. Consequences for single wind turbines and wind farms Reference [6] Søndergaard, Bo and Madsen, Kaj Dam Low Frequency Noise from Large Wind Turbines Results from sound power measurements. AV 136/08 Rev. 1. DELTA December 2008 As a general rule larger wind turbines make more noise than smaller wind turbines. The increase in noise with the turbine size is less for larger wind turbines than for smaller wind turbines. This can be seen in Figure 17, where the sound power level L WA and the low frequency part of the sound power level L WA,LF for different wind turbines are shown as a function of produced power.
Page 34 of 43 L WA and L WALF 120 2 MW 110 100 db re 1 pw 90 LWA LWALF 80 70 60 Wind Turbine Power Figure 17 Sound power level L WA and the low frequency sound power level L WA,LF for different wind turbines. LpA and LpALF Outside and In-door at 4 times total height 50 2 MW 45 40 db re 20 upa 35 30 25 LpA LpALF LpALF, in-door 20 15 10 Produced Power Figure 18 Predicted noise at 4 times total height for different wind turbines. The indoor noise is based on the average noise insulation from the measurements from [3] and using 3Dcorner positions. No correction is made to the noise insulation. The low frequency noise from the wind turbines above 2 MW does not deviate significantly from the wind turbines below 2 MW.
Page 35 of 43 According to Danish rules a wind turbine shall be at least 4 times the total height of the wind turbine from the nearest residence, the minimum distance. A comparison of the total noise and the low frequency noise at the nearest residence is shown in Figure 18. The calculation is based on the nord2000 method, not to underestimate the noise at the lowest frequencies. The indoor noise level calculations are based on the average noise insulation from the measurements from [3] and using 3D-corner positions. No correction is made to the noise insulation. The 3D-corner position method for determining the sound insulation differs from the current Danish regulations and the absolute values of the indoor noise levels cannot be compared to the Danish recommended limits for low frequency industrial noise. In Figure 19 the frequency distribution of the noise at 4 times the total height is shown. At high frequencies the air absorption reduces the noise more with increasing turbine size, because of the increasing minimum distance. Sound Pressure spectra At 4 times total height 40 L WA,1/3-okt - L WA [db re 1pW] 30 20 10 0 10-10 -20-30 -40 12.5 16 20 25 31.5 40 50 63 80 100 125 160 200 200 kw: 4 200-1000 kw: 23 1000-2000 kw: 10 > 2000 kw: 4 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000 =<200 200-1000 1000-2000 Project > 2MW 1/3-octaveband centerfrequency [Hz] Figure 19. Average spectra at 4 times the total height. The spectrum for the project wind turbine does not deviate from the values from the other groups except the wind turbines below 200 kw. In Table 2 a comparison of L pa outside a house, L pa,lf outside a house and L pa,lf inside a house at the minimum distance for single wind turbines can be seen. The calculation is based on the nord2000 method, not to underestimate the noise at the lowest frequencies. The indoor noise is based on the average noise insulation from the measurements from [3] and using 3D-corner positions. No correction is made to the noise insulation. The 3Dcorner position method for determining the sound insulation differs from the current Dan-
Page 36 of 43 ish regulations and the absolute values of the indoor noise levels cannot be compared to the Danish recommended limits for low frequency industrial noise. Apart from the smallest wind turbines the differences between the groups are small, less than 2.5 db. Notice that some of the large wind turbines have an amount of low frequency audible tones that might prevent them from being erected at the minimum distance, thus making this comparison less valid. L pa Outside L palf Outside L palf Inside < 200 43.3 27.3 12.6 200-1000 39.1 29.7 15.4 1000-2000 39.5 30.4 16.0 >2000 (project) 38.2 32.0 17.9 Table 2 Noise levels at minimum distance for single wind turbines. L pa, L pa,lf and L pa,lf inside a house in db re 20 µpa. An average value is applied for the insulation of the house. For wind farms the total noise at the nearest residence can be as much as 44 db(a) according to Danish rules. The effect of taking 44 db(a) as the value for L pa outside the house and recalculating the other values in Table 2 increases the low frequency noise outside and inside as can be seen in Table 3. L pa Outside L palf Outside L palf Inside < 200 44.0 29.0 14.3 200-1000 44.0 34.6 20.3 1000-2000 44.0 34.9 20.5 >2000 (project) 44.0 37.8 23.7 Table 3 Values from Table 2 recalculated for a wind farm situation with 44 db(a) outside the residence. Apart from the smallest wind turbines the difference between the groups of turbines sorted by produced power is up to 3.4 db for L pa,lf inside the house. For the project wind turbines low frequency audible tones in the noise influence the result. If the low frequency tones found in these wind turbines are reduced to a level where there is no penalty according to Danish rules, the level of the low frequency noise is expected to drop around 3 db for the large wind turbines in table 2 and Table 3. In this case, there is virtually no increase in the low frequency noise for the large wind turbines.
