Project Report. EFP-06 project Low Frequency Noise from Large Wind Turbines. Measurements of Sound Insulation of Facades

Transcription

1 We help ideas meet the real world Project Report EFP-06 project Low Frequency Noise from Large Wind Turbines Measurements of Sound Insulation of Facades Client: Danish Energy Authority Page 1 of April 2008 DELTA Danish Electronics, Light & Acoustics Venlighedsvej Hørsholm Danmark Tlf. (+45) Fax (+45)

2 Page 2 of 27 Title Low Frequency Noise from Large Wind Turbines Measurements of Sound Insulation of Facades Journal no. Project no. Our ref. A /08 DH/BSG/lm Client Danish Energy Authority Amaliegade Copenhagen K Client ref. Contract no.: / 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 / 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. DELTA, 30 April 2008 Dan Hoffmeyer Acoustics Bo Søndergaard Acoustics

3 Page 3 of 27 Contents Preface Summary Introduction Aim Measurement Methods ISO method Low Frequency Method Specified Method to be used in this Project for Measuring the Outdoor/Indoor Level Difference Measurements Buildings Used for the Measurements Description of the Houses Measurement Results Uncertainty Conclusions References...26 Appendix A Measurement Equipment...27

4 Page 4 of Summary The sound insulation of the facade is the last part of the sound propagation from a wind turbine to a receiver inside a building. The aim is to specify a method for measurement of airborne sound insulation of building facades at low frequencies and investigate the sound insulation at low frequencies of a number of typical buildings in Denmark. The results of the work should give typical data of the sound insulation at low frequencies for use within the EFP-06 project at calculations of indoor noise levels from wind turbines particularly with respect to low frequencies. It has been chosen to express the sound insulation as the outdoor/indoor sound level difference of the whole building façade instead of using the weighted sound reduction index R w of façade elements, for example windows. In addition the method specified is adjusted to describe indoor annoyance at low frequencies. A measurement method for sound insulation of building facades in the low frequency range 8 Hz Hz with respect to noise from large wind turbines has been specified. The method is based on the existing ISO method for the normal building acoustics frequency range and the low frequency method described in a report from the Danish Environmental Protection Agency. The most important deviation from these methods is the use of 3D positions in the room four positions very near to the three-dimensional corners where two walls and the ceiling or the floor meet. The specified measurement method was used for measurements of the outdoor/indoor level difference for the building facades of 10 rooms in five different types of singlefamily houses, representing possible buildings situated in areas near large wind turbines in the open land. The precision of the prescribed measurement method is not known, but a 90% uncertainty around +/- 5 7 db may be realistic. In the frequency range 8 Hz to 50 Hz the choice of 3D positions seems to have very little influence on the results compared to positions in accordance with the existing methods. At higher frequencies the measured level differences from the 3D positions as expected are 5-10 db lower.

5 Page 5 of Introduction The present report gives a discussion of a suitable measurement method to be used for determination of the airborne sound insulation of building facades in the low frequency range Hz and presents measurement results for specific houses.. 2. Aim The aim is to specify a method for measurements of airborne sound insulation of building facades at low frequencies and investigate the sound insulation at low frequencies of a number of typical buildings in Denmark. The sound insulation of the facade is the last part of the sound propagation from a wind turbine to a receiver inside a building. The results of the work should give typical data of the sound insulation at low frequencies for use within the EFP-06 project at calculations of indoor noise levels from wind turbines particularly with respect to low frequencies. It has been chosen to express the sound insulation as the outdoor/indoor sound level difference of the whole building façade instead of using the weighted sound reduction index R w of façade elements, for example windows. In addition the method specified is adjusted to describe indoor annoyance at low frequencies. 3. Measurement Methods Two existing measurement methods for sound insulation of building facades are used as background for the method specified for this project. These two methods both including the possibility of measuring the outdoor/indoor level difference of a whole building façade are first mentioned in brief and then the specified method to be used is discussed. 3.1 ISO method The international standard EN ISO 140-5:1998 [1] for field measurements of airborne sound insulation of facade elements and facades is intended for use in the frequency range from 50 Hz to 5000 Hz. The standard deals with eight different measurement methods, element methods for measurements of facade elements (sound reduction index), global methods for whole facades (level difference), measurements using loudspeaker noise, and measurements with traffic as sound source.

