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Project Report
EFP-06 project
Low Frequency Noise from Large Wind Turbines
Sound Power Measurement Method
Client: Danish Energy Authority
AV 135/08
Page 1 of 26
30 April 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
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Title
Low Frequency Noise from Large Wind Turbines
Sound Power Measurement Method
Journal no.
AV 135/08
Project no.
A401929-06
Our ref.
BSG-CRP/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 033001/33032008. 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
Bo Søndergaaard
Acoustics
Carsten Ryom
Acoustics
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Contents
Preface
2
0.
Summary.......................................................................................................................4
1.
Introduction ..................................................................................................................5
2.
Discussion of the measurement method .....................................................................5
2.1 History ....................................................................................................................5
2.2 The wind turbine measurement method - IEC 61400-11:2002 edition 2.1 ............6
2.3 The microphone height and ground board..............................................................7
2.4 Horizontal directivity..............................................................................................9
2.5 Vertical directivity ................................................................................................10
2.6 Wind induced noise ..............................................................................................11
2.7 Calibration of a secondary wind screen for wind turbine testing .........................15
2.8 Air absorption .......................................................................................................15
2.9 Background noise .................................................................................................16
2.10 Instrumentation used for carrying out measurements...........................................17
2.11 Uncertainty ...........................................................................................................18
3.
Recommendation........................................................................................................19
4.
References ...................................................................................................................21
Annex A
Procedure for Calibration of Secondary Wind Screen..............................22
Annex B
Insertion Loss Values for the Secondary Wind Screen DELTA
type H as an average of all measurements ...................................................26
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Summary
A major purpose of EFP-06 project: “Low Frequency Noise from Large Wind Turbines” is
to give recommendations on how to predict the amount of low frequency noise at
neighbours to wind turbines and wind farms. To be able to do this there has to be a reliable
measurement method available for determination of the sound power levels used in the
noise predictions.
The present report discusses the possibilities for using the existing standard IEC 61400-11
in determining the sound power level in the low frequency range for a wind turbine and
recommends a method. As IEC 61400-11 is the generally accepted method for declaration
of noise from wind turbines and for calculation of the noise at the neighbouring residences,
it was decided already in the project formulation to focus on extending this method into
the low frequency range.
The result of this method is an 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.
Several potential limitations of the method are discussed and the main finding is that a
secondary wind screen must be applied to improve the signal to noise ratio for wind induced noise to the microphone, especially at low frequencies.
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Introduction
The present report discusses the possibilities for using the existing standard IEC 61400-11
[1] in determining the sound power level in the low frequency range for a wind turbine and
recommends a method. As IEC 61400-11 is the generally accepted method for declaration
of noise from wind turbines and for calculation of the noise at the neighbouring residences
it was decided already in the project formulation to focus on extending this method into
the low frequency range.
A major purpose of EFP-06 project: “Low frequency noise from large wind turbines” is to
give recommendations on how to predict the amount of low frequency noise at neighbours
to wind turbines and wind farms. To be able to do this there has to be a reliable measurement method available for determination of the sound power levels used in the noise predictions.
At the moment there is a widely accepted measurement method described in the measurement standard IEC 61400-11:2002 edition 2.1. This measurement method has since 1998,
where the first version of the standard was published, been the preferred method around
the world, when it comes to comparison of the noise emission from different wind turbines, and when input to noise predictions are needed. The result of this method is an 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.
As this method is widely accepted and already standardized, the background and assumptions leading to this method will be discussed and investigated for arguments preventing a
generalization into the low frequency range.
2.
Discussion of the measurement method
2.1
History
The IEC method [1] is the most used standard for measuring the emitted sound power
level from wind turbines. Measurements according to IEC 61400-11 give the sound power
level in the frequency range from 50 Hz to 10 kHz thus covering most of the low frequency range. In annex A of [1] it is recommended to use the standard down to 20 Hz in
case of significant low frequency noise. Thus the frequency range for the method is already covering most of the low frequency range which is defined as 10 Hz to 160 Hz. In
this EFP-06-project it is necessary to make measurements to at least 10 Hz and preferably
even lower.
