IEC 61400-11:2002

INTERNATIONAL IEC
STANDARD
61400-11
Second edition
2002-12
Wind turbine generator systems –
Part 11:
Acoustic noise measurement techniques
Aérogénérateurs –
Partie 11:
Techniques de mesure du bruit acoustique
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INTERNATIONAL IEC
STANDARD
61400-11
Second edition
2002-12
Wind turbine generator systems –
Part 11:
Acoustic noise measurement techniques
Aérogénérateurs –
Partie 11:
Techniques de mesure du bruit acoustique
 IEC 2002  Copyright - all rights reserved
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– 2 – 61400-11  IEC:2002(E)
CONTENTS
FOREWORD . 4
INTRODUCTION .5
1 Scope . 6
2 Normative references. 6
3 Definitions . 7
4 Symbols and units . 8
5 Outline of method . 9
6 Instrumentation.10
6.1 Acoustic instruments.10
6.2 Non-acoustic Instruments .11
6.3 Traceable calibration .12
7 Measurements and measurement procedures .12
7.1 Measurement positions.12
7.2 Acoustic measurements.13
7.3 Non-acoustic measurements.15
8 Data reduction procedures.17
8.1 Wind speed .17
8.2 Correction for background noise .18
8.3 Apparent sound power levels.18
8.4 One-third octave band levels .19
8.5 Tonality .19
8.6 Directivity (optional).22
9 Information to be reported.23
9.1 Characterisation of the wind turbine.23
9.2 Physical environment.24
9.3 Instrumentation.24
9.4 Acoustic data.24
9.5 Non-acoustic data.25
9.6 Uncertainty .25
Annex A (informative) Other possible characteristics of wind turbine noise emission
and their quantification.35
Annex B (informative) Criteria for recording/playback equipment.37
Annex C (Informative) Assessment of turbulence intensity .39
Annex D (informative) Assessment of measurement uncertainty.40
Bibliography.43
Figure 1 – Mounting of the microphone .26
Figure 2 − Picture of microphone and board .27
Figure 3 − Standard pattern for microphone measurement positions (plan view).28
Figure 4 − Illustration of the definitions of R and slant distance R .29
0 1
61400-11  IEC:2002(E) – 3 –
Figure 5 − Allowable region for meteorological mast position as a function of β –
plan view.30
Figure 6 − Allowable range for anemometer height – cross section .31
Figure 7 – Workflow chart for tonality procedure .32
Figure 8 – Illustration of L level in the critical band .33
70 %
Figure 9 – Illustration of lines below the L + 6dB criterion .33
70 %
Figure 10 – Illustration of L level and lines classified as masking.34
pn,avg
Figure 11 – Illustration of classifying all spectral lines .34
Figure B.1 − Tolerances for frequency characteristic, IEC 60651 type 1 .37
Table 1 − Roughness length.18
Table 2 − Frequency resolution .19
Table D.1 − Examples of possible values of type B uncertainty components relevant for
apparent sound power level .41

– 4 – 61400-11  IEC:2002(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND TURBINE GENERATOR SYSTEMS –
Part 11: Acoustic noise measurement techniques
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International
Organization for Standardization (ISO) in accordance with conditions determined by agreement between the
two organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61400-11 has been prepared by IEC technical committee 88: Wind
turbine systems.
This second edition of IEC 61400-11 cancels and replaces the first edition published in 1998
and constitutes a technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
88/166/FDIS 88/171/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged
until 2004. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
61400-11  IEC:2002(E) – 5 –
INTRODUCTION
The purpose of this part of IEC 61400 is to provide a uniform methodology that will ensure
consistency and accuracy in the measurement and analysis of acoustical emissions by wind
turbine generator systems. The standard has been prepared with the anticipation that it would
be applied by:
• the wind turbine manufacturer striving to meet well defined acoustic emission performance
requirements and/or a possible declaration system;
• the wind turbine purchaser in specifying such performance requirements;
• the wind turbine operator who may be required to verify that stated, or required, acoustic
performance specifications are met for new or refurbished units;
• the wind turbine planner or regulator who must be able to accurately and fairly define
acoustical emission characteristics of a wind turbine in response to environmental
regulations or permit requirements for new or modified installations.