6 Page 6 of 27 The loudspeaker methods define a loudspeaker position outside the building with the angle of sound incidence equal to approx. 45 o. The outdoor sound pressure level is determined either directly on the facade element or 2 m in front of the facade. The indoor level is measured in at least five positions distributed throughout the room and spaced uniformly. The minimum separating distance between any microphone position and room boundaries is 0.5 m. 3.2 Low Frequency Method The other method is described and used in Working Report No. 10, 1997 from the Danish Environmental Protection Agency [2]. The method is dedicated to low frequency measurements of sound insulation. The results of measurements of low frequency sound insulation of several buildings described in the working report are used in a calculation method for the indoor noise level from high speed ferries at low frequencies. The method uses a loudspeaker placed outside the building, and the outdoor sound pressure level is determined directly on the facade. The indoor level is measured in three positions. One position is in a corner at the facade, m from the facade and m above the floor. The other positions are chosen to represent typical habitation in the room, at least 0.5 m from walls and large pieces of furniture and m above the floor. These three positions are in accordance with the Danish guidelines on environmental low frequency noise, infrasound and vibration [3]. The guidelines has a supplementary possibility in small rooms (less than about 20 m 2 ) where the noise can be measured at two positions in different corners, m from the adjoining walls and m above the floor. 3.3 Specified Method to be used in this Project for Measuring the Outdoor/Indoor Level Difference The specified method differs from the method from the Danish Environmental Protection Agency mainly in the choice of indoor positions. In agreement with the project partner Department of Acoustics, Aalborg University, their findings about indoor measurements of sound at low frequencies up to 200 Hz [4] were worked into the method for measurements of airborne sound insulation of building facades at low frequencies. This introduces measurements in three-dimensional corners. In the 3D positions the microphone is placed only m from a three-dimensional corner where two walls and the ceiling or the floor meet. It has been found that these positions give a better estimate of the low frequency high-level areas in a room than other methods. Whereas the traditional sound insulation methods aim to describe the energetic space averaged sound pressure level in the room, the method specified aims to represent the highest sound pressure levels to be found in the room. The use of the 3D positions as specified in this method aims to measure a sound pressure level only exceeded in 10% of the room volume (10% exceedance level). This level has been shown to be 3-4 db higher than the energetic space averaged sound pressure level of the room [4].

7 Page 7 of 27 The use of a nearby outdoor loudspeaker giving a spherical sound field to represent a plane sound field propagating from a far away wind turbine has been discussed. Papers on this topic hasn t been found, but the influence is expected to be of minor order assuming that the sound field from the loudspeaker using a 45 o horizontal angle of sound incidence propagates over the façade in the same way as a plane sound field arriving in an oblique angle to the plane of the facade. There are different ways of background noise correction for the indoor levels. In [2] differences between the signal and the background noise combined and the background noise level down to 1.3 db are accepted, corresponding to a maximum correction of approx. 6 db. This is done in order to get more results at the lower frequencies. Following the ISO method [1] differences should be more than 6 db. For lower differences corrections are limited to 1.3 db, corresponding to a difference of 6 db. For the specified method is has been decided to follow the ISO method, with the addition that results with differences less than 1.3 db are discarded. This correction method gives the lowest level differences of the facades, thus giving the most conservative (highest) values when used for calculation of the indoor noise level. From the measurements in this project it can be seen that the differences between the signal and the background noise combined and the background noise level (SNR) are less than 6 db mainly in the one-third octave bands with centre frequencies 8 Hz and 10 Hz. In these frequency bands the correction of the results is limited to 1.3 db or the results are discarded. The specified method for measuring the outdoor/indoor level difference for a building facade at low frequencies can be summarized as follows: A global loudspeaker method, airborne sound insulation level difference - of the whole building facade. Consequently there are no corrections for the area of the test specimen. Loudspeaker placed on the ground outside the building with the angle of sound incidence equal to approx. 45 o to the centre of the facade. The distance at right angles to the facade should be at least 5 m. Broadband noise limited to the low frequency range up till 250 Hz. Equalized to compensate for the loudspeaker characteristics. Measurements in one-third octave bands with centre frequencies from 8 to 200 Hz. Outdoor microphone fastened directly to the facade (+6 db measurement) approx. 1.5 m above the floor in the receiving room. Indoor microphone positions shall be 3D positions. Four arbitrary selected positions shall be used in the receiving room. Positions very near to windows or facade doors representing a strong source should be avoided. The 3D positions should be chosen to represent most of the walls and both ceiling and floor in the room.