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The measurement method has been developed in the 1980’s where wind turbines were becoming a natural part of the environment and the need for regulation arose. It is partly
based on references [2] and [3] and the ideas are taken from the Nordic Large Source
method [4] and [5]. All the methods are inspired by the ISO 3740 series. The general idea
behind the ISO 3740 methods is to measure in the far field avoiding local phenomena in
the noise radiation. In this respect it is assumed that far-field conditions are achieved at a
distance larger than the characteristic dimension d0 of a “reference” box, a rectangular box,
exactly enclosing the source in question.
The measurement surface is hemispherical and the microphone positions are at a horizontal distance of 0.8 times R, and the microphone height is 0.6 times R, where R is the radius
of the hemisphere. R is larger than d0.
The result is the sound power level of an equivalent point source. Horizontal directivity is
included in the far-field measurement methods. The methods give the sound power level in
1/1-octave from 31.5 Hz to 8000 Hz. This is the frequency range generally used when describing environmental noise and covers most of the low frequency range. No assumptions
are made on the frequency range in developing these standards.
2.2
The wind turbine measurement method - IEC 61400-11:2002 edition 2.1
The noise from a wind turbine changes with the wind speed. For that reason the noise and
the wind speed is measured in parallel and the noise is given as a function of the wind
speed. In the first version of the standard the wind speed was measured with an anemometer at 10 m height upwind from the wind turbine. This was later changed due to a poor correlation of the wind speed at 10 m height and at hub height for wind turbines with a hub
height above 50 m. The wind speed is now determined in part from the produced power
and a certified power versus wind curve and partly from the anemometer on the nacelle.
The wind speed is still measured at 10 m height when the background noise is measured,
as the power method does not apply when the wind turbines are parked.
The geometrical setup of the measurement method is shown in Figure 1. The measurement
distance d = hub height + half a rotor diameter.
The microphone is put on a board on the ground mounted with a half standard wind
screen. The ground board used is shown in Figure 5 and Figure 6 and has a diameter of 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.
To minimize influence due to the edges of the reflecting board on the measurement results,
it shall be ensured that the board is positioned flat on the ground. Any edges or gaps under
the board should be levelled out by means of soil. The inclination angle of the line from
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the rotor centre to the microphone shall be between 25° and 40° from horizontal. This may
require adjustment of the measurement distance within an allowed tolerance of ± 20%.
Figure 1
Measurement setup. From [1]
2.3
The microphone height and ground board
In the standards behind IEC 61400-11 the microphones are positioned at a fixed height
above the ground as mentioned in paragraph 2.1 and a ground correction is applied when
calculating the sound power level. This would imply that the microphone should be put at
a height of approximately 100 m with present day wind turbines.
This would make noise measurements on wind turbines very complicated and consequently only a small number of measurements would be made. Furthermore, wind noise
would increase in the measurement systems as the reference wind speed of 8 m/s at 10 m
height corresponds to a wind speed at a 100 m of more than 11 m/s. The signal to noise
ratio at the lowest and highest frequencies would be reduced significantly.
The microphone height that would be possible for most measuring institutes is 10 m which
still causes problems concerning the wind induced noise, without giving further information about the noise propagating to the neighbours than the microphone on the ground
board gives. Moving further away from the wind turbine to obtain the noise radiated in the
direction of the neighbours at another inclination angle to the nacelle is another possibility.
The distances would have to be increased from 100 - 150 m to at least 500 m to create a
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significant change in inclination angle to the wind turbine nacelle. At these distances the
background noise will most often be higher than the noise from the wind turbine at the
lowest and highest frequencies. This is assuming that the noise radiation does not increase
significantly at higher elevations.
On the ground board the wind induced noise is low as the wind speed is low close to the
ground. Even in this situation experience shows that the signal to noise ratio at the lowest
and highest frequencies can be relatively poor.