Page 37 of 43 7. Conclusions The project on Low Frequency Noise from Large Wind Turbines comprises a series of investigations carried out to give increased knowledge on low frequency noise from wind turbines. This report gives an overview of the part of the project dealing with objective measures such as noise generation, noise measurements and results, noise propagation, audibility and masking. From these investigations a consistent method on how to predict low frequency noise levels at neighbour positions indoor and outdoor is suggested. Furthermore a method on evaluation of the audibility and masking of the low frequency noise is proposed. Annoyance of low frequency wind turbine noise will be discussed in another part of the project carried out by Section of Acoustics at Aalborg University and DELTA. Only a few results on annoyance are commented on in the present report on the basis of measurements made at neighbours to wind turbines. The results in this report are valid for the typical three-bladed wind turbine with the rotor positioned upwind of the tower. This type of wind turbines has been used for more than 20 years and is the most common wind turbine produced and erected. The project deals with noise from large wind turbines. As most wind turbines with a nominal power up to 2 MW are considered as commercially available turbines, large wind turbines in this project are defined as turbines with a nominal power above 2 MW. The large wind turbines investigated in this project are all prototypes at an early stage of development, subject to changes also with respect to the noise emission. From the measurements in this project on these large wind turbines it was found that there were tones in the noise at low frequencies for several of the investigated turbines. This is not unusual for prototypes and usually the fully developed commercial wind turbines are improved on the noise emission, especially concerning audible tones in the noise. Audible tone noise from wind turbines is considered unacceptable in most countries and results in a penalty when evaluated against noise limits. This encourages the manufacturers to reduce this type of noise. The following conclusions can be made on the investigations carried out in this project: Infrasound The wind turbines do not emit audible infrasound. This conclusion is found as the general conclusion among researchers and technicians through a literature study. The levels are far below the hearing threshold. This conclusion is further substan-
Page 38 of 43 tiated by model calculations made by RISØ DTU and the measurements made on large wind turbines in this project Changes of noise characteristics with increasing size of wind turbines: The emitted sound power level from the wind turbines increases with the nominal power of the turbines i.e. the turbine size. It is seen that this increase is less for the group of wind turbines with a nominal power above 1 MW than for the group of turbines with a nominal power below 1 MW The frequency spectra of the aerodynamic noise from the rotor blades of the largest wind turbines does not deviate significantly from the spectra for smaller wind turbines. This means that for the aerodynamic noise the low frequency range is not more prominent for large turbines than for small turbines For the investigated large prototype turbines it is observed that the relative amount of low frequency noise is slightly higher than for smaller wind turbines. This is mainly caused by gear tones at frequencies below 200 Hz. An increase of 2 3 db is seen for the noise in this frequency range for the large turbines. It is expected that when the necessary reduction of the tones is made for the large turbines this increase will be less than 1 db Consequences for wind farms and indoor noise levels at neighbours The low frequency noise levels indoors at neighbours to wind farms are not expected to increase when comparing large wind turbines to small turbines. This is valid for houses at the minimum distance of 