8 Page 8 of 27 Background noise shall be measured and corrections for background noise applied. Differences between the signal and the background noise combined and the background noise level should be more than 6 db. For results with differences less than or equal to 6 db corrections are limited to 1.3 db, corresponding to a difference of 6 db. Results with differences less than 1.3 db are discarded. The level difference for each position shall be calculated as the difference between the outdoor level minus 6 db and the background noise adjusted indoor level. The correction for the facade-reflection to the free-field value is specified as minus 6 db although the influence of the facade is frequency dependent at low frequencies. This matter is discussed in [2] and the correction method is taken from there. The level differences from the four positions are averaged on energy basis (reciprocally power averaged level differences). There is no correction for the indoor room acoustic environment, e.g. no corresponding reference value (normalized, standardized). 4. Measurements The measurements of outdoor/indoor level difference for a building facade at low frequencies were performed using the above specified method. The measurements were made in November-December An example of a 3D position is shown in Figure 1.

9 Page 9 of 27 Figure 1 Example of a 3D position with the microphone placed in the three-dimensional corner where two walls and the ceiling meet. In addition to the four 3D positions in the specified method, five positions giving the possibility of representing the ISO method [1] and the low frequency method [2] was measured two corner positions and three typical habitation positions. The measurement equipment is listed in Appendix A. The low frequency sound field was generated from the loudspeaker outside the building. The broadband signal, limited and equalized, was played from a CD. The sound pressure levels were recorded simultaneously in approx. 2 minutes from four microphones, one outside position and three inside positions at a time. The recorded signals were analysed using the noiselab software from DELTA. The equivalent sound pressure levels per one-third octave band from 8 Hz to 200 Hz were found for a period of approx. 60 seconds. The correction for different calibration levels, background noise, and building reflection, as well as the averaging on energy basis was performed in a spreadsheet.

10 Page 10 of Buildings Used for the Measurements Measurements have been performed in five different types of single-family houses, representing possible buildings situated in areas near large wind turbines in the open land in Denmark. In addition to newer single storey houses with heavy-weight façades an old, partly two-storey farm house was chosen. Two houses with light-weight facades represent holiday houses as well as permanent residences. The measurements are focused on the transmission through the facades to rooms in the ground floor. The selected houses - only one of them having flat-topped roof - are expected to have a higher sound insulation for the combined roofing, roof space, and ceiling than for the façade. For each house measurements of the sound insulation of the facade were made both for the living room and for a small-sized room. 5.1 Description of the Houses The houses are named by their location. The house types and some details about the buildings are listed below. The rooms in all the houses had horizontal ceilings. The height of the rooms was approx. 2.4 m.

11 Page 11 of 27 Figure 2 Location: Værløse. Location: Type of house: Facade: Windows: Living room: Small-sized room: Værløse Single-storey house from the seventies Light-weight facade, partly with brick siding. Large window areas Windows and terrace doors with insulating glass units Approx. 50 m 2, L-shape, L x W 10.2 m x 4.2/5.7 m Approx. 11 m 2, rectangular shape, L x W 3.4 m x 3.2 m

12 Page 12 of 27 Figure 3 Location: Vejby. Location: Type of house: Façade: Windows: Living room: Small-sized room: Vejby New single-storey wooden house Light-weight facade with wood casing. Ordinary window areas Windows and terrace doors with triple layer insulating glass units Approx. 33 m 2, rectangular shape, L x W 8.4 m x 3.9 m Approx. 12 m 2, rectangular shape, L x W 3.9 m x 3.1 m

13 Page 13 of 27 Figure 4 Location: Tulstrup. Location: Type of house: Facade: Windows: Living room: Small-sized room: Tulstrup Single-storey house from the eighties Heavy-weight facade made of brick. Ordinary window areas Windows and terrace doors with insulating glass units Approx. 31 m 2, rectangular shape, L x W 7.6 m x 4.1 m Approx. 12 m 2, rectangular shape, L x W 3.9 m x 3.0 m

14 Page 14 of 27 Figure 5 Location: Slangerup Location: Type of house: Facade: Windows: Living room: Small-sized room: Slangerup New single-storey house Heavy-weight façade made of brick. Ordinary/Large window areas Windows and terrace doors with insulating glass units Approx. 54 m 2, rectangular shape, L x W 10.8 m x 5.0 m Approx. 9 m 2, rectangular shape, L x W 3.1 m x 2.9 m

15 Page 15 of 27 Figure 6 Location: Helsinge Location: Type of house: Façade: Windows: Living room: Small-sized room: Helsinge Earlier two-storey farm house Heavy-weight facade made of brick. Ordinary/large window areas Windows and terrace doors with insulating glass units Approx. 55 m 2, L-shape, L x W 10.1 m x 3.8/7.5 m Approx. 13 m 2, rectangular shape, L x W 3.7 m x 3.6 m 6. Measurement Results The measurement results obtained with the specified method are shown in Table 1. All values are outdoor/indoor level differences after correction for the facade reflection, e.g. the level difference between the free-field value outside and the value in the 3D positions inside.