For practical reasons the ground board microphone is chosen knowing that a possible vertical directivity of the source is not taken into account. This is discussed further in paragraph 2.5 of this report.
Apart from reducing the wind induced noise in the measurement system the position on
the ground board also simplifies the ground influence as it is assumed to be + 6 dB at all
frequencies independent of the surrounding terrain. The ground board restricts the distances that can be applied due to the angle of incidence to the board.
In the IEA method from 1988 [2] a test of the reflection of the ground board is presented.
Measurements on a board on a lawn are compared to measurements in an anechoic chamber for different angles of incidence. The source is a reference sound power source B&K
4205. From 50 Hz to 8 kHz there is a maximum deviation of ± 2 dB relative to a +6 dB
reflection. The results are subject to some uncertainty as the microphone is moved from
the board to the anechoic chamber.
Similar conclusions can be found in [6] and [7] where it is also concluded that the optimal
position of the microphone is close to the centre of the board and that a variation of the
size of the board up to 2 m diameter does not change the reflection significantly.
From calculations with Nord2000 the ground board reflections are expected to be + 6 dB
within 1 dB up to 2000 Hz for a standard geometry used in measurements according to
IEC 61400-11. This is shown in Figure 2 where a calculation with and without the microphone board is compared. The Nord2000 model is giving exact results for a situation without impedance jumps like a grass field. It can be seen that for the 2 situations the ground
effect is almost the same and close to +6 dB below 100 Hz. Above 100 Hz the ground effect for the grass field drops while the grass field with the microphone remains close to +6
dB up to 3 kHz where it drops below +5 dB.
The Nord2000 model is described in detail in another part of this project [10].
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Ground effect
IEC61400-11 geometry
7
6
5
4
[dB]
3
Grass
2
Grass and board
1
10000
6300
4000
2500
1600
1000
630
400
250
160
100
63
40
25
16
-1
10
0
-2
-3
1/3-octave [Hz]
Figure 2
Ground effect for the measurement geometry for wind turbine noise measurements on a
grass field with and without the microhone board according to Nord2000
2.4
Horizontal directivity
The variation in emitted noise around the wind turbine is tested through measurements at 3
extra positions distributed as shown in Figure 3.
The measurements in these non-reference positions shall be made simultaneously with corresponding measurements in the reference position. The measurements in the three nonreference positions may be made individually, but each one shall be made simultaneously
with measurement in the reference position. No measurements are made in the rotor plane
as it is well known that the noise radiation is lower in this direction. Measurements in the
non-reference positions are optional according to the standard, but will be carried out for
the majority of the measurements in this project to investigate the horizontal directivity of
the turbines included in the survey of this project.
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Wind direction
3
Tower vertical
centerline
Ro
60°
60°
2
4
1
Optional measuring
positions
Reference position 1
IEC 3194/02
Figure 3
Plan view showing the measurement positions. From [1]
2.5
Vertical directivity
In [3] the vertical directivity was investigated for a small wind turbine with a hub height of
22.5 m. The conclusion then was that there were variations up to 5 dB in single octave
bands and that the highest sound power levels were achieved at angles of incidence from
30° to 50° to horizontal. Variations for octave band 63 Hz for angles 30 and 10 were found
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to be within 2 dB at a wind speed of 8 m/s. The allowed range in IEC 61400-11:2002 ed.
2.1 for the angle of incidence during measurement is 25° to 40°.
Present day wind turbines are different from then and the primary noise is now aerodynamic noise from the rotor, but the directivity is expected to be less pronounced at lower
frequencies due to the increasing wavelength. The fact that no significant low frequency
horizontal directivity was found in the measurements made in this project supports this
evaluation.
No recent results discussing the vertical directivity of a wind turbine are available and the
results from the measurements on the ground board is assumed to be representative for the
noise propagating to the neighbours.