This standard provides guidance in the measurement, analysis and reporting of complex
acoustic emissions from wind turbine generator systems. The standard will benefit those
parties involved in the manufacture, installation, planning and permitting, operation,
utilization, and regulation of wind turbines. The measurement and analysis techniques
recommended in this document should be applied by all parties to insure that continuing
development and operation of wind turbines is carried out in an atmosphere of consistent and
accurate communication relative to environmental concerns. This standard presents
measurement and reporting procedures expected to provide accurate results that can be
replicated by others.
– 6 – 61400-11  IEC:2002(E)
WIND TURBINE GENERATOR SYSTEMS –
Part 11: Acoustic noise measurement techniques
1 Scope
This part of IEC 61400 presents measurement procedures that enable noise emissions of a
wind turbine to be characterised. This involves using measurement methods appropriate to
noise emission assessment at locations close to the machine, in order to avoid errors due to
sound propagation, but far enough away to allow for the finite source size. The procedures
described are different in some respects from those that would be adopted for noise
assessment in community noise studies. They are intended to facilitate characterisation of
wind turbine noise with respect to a range of wind speeds and directions. Standardisation of
measurement procedures will also facilitate comparisons between different wind turbines.
The procedures present methodologies that will enable the noise emissions of a single wind
turbine to be characterised in a consistent and accurate manner. These procedures include
the following:
• location of acoustic measurement positions;
• requirements for the acquisition of acoustic, meteorological, and associated wind turbine
operational data;
• analysis of the data obtained and the content for the data report; and
• definition of specific acoustic emission parameters, and associated descriptors which are
used for making environmental assessments.
The standard is not restricted to wind turbines of a particular size or type. The procedures
described in this standard allow for the thorough description of the noise emission from a wind
turbine. If, in some cases, less comprehensive measurements are needed, such measure-
ments are made according to the relevant parts of this standard.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60386:1972, Method of measurement of speed fluctuations in sound recording and
reproducing equipment
IEC 60651:1979, Sound level meters
IEC 60688:1992, Electrical measuring transducers for converting a.c. electrical quantities to
analogue or digital signals
IEC 60804:2000, Integrating-averaging sound level meters
IEC 60942:1997, Electroacoustics – Sound calibrators
IEC 61260:1995, Electroacoustics – Octave-band and fractional-octave-band filters
IEC 61400-12:1998, Wind turbine generator systems – Part 12: Wind turbine power
performance testing
61400-11  IEC:2002(E) – 7 –
3 Definitions
For the purposes of this standard, the following definitions apply:
3.1
apparent sound power level
L (in dB re. 1 pW)
WA
the A-weighted sound power level re. 1 pW of a point source at the rotor centre with the same
emission in the downwind direction as the wind turbine being measured, L is determined at
WA
each wind speed integer from 6 to 10 m/s
3.2
audibility criterion
L (in dB re. 20 µPa)
a
a frequency dependent criterion curve determined from listening tests, and reflecting the
subjective response of a ‘typical’ listener to tones of different frequencies
3.3
A-weighted or C-weighted sound pressure levels
L or L , respectively (in dB re. 20 µPa)
A C
sound pressure levels measured with the A or C frequency weighting networks specified in
IEC 60651
3.4
directivity
ΔΔΔΔ (in dB)
i
the difference between the A-weighted sound pressure levels measured at measurement
positions 2, 3, and 4 and those measured at the reference position 1 from the turbine
corrected to the same distance from the wind turbine rotor centre
3.5
inclination angle
φφ (in °)
φφ
the angle between the plane of the microphone board and a line from the microphone to the
rotor centre
3.6
reference distance
R (in m)
the nominal horizontal distance from the centre of the base of the wind turbine to each of the
prescribed microphone positions