16 Page 16 of Location Værløse Living room * -3.7 * Værløse Small-sized room * 11.1 * Vejby Living room * Vejby Small-sized room * Tulstrup Living room * -1.0 * 12.0 * Tulstrup Small-sized room * 16.5 * 15.3 * Slangerup Living room * 8.5 * Slangerup Small-sized room 9.0 * Helsinge Living room * 0.3 * Helsinge Small-sized room Table 1 Measurement results for five living rooms and five small-sized rooms. Outdoor/indoor level differences in db per one-third octave measured with the specified method. * Indoor SNR between 1.3 db and 6 db, correction limited to 1.3 db. The measurement results for the living rooms in the five houses are shown in Figure 7, too. Similarly, the results for the small-sized rooms are shown in Figure 8. The variation among the results from the measured houses is high. 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 and sizes and the measured level differences at low frequencies. An average value of the level differences of the different house and room types may be calculated, but it is decided to present all data for each house and room type. Figure 9 to Figure 18 show the level differences for each location and room and for all four 3D positions (in order to visualise the differences between the positions) and the energy based average value of the four level differences (reciprocally power averaged level differences). As the positions are not all measured simultaneously the method prescribes to calculate the level difference for each 3D position one by one. It can be seen that in each room the variations between the results from the four 3D corners are somewhat smaller than the variations between the measured houses. At the frequencies 8 Hz and 10 Hz the correction for background noise is limited to 1.3 db for most of the results, see Table 1. Additionally seven of the average values are calculated only on the basis of to or three 3D positions in a room because some results are discarded. Totally 18 results (out of 120) from individual 3D positions at the frequencies 8 Hz to 12.5 Hz were discarded and two average values at 8 Hz are omitted, see Figure 9 to Figure 18. Consequently the results as to be expected are less reliable at these lowest frequencies.

17 Page 17 of Level Difference [db] Værløse Vejby Tulstrup Slangerup Helsinge Figure 7 Measurement results for five living rooms. Outdoor/indoor level differences in db per one-third octave measured with the specified method Level Difference [db] Værløse Vejby Tulstrup Slangerup Helsinge Figure 8 Measurement results for five small-sized rooms. Outdoor/indoor level differences in db per one-third octave measured with the specified method.

18 Page 18 of Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 9 Location: Værløse. Measurement results for the living room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 10 Location: Værløse. Measurement results for the small-sized room. Outdoor/indoor level differences in db per one-third-octave measured with the specified method. Results are shown for each 3D position and for the average value.

19 Page 19 of Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 11 Location: Vejby. Measurement results for the living room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 12 Location: Vejby. Measurement results for the small-sized room. Outdoor/indoor level differences in db per one-third-octave measured with the specified method. Results are shown for each 3D position and for the average value.

20 Page 20 of Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 13 Location: Tulstrup. Measurement results for the living room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 14 Location: Tulstrup. Measurement results for the small-sized room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value.

21 Page 21 of Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 15 Location: Slangerup. Measurement results for the living room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 16 Location: Slangerup. Measurement results for the small-sized room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value.

22 Page 22 of Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 17 Location: Helsinge. Measurement results for the living room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value Level difference [db] D-a 3D-b 3D-c 3D-d average 3D Figure 18 Location: Helsinge. Measurement results for the small-sized room. Outdoor/indoor level differences in db per one-third octave measured with the specified method. Results are shown for each 3D position and for the average value.