2.6
Wind induced noise
As the ground board simplifies the ground reflection and no arguments have been found
behind the standard that excludes low frequencies, the main problem is low frequency
pressure variations at the microphone position caused by the wind. Usually a 20 Hz high
pass filter and a half standard wind screen is sufficient to deal with this.
It is important that the wind shield is fitted carefully to avoid wind between the wind
screen and the board. This becomes even more important if the high pass filter is not applied. From experience it is possible to make ground board measurements without extra
protection against the wind even at wind speeds of 10 – 15 m/s at 10 m height. However
even at moderate wind speed transient wind burst can result in overload of the measurement system.
Given below is a general description taken from [1] of a secondary wind screen that can be
applied to reduce wind induced noise in the microphone.
The secondary windscreen may be used when it is necessary to obtain an adequate signal-to-noise ratio at low frequencies in high winds. For example, it could
consist of a wire frame of approximate hemispherical shape, at least 450 mm in
diameter, which is covered with a 13 mm to 25 mm layer of open cell foam with a
porosity of 4 to 8 pores per 10 mm.
This secondary hemispherical windscreen shall be placed symmetrically over the
smaller primary windscreen.
If the secondary windscreen is used, the influence of the secondary windscreen
on the frequency response must be documented and corrected for.
From the JOULE project JOR3-CT95-0065 [8] DELTA has experience with different types
of wind screens. An almost spherical wire frame with a cover of Rycote Windjammer
cloth was used for noise measurements from wind farms at heights around 1.6 m above
terrain with good results. The investigated frequency range was from 20 Hz to 10 kHz.
The wind screen is shown in Figure 4.
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Dimitris Theofiloyannakos has as part of the JOULE project investigated the insertion loss
and the wind induced noise reduction of different types of secondary wind screens for free
field measurements. The results are published in [9] and shows that the “Large Rycote”
wind screen gives a significant reduction of the wind induced noise of approximately 20
dB between 50 Hz and 100 Hz for wind speeds of 8 m/s. Both the wind and noise measurements were made 1.9 m above the ground in a quiet area in Greece.
This design of a secondary wind screen was better than the rest of the tested wind screens
at reducing the wind induced noise, but also had a larger insertion loss.
If a secondary wind screen is used an improvement of the signal to wind induced noise
ratio can be expected especially at low frequencies based on the experiences from the
JOULE project JOR3-CT95-0065 [8]
Figure 4
Immission measurements with a spherical secondary windscreen
A hemispherical wind screen based on the experiences from the JOULE project has been
made in the framework of this project and tested in measurements. No high pass filtration
was used during measurements meaning that data from about 4 Hz is available. So far
noise measurements on wind turbines have been made up to average wind speeds over 1
minute of 16 - 18 m/s. The wind screen is shown in Figure 5 and Figure 6. The wire frame
is heavy enough to be kept in position and no extra fixation is necessary. As the wind in-
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duced noise at the ground is low the improvement in signal to wind induced noise ratio is
not improved at the same rate as for the spherical wind screen used at higher positions.
Figure 5
DELTA H wind screen with Reinhardt cloth. The wire frame can be seen.
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Figure 6
DELTA H wind screen used in the project with a Reinhardt cloth
No measurements of the wind induced noise reduction have been made in the project, but
the increased signal to wind induced noise ratio is indicated in Figure 7 where results from
measurements carried out in another part of this project with a secondary wind screen is
shown. Usually the background noise is dominating from around 50 Hz, but in this case
results can be achieved down to 25 Hz.
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Sound Pressure Level
9 m/s
50
40
20
10
Total Noise
0
10
00
20
00
40
00
80
00
50
0
25
0
12
5
63
32
16
Background Noise
8
-10
4
LpA [dB re 20uPa]
30
-20
-30
-40
-50
Frequency [Hz]
Figure 7
Typical results from measurements where a secondary wind screen is used. The background noise is dominating below 20 Hz and has a strong influence at and above 4 kHz
2.7
Calibration of a secondary wind screen for wind turbine testing
The secondary wind screen DELTA type H has an insertion loss that has to be measured
and corrected for. A procedure for measuring the insertion loss valid for wind turbine testing is described in Annex A. In Annex B specific insertion loss values for the wind screen
are tabulated.