4 times the turbine total height as laid out in Danish regulations. To fulfill this it is provided that tones are reduced for the large turbines before placed on the market Annoyance from wind turbine noise From the results of a measurement campaign carried out at a neighbour to the Høvsøre test site there was found a good correlation between registered annoyance by the neighbour and a the occurrence of a low frequency tone in the noise from one of the wind turbines The part of the project carried out at Aalborg University will be dealing with annoyance of low frequency wind turbine noise in more detail
Page 39 of 43 Methods for measurement and prediction of low frequency noise from wind turbines In the project a set of tools for predicting the low frequency noise at the neighbours outdoors and indoors have been developed. The method differs from the procedure recommended by the Danish Environmental Protection Agency which has to be considered when comparing results with recommended noise limits Even though no major increase in low frequency noise was found in this investigation it is recommended for future measurements to extend the frequency range down to at least 20 Hz A method for evaluating the audibility and masking of low frequency noise has been suggested in this project
Page 40 of 43 7.1 Konklusion (in Danish) Projektet Lavfrekvent Støj fra Store Vindmøller omfatter en række undersøgelser med det formål at skabe en forøget viden om lavfrekvent støj fra vindmøller. Denne rapport giver et overblik over den del af projektet, der omhandler objektive begreber som støjgenerering, støjmålinger og resultater, hørbarhed og maskering. På baggrund af disse undersøgelser er der foreslået en metode til prædiktering af det lavfrekvente støjniveau såvel udendørs som indendørs ved naboer til vindmøller. Derudover er der foreslået en metode til vurdering af hørbarhed og maskering af lavfrekvent støj. Genevirkningen af lavfrekvent vindmøllestøj undersøges i en anden del af projektet af af Afdeling for Akustik på Aalborg Universitet og DELTA. Der er kun nogle få kommentarer til genevirkning af lavfrekvent vindmøllestøj i denne rapport baseret på målinger foretaget ved naboer til vindmøller. Resultaterne i denne rapport gælder for den typiske tre-bladede vindmølle med rotoren placeret på opvindssiden af tårnet. En type der er blevet anvendt i mere end 20 år og er den mest almindelige producerede og opstillede vindmølle. Projektet omhandler lavfrekvent støj fra store vindmøller. Da de fleste vindmøller med en nominel effekt op til 2 MW ses som kommercielt tilgængelige produktionsmøller, er store vindmøller i dette projekt defineret som vindmøller med en nominel effekt over 2 MW. De store vindmøller, der er undersøgt i dette projekt, er alle prototype på et tidligt trin i udviklingsfasen. Vindmøllerne undergår som prototyper en udvikling også hvad angår støjemission. Fra målingerne gennemført i dette projekt kunne det konstateres, at støjen fra flere af vindmøllerne indeholdt toner i det lavfrekvente område. Dette er ikke usædvanligt for prototyper, og almindeligvis er de færdigudviklede vindmøller forbedret med hensyn til støjemission og specielt med hensyn til hørbare toner i støjen. Dette skyldes, at hørbare toner i støjen opfattes som uacceptable i mange lande og fører til et straftillæg, når der sammenlignes med støjgrænser. Dette motiverer vindmølleproducenterne til at fokusere på dæmpning af tonestøj. Følgende konklusioner kan drages på de gennemførte undersøgelser: Infralyd Vindmøllerne udstråler ikke hørbar infralyd. Denne konklusion kan findes som den generelle konklusion bland forskere og teknikere i et gennemført litteraturstudium. Niveauerne er langt under høretærsklen. Konklusionen er yderligere understøttet af modelberegninger udført af RISØ DTU samt målingerne udført i projektet