23 Page 23 of 27 In Figure 19 and Figure 20 examples of the measurement results obtained with three possibilities concerning indoor microphone positions are given. The measurement results show the outdoor/indoor level differences measured with the specified method (3D positions), with positions fulfilling the low frequency method [2], and with indoor positions in accordance with the ISO method [1]. The positions fulfilling the low frequency method are two corner positions and one typical habitation position three positions in accordance with the Danish guidelines on environmental low frequency noise, infrasound and vibration [3]. The positions in accordance with the ISO method are two corner positions and three typical habitation positions all five positions distributed throughout the room and spaced uniformly fulfilling the minimum separating distance between any microphone position and room boundaries of 0.5 m. The results show only small differences at frequencies 8 Hz - 50/63 Hz between the three kinds of indoor positions, but at higher frequencies the 3D method gives 5-10 db lower values than the others. These results are representative of the other locations as well. Since the measurements following the low frequency method and the ISO method both includes corner positions (approx. 0.5 m from room boundaries) it is to be expected that the values in the lower part of the frequency range are almost equal to the results obtained with the specified 3D method. Similarly the differences between the results at higher frequencies above 50 Hz are to be expected as the 3D position method in this frequency range are expected to have a target value at least 3-4 db higher than the energetic space averaged sound pressure level in the room.

24 Page 24 of Level difference [db] average 3D average LFM average ISO Figure 19 Location: Værløse. Measurement results for the living room. Outdoor/indoor level differences in db per one-third octave measured with the specified method (3D positions), with positions fulfilling the low frequency method [2], and with indoor positions in accordance with the ISO method [1] Level difference [db] average 3D average LFM average ISO Figure 20 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 [2], and with the ISO method [1].

25 Page 25 of Uncertainty Estimation of the uncertainty of this kind of sound insulation measurements is discussed in [1] and [2]. In addition to the usual uncertainty for building acoustics field measurements, the low frequency conditions, the special 3D positions, and the corrections to free-field conditions at the façade contribute to the uncertainty of the level differences for each house and room. In [1] it is stated that the reproducibility of the global loudspeaker method has been shown to be about 2 db. The correction to free-field conditions at low frequencies will give a contribution to the standard uncertainty of 1.7 db [2]. According to [2] the overall 90% uncertainty may be in the range +/- 5 7 db, the highest value at the lowest frequencies. 7. Conclusions A measurement method for sound insulation of building facades in the low frequency range 8 Hz Hz with respect to noise from large wind turbines has been specified. The method is based on the existing ISO method for the normal building acoustics frequency range [1] and the low frequency method described in a report from the Danish Environmental Protection Agency [2]. The most important deviation from these methods is the use of 3D positions [4] in the room positions very near to the three-dimensional corners where two walls and the ceiling or the floor meet. The specified measurement method was used for measurements of the outdoor/indoor level difference for the building facades of 10 rooms in five different types of singlefamily houses, representing possible buildings situated in areas near large wind turbines in the open land. The measurement results in the frequency range 8 Hz Hz are presented in a table and graphically. The precision of the prescribed measurement method is not known, but a 90% uncertainty around +/- 5 7 db may be realistic. In the frequency range 8 Hz to 50 Hz the choice of 3D positions seems to have very little influence on the results compared to positions in accordance with the existing methods. At higher frequencies the measured level differences from the 3D positions as expected are 5-10 db lower. Selected results from this work will be used within the EFP-06 project at calculations of indoor noise levels from wind turbines particularly with respect to low frequencies.

26 Page 26 of References [1] EN ISO 140-5:1998, Acoustics Measurement of sound insulation in buildings and of building elements Part 5: Field measurements of airborne sound insulation of facade elements and façades, 1998 [2] DELTA Akustik & Vibration, Vurdering af lavfrekvent støj fra færger 2, Working Report from the Danish Environmental Protection Agency no (in Danish) [3] Lavfrekvent støj, infralyd og vibrationer i eksternt miljø, Information from the Danish Environmental Protection Agency no (in Danish) [4] Pedersen, S, Møller, H and Persson, K Indoor measurements of noise at low frequencies problems and solutions, Journal of Low Frequency Noise, Vibration and Active Control, Volume 26 number

27 Page 27 of 27 Appendix A Measurement Equipment Instrument Type A&V No. Sound source: CD player Discman Preamplifier LI 677L Amplifier rack Meyer 6001 Loudspeaker Meyer 650-R2 Outdoor: Microphone B&K L Microphone Preamplifier B&K L Microphone Power Supply B&K L Indoor: Microphone B&K L Microphone Preamplifier B&K L Microphone B&K L Microphone Preamplifier B&K L Microphone Power Supply B&K L Microphone B&K L Analyzer B&K L Acoustic Calibrator B&K L Hard Disk Recorder HDR 744T 1348L