The procedure describes how to measure the insertion loss for a secondary wind screen
using a measurement geometry corresponding to the wind turbine noise measurement
situation using a loudspeaker as a sound source.
2.8
Air absorption
In [3] it is stated that correction for ground and air absorption should be included in calculation of the sound power level of the source, but the air absorption is not included according to [1] leading to an underestimation of the sound power level at frequencies above 1
kHz. The effect is less than 1 dB on the A-weighted sound power level.
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The signal to noise ratio is poor at frequencies above 4 kHz, meaning that the noise measured at these frequencies is often dominated by the background noise and it is not possible
to include the air absorption at these frequencies when calculating the sound power level.
This can be seen from the measurements carried out in this project, and an example is
shown in Figure 7. The problem is discussed at the moment in an ongoing revision of the
measurement standard. In relation to this project primarily concerned with low frequency
noise this is of minor concern. The predicted noise at neighbours is dominated by frequencies below 2 kHz, and would be so even if the air absorption was included.
2.9
Background noise
In IEC 61400-11 the 1/3-octave spectra are determined from a small number of 1 minute
averages at each wind speed. The procedure is:
“The one-third octave band spectrum of the noise from the wind turbine in the reference
position shall be determined as the energy average of at least three spectra, each measured
over at least 1 min at each integer wind speed. As a minimum, one-third octave bands with
centre frequencies from 50 Hz to 10 kHz, inclusive, shall be measured.
Background measurements with the wind turbine stopped shall satisfy the same requirements”
Correction for background noise in 1/3-octave bands is made as follows:
“Using the methods specified in the relevant following paragraphs 8.3 to 8.5 (of IEC
61400-11:2002 ed. 2.1), all measured sound pressure levels shall be corrected for the influence of background noise. For average background sound pressure levels that are 6 dB
or more below the combined level of the wind turbine and background, the corrected value
can be obtained using the following equation:
⎡ (0,1 Ls+n )
− 10 (0,1 Ln ) ⎤⎥
L s = 10 lg ⎢10
⎣
⎦
(8)
where
LS
is the equivalent continuous sound pressure level, in dB, of the wind turbine operating alone;
LS+n is the equivalent continuous sound pressure level, in dB, of the wind turbine plus
background noise;
Ln
is the background equivalent continuous sound pressure level, in dB.
If the equivalent continuous sound pressure level of the wind turbine plus background
noise, LS+n, is less than 6 dB but more than 3 dB higher than the background level, LS+n is
corrected by subtraction of 1.3 dB, but the corrected data points are marked with an asterisk, “ * ”. These data points shall not be used for the determination of the apparent sound
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power level or directivity. If the difference is less than 3 dB, no data points shall be reported, but it shall be reported that the wind turbine noise was less than the background
noise.”
2.10
Instrumentation used for carrying out measurements
In IEC 61400-11 ed. 2.1 the following is stated about instruments for acoustic measurements:
Acoustic instruments
The following equipment is necessary to perform the acoustic measurements as
set forth in this standard.
Equipment for the determination of the equivalent continuous A-weighted sound
pressure level
The equipment shall meet the requirements of a type 1 sound level meter according to IEC 60804. The diameter of the microphone shall be no greater than
13 mm.
Equipment for the determination of one-third octave band spectra
In addition to the requirements given for type 1 sound level meters, the equipment shall have a constant frequency response over at least the 45 Hz to
11 200 Hz frequency range. The filters shall meet the requirements of IEC 61260
for Class 1 filters.
The equivalent continuous sound pressure levels in one-third octave bands shall
be determined simultaneously with centre frequencies from 50 Hz to 10 kHz. It
may be relevant to measure the low-frequency noise emission of a wind turbine.