Page 41 of 43 Ændring i støjens karakteristika med øget vindmøllestørrelse Det udstrålede lydeffektniveau fra vindmøllerne stiger med vindmøllernes nominelle effekt (vindmøllestørrelse). Denne stigning er mindre for gruppen af vindmøller med nominel effekt over 1 MW end for gruppen af vindmøller med nominel effekt under 1 MW Frekvensspektrene af den aerodynamiske støj fra vindmøllevingerne afviger for de store vindmøller ikke væsentligt fra frekvensspektrene for de mindre vindmøller. Det betyder, at den lavfrekvente aerodynamiske støj ikke er mere fremtrædende for store møller end for mindre møller For de undersøgte store prototype vindmøller er der konstateret en svag stigning i den relative mængde af lavfrekvent støj sammenlignet med mindre vindmøller. Stigningen skyldes hovedsageligt gear toner ved frekvenser under 200 Hz. Der kan ses en stigning for de store vindmøller i størrelsesordenen 2 3 db for støjen i dette frekvensområde. Det forventes, at stigningen vil være mindre end 1 db når den nødvendige reduktion af tonerne er udført Konsekvenser for vindmølleparker og det indendørs støjniveau ved naboer Det lavfrekvente støjniveau indendørs ved naboer til vindmøller forventes ikke at stige, når der ses på sammenligning af store vindmøller med mindre vindmøller. Dette gælder for huse i minimumsafstanden 4 gange vindmøllens totalhøjde som beskrevet i de danske regler og under forudsætning af, at tonerne reduceres inden vindmøllerne kommer på markedet Genevirkning af vindmøllestøj På baggrund af en måleserie udført hos en nabo til Høvsøre blev der konstateret en sammenhæng mellem den registrerede gene hos naboen og forekomsten af en lavfrekvent tone i støjen fra en af vindmøllerne Den del af projektet, som udføres på Aalborg Universitet vil beskæftige sig mere detaljeret med genevirkningen af lavfrekvent støj fra vindmøller Metoder til måling og prædiktering af lavfrekvent støj fra vindmøller I projektet er der udviklet et sæt af metoder til prædiktering af den lavfrekvente støj fra vindmøller ved naboer både udendørs og indendørs. Metoderne afviger fra proceduren anbefalet af Miljøstyrelsen og dette skal der tages hensyn til, hvis de absolutte resultater af beregningerne i dette projekt sammenlignes med de anbefalede støjgrænser
Page 42 of 43 Selvom der ikke er konstateret en væsentlig stigning i den lavfrekvente støj fra vindmøller i denne undersøgelse, anbefales ved fremtidige støjmålinger at frekvensområdet udvides ned til mindst 20 Hz Der er i projektet foreslået en metode til vurdering af hørbarhed og maskering af lavfrekvent støj i forbindelse
Page 43 of 43 8. References [1] Søndergaard, Bo and Ryom, Carsten Low Frequency Noise from Large Wind Turbines - Sound Power measurement method. AV 135/08. DELTA April 2008 [2] Plovsing, Birger Low Frequency Noise from Large Wind Turbines Selection of a Propagation Model. AV 1096/08. DELTA April 2008 [3] Hoffmeyer, Dan and Søndergaard, Bo Low Frequency Noise from Large Wind Turbines Measurements of Sound Insulation of Facades. AV 1097/08. DELTA April 2008 [4] Pedersen, Torben Holm Low Frequency Noise from Large Wind Turbines A procedure for evaluation of the audibility for low frequency sound and a literature study. AV 1098/08. DELTA April 2008 [5] Søndergaard, Lars S. and Søndergaard, Bo Low Frequency Noise from Large Wind Turbines Background noise measurement and evaluation. AV 138/08. DELTA April 2008 [6] Søndergaard, Bo and Madsen, Kaj Dam Low Frequency Noise from Large Wind Turbines Results from sound power measurements. AV 136/08 Rev. 1. DELTA December 2008 [7] Søndergaard, Bo Low Frequency Noise from Large Wind Turbines Results from previous sound power measurements. AV 137/08. DELTA April 2008 [8] Søndergaard, Lars Sommer, Madsen, Kaj Dam and Ryom, Carsten Low Frequency Noise from Large Wind Turbines Noise monitoring af Høvsøre. AV 139/08. DELTA April 2008 [9] Pedersen, Troels Friis Low Frequency Noise from Large Wind Turbines - Analysis of meteorological parameters during noise monitoring at Risø DTU Test Site at Høvsøre. Risø-R- 1643(EN) 2008 [10] Helge Aagaard Madsen Low frequency noise from MW wind turbines mechanisms of generation and its modeling Risø-R-1637(EN). April 2008