In such cases, a wider frequency range is necessary as discussed in Annex A.
Equipment for the determination of narrow band spectra
The equipment shall fulfil the relevant requirements for IEC 60651 type 1 instrumentation in the 20 Hz to 11 200 Hz frequency range
In order to obtain consistency the demands given above have to be fulfilled. Further more,
the requirements for the frequency range have to be extended down to at least 3-4 Hz to
cover the low frequency region. Most of the infrasound region is covered as well.
Different strategies are used by different laboratories from a full processing of the measurements in the field over partly processing the data in the field and partly in the laboratory to just recording in the field and conducting all the analysis in the laboratory.
This means that different types of instruments are used. DELTA uses a PC-based system
with a full processing of the data in the field, and storage of all data on the PC making reprocessing possible. This ensures full synchronization of all parameters measured. This
type of system consists of microphones, preamplifiers, data acquisition cards with A/Dconverters and a laptop computer.
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The individual elements of the measurement chain as well as the entire measurement chain
must comply with the above requirements. In Table 1 is shown an example of the equipment used by DELTA in this project.
Equipment
Make
Type
Calibrator
Brüel & Kjær
4230
1/2” microphone
G.R.A.S.
40AE
Preamplifier
G.R.A.S.
26CA
Cup anemometer
Risø
P2546
Windvane
Vector Instruments
W200P
Measurement system
DELTA
Wind Turbine 2.0
Data Acquisation Card
National Instruments
NI 9233
Data Acquisation Card
National Instruments
NI 9215
Data Acquisation Card
National Instruments
NI 9215
Table 1
Example of measurement equipment used by DELTA
Another possibility is to use analyzers, sound level meters and recorders to record and do a
part processing in the field along with using analyzers e.g. FFT-analyzers in the laboratory. Care should be taken to ensure that the frequency range and dynamic range of the
equipment is sufficient.
Digital technique has made it possible to buy noise measurement equipment with a lot of
features at low prices. For that reason it is even more important to check if the instruments
comply with the requirements. Not all producers focus on measuring low frequency noise
and noise at low levels. Especially if a sound level meter is used as input to a recorder
(DAT or harddisc) it is important to check that the output level is high enough to make use
of the dynamic range of the recorder.
It has not been the intention to recommend specific instruments in this section but only to
give advice on the type of problems to be aware of.
Equipment for measuring wind speed and produced power does not depend on the frequency range under investigation and requirements can be found in [1].
2.11
Uncertainty
Guidelines on how to calculate the uncertainty on the reported sound power level are given
in the standard [1]. Typically the standard deviation on the average of the total noise in
each 1/3-octave band at the low frequencies varies between 1 and 3 dB. This applies to
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the background noise as well. Including the uncertainty from the measurement equipment
the total uncertainty on the background noise corrected sound power spectra can easily be
of the order of magnitude of 5 dB at the lowest frequencies. At frequencies where the
background noise is less than 3 dB below the total noise the total noise level is most often
used and the uncertainty cannot be estimated.
3.
Recommendation
The purpose of the noise measurements is to deliver sound power levels to be used in
noise predictions. Measurement methods using free field measurements at some height
above the ground introduces problems with wind induced noise, ground effects and background noise in the effort of finding the sound power level relevant for the neighbours.
The background noise problems are expected to be worst at frequencies below 50 Hz and
above 4 kHz.
The general recommendation is therefore to use the IEC 61400-11:2002 ed. 2.1 [1] and a
secondary wind screen when measuring at low frequencies and/or at high wind speeds.
Different types of wind screens can be used as long as the insertion loss is known and corrected for.
The apparent Sound Power Level, LW,oct,c of the wind turbine as an equivalent point source
is calculated at the integer wind speeds, c, from the measurements as follows in each 1/3octave band:
L W,oct ,c
= Leq,oct,c
⎡ 4 π R2 ⎤
1 ⎥
− 6 + 10 lg⎢
⎢ S ⎥
⎢⎣ 0 ⎥⎦
Equation 1
where
Leq,oct,c is the background noise corrected sound pressure level in 1/3-octaves at the integer wind speeds and under reference conditions;
R1
is the slant distance in meters from the rotor centre to the microphone and
S0
is a reference area, S0 = 1 m2.
The 6 dB constant in Equation 1 accounts for the approximate pressure doubling that occurs for the sound level measurements on a ground board.
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To investigate horizontal directivity of the wind turbine additional ground measurement
positions can be applied according to [1].
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References
[1]
IEC 61400-11:2002 Wind turbine generator systems – Part 11:Acoustic noise measurement techniques.
[2]
IEA Expert Group Study on Recommended Practices for Wind Turbine Testing and
Evaluation, 4. Acoustics “Measurement of Noise Emission from Wind Turbines”.
2.ed 1988.
[3]
Jakobsen J., Andersen B. “Noise Emission from Wind Turbine Generators – A
measurement method” Danish Acoustical Institute. Report No 109 1983
[4]
Jonasson H.,Eslon L. “Determination of sound power levels of external noise
sources” Statens Provningsanstalt. Technical Report SP-RAPP 1981:45
[5]
Lahti T., Tuominen H. “Measurement procedure for the emission of external noise
from large industrial sources.” Technical Research Centre of Finland 1982.
[6]
Nii Yoshinori: “Ground effects upon sound pressure levels on a board for wind turbine noise measurements” Proc. INTERNOISE 2000, Vol 6, pp. 3798-3802.
[7]
Nii Yoshinori: “Sound level distributions on a circular ground board for wind turbine noise measurements”. Noise Control Eng. J. 50 (3) , 2002 May-June
[8]
Jakobsen, Jørgen: AV 50/97 “Investigations of Wind Screens, Insertion Loss and
Attenuation of Wind Noise, Note 1, JOR3-CT95-0065” DELTA 1997.
[9]
Theofiloyannakos D, Kragh J, Fragoulis A. “Investigation of the reduction of the
wind induced microphone noise by the use of supplementary wind screens. EWEC
1997.
[10] Plovsing, Birger
Low Frequency Noise from Large Wind Turbines – Selection of a Propagation
Model. AV 1096/08. DELTA April 2008
DELTA
Acoustics & Electronics
AV 135/08
Page 22 of 26
Annex A Procedure for Calibration of Secondary Wind Screen
Calibration of the secondary wind screen DELTA type H used for the measurements of
noise on the ground plane must be calibrated so that insertion loss values are available for
correction of the measurement results.
The measurement setup is similar with regard to the angle of incidence of the sound to the
measurement board as the measurement situation for wind turbine noise measurements.
A test microphone and a reference microphone are put on two separate measurement
boards, placed side by side at a horizontal distance of 6 m from the loudspeaker. The loudspeaker is put on a stand at a height of 4 m. The loudspeaker height is varied by ± 20 %,
thereby simulating the allowed variation in inclination angle to be used for the measurements according to [1] (In the standard given as an allowable variation in measurement
distance). The loudspeaker emits pink noise, and is directed towards the two microphones.
The purpose of the reference microphone is to monitor the noise from the loudspeaker during the measurements, looking for variation in the noise emission. A half standard wind
screen is applied on each of the two microphones.
The secondary wind screen is applied to the test microphone. See Figure 8. Noise is emitted from a loudspeaker and the resulting sound pressure levels at the microphone positions
are recorded for 1 - 2 minutes. The secondary wind screen is removed from the test microphone and another recording is made. This is repeated 3 times. The background noise is
measured before and after these measurements. This procedure is repeated with 3 different
heights of the loudspeaker. 4.8 m, 4.0 m and 3.2 m. All measurements are made in 1/3octave bands.
DELTA
Acoustics & Electronics
AV 135/08
Page 23 of 26
Figure 8
Measurement board with microphone and secondary wind screen DELTA H fitted. Part of
the plate with the reference microphone can be seen to the left.
The insertion loss can then be determined as the level difference with and without the secondary wind screen as an arithmetic average for the 9 measurements. The standard deviation must be calculated as well. As the result is a small difference between high sound
pressure levels it is sensitive to small variations in the source output and the transfer function (from meteorology etc.) Therefore it is necessary to normalize the level differences
with respect to the variation in the levels measured with the control microphone.
The background noise in each 1/3-octaveband must be at least 3 dB below the noise with
the loudspeaker on. For 1/3-octave bands where this is not the case the insertion loss cannot be reported.
An example of the insertion loss is shown in Figure 9.
DELTA
Acoustics & Electronics
AV 135/08
Page 24 of 26
Insertion loss
Average of 3 heights
4
2
1
63
00
80
00
10
00
0
50
00
40
00
31
50
25
00
20
00
16
00
12
50
80
0
10
00
63
0
50
0
40
0
31
5
25
0
20
0
12
5
16
0
80
10
0
63
50
40
25
31
.5
0
20
Lp,screen-Lp,noscreen [dB re 20 uPa]
3
-1
-2
Figure 9
Example of insertion loss for a secondary DELTA H wind shield.
Details from the measurements can be found in Figure 10 to Figure 12.
Insertion loss 4.0 m
6
4
3
Measurement 1
Measurement 2
Measurement 3
Average
2
1
80
10
0
12
5
16
0
20
0
25
0
31
5
40
0
50
0
63
0
80
0
10
00
12
50
16
00
20
00
25
00
31
50
40
00
50
00
63
00
80
00
10
00
0
63
40
50
25
31
.5
0
20
Lp,screen-Lp,noscreen [dB re 20 uPa]
5
-1
-2
Figure 10
Insertion loss measurements for height 4.0 m
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Acoustics & Electronics
AV 135/08
Page 25 of 26
Insertion loss 3.2 m
6
4
3
Measurement 1
Measurement 2
Measurement 3
Average
2
1
63
80
10
0
12
5
16
0
20
0
25
0
31
5
40
0
50
0
63
0
80
0
10
00
12
50
16
00
20
00
25
00
31
50
40
00
50
00
63
00
80
00
10
00
0
40
50
25
31
.5
0
20
Lp,screen-Lp,noscreen [dB re 20 uPa]
5
-1
-2
Figure 11
Insertion loss measurements for height 3.2 m
Insertion loss 4.8 m
6
4
3
Measurement 1
Measurement 2
Measurement 3
Average
2
1
63
80
10
0
12
5
16
0
20
0
25
0
31
5
40
0
50
0
63
0
80
0
10
00
12
50
16
00
20
00
25
00
31
50
40
00
50
00
63
00
80
00
10
00
0
40
50
25
31
.5
0
20
Lp,screen-Lp,noscreen [dB re 20 uPa]
5
-1
-2
Figure 12
Insertion loss measurements for height 4.8 m
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Acoustics & Electronics
AV 135/08
Page 26 of 26
Annex B Insertion Loss Values for the Secondary Wind Screen
DELTA type H as an average of all measurements
Frequency
1/3 octave band, [Hz]
Insertion Loss [dB]
Standard deviation [dB]
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
0.1
0.2
0.1
0.1
0.1
0.0
0.2
0.2
0.1
-0.1
-0.3
0.0
0.3
0.6
1.2
1.7
1.7
0.7
1.3
1.7
1.6
2.3
2.6
2.1
0.8
-0.1
0.7
1.6
0.2
0.4
0.2
0.2
0.3
0.3
0.2
0.1
0.1
0.2
0.3
0.2
0.2
0.2
0.2
0.2
0.4
0.3
0.7
0.5
0.5
0.4
0.6
0.9
1.3
0.7
1.0
1.9
Insertion loss values given in 1/3-octaveband for secondary windscreen DELTA type H
measured according to Annex A.