3.7
reference height
z (in m)
ref
a height of 10 m used for converting wind speed to reference conditions
3.8
reference roughness length
z (in m)
0ref
a roughness length of 0,05 m used for converting wind speed to reference conditions
3.9
sound pressure level
L (in dB re. 20 µPa)
p
10 times the log of the ratio of the mean-square sound pressure to the square of the
reference sound pressure of 20 µPa

– 8 – 61400-11  IEC:2002(E)
3.10
standardized wind speed
−1
V (in ms )
s
wind speed converted to reference conditions (height 10 m and roughness length 0,05 m)
using a logarithmic profile
3.11
tonal audibility ΔΔΔΔL (in dB)
a,k
The difference between the tonality and the audibility criterion at integer wind speeds k = 6, 7,
8, 9, 10
3.12
tonality ΔΔΔΔL (in dB)
k
the difference between the tone level and the level of the masking noise in the critical band
around the tone at integer wind speeds k = 6, 7, 8, 9, 10
4 Symbols and units
D rotor diameter (horizontal axis turbine) or equatorial diameter (vertical
axis turbine) (m)
H height of rotor centre (horizontal axis turbine) or height of rotor equatorial
plane (vertical axis turbine) above local ground near the wind turbine (m)
L or L A or C-weighted sound pressure level (dB)
A C
L equivalent continuous A-weighted sound pressure level at each integer
Aeq,k
wind speed, where k = 6, 7, 8, 9, 10 (dB)
L equivalent continuous A-weighted sound pressure level corrected for
Aeq,c,k
background noise at each integer wind speed and corrected to reference
conditions, where k = 6, 7, 8, 9, 10 (dB)
L equivalent continuous A-weighted sound pressure level in position ‘i ’
Aeq,i
corrected for background noise where i = 1, 2, 3, or 4 (dB)
L equivalent continuous sound pressure level of the background noise (dB)
n
th
L sound pressure level of masking noise within a critical band in the ‘j ’
pn,j,k
th
spectra at the ‘k ’ wind speed, where j = 1 to 12 and k = 6, 7, 8, 9, 10 (dB)
L average of analysis bandwidth sound pressure levels of masking in the
pn,avg,j,k
th th
‘j ’ spectra at the ‘k ’ wind speed, where j = 1 to 12 and k = 6, 7, 8, 9,
10 (dB)
th th
L sound pressure level of the tone or tones in the ‘j ’ spectra at the ‘k ’
pt,j,k
wind speed, where j = 1 to 12 and k = 6, 7, 8, 9, 10 (dB)
L equivalent continuous sound pressure level of only wind turbine noise (dB)
s
L equivalent continuous sound pressure level of combined wind turbine and
s+n
background noise (dB)
L apparent sound power level, where k = 6, 7, 8, 9, 10 (dB)
WA,k
P measured electric power (W)
m
P normalised electric power (W)
n
61400-11  IEC:2002(E) – 9 –
R slant distance, from rotor centre to actual measurement position ‘i’,
i
where i = 1, 2, 3, or 4 (m)
R reference distance (m)
2 2
S reference area, S = 1 m (m )
0 0
T air temperature (C)
C
T air temperature (K)
K
U , U uncertainty components (dB)
A B
V wind speed at hub height, H (m/s)
H
V derived wind speed from power curve (m/s)
D
V wind speed at height, z (m/s)
z
V standardized wind speed (m/s)
s
f frequency of the tone (Hz)
f centre frequency of critical band (Hz)
c
p atmospheric pressure (kPa)
z roughness length (m)
z reference roughness length, 0,05 m (m)
0ref
z anemometer height (m)
z reference height for wind speed, 10 m (m)
ref
β angle used to define allowable area for anemometer mast location (°)
κ the ratio of standardised wind speed and measured wind speed
th
Δ directivity at ‘i ’ position, where i = 2, 3, or 4 (dB)
i
th th
Δ tonality of the ‘j ’ spectra at ‘k ’ wind speed, where j = 1 to 12 and
Ltn,j,k
k = 6, 7, 8, 9, 10 (dB)
φ inclination angle (°)
5 Outline of method
This Part of IEC 61400 defines the procedures to be used in the measurement, analysis and
reporting of acoustic emissions of a wind turbine. Instrumentation and calibration
requirements are specified to ensure accuracy and consistency of acoustic and non-acoustic
measurements. Non-acoustic measurements required defining the atmospheric conditions
relevant to determining the acoustic emissions are also specified. All parameters to be
measured and reported are identified, as are the data reduction methods required for
obtaining these parameters.
Application of the method described in this International Standard provides the apparent A-
weighted sound power levels, spectra, and tonality at integer wind speeds from 6 to 10 m/s of
an individual wind turbine. Optionally, directivity may also be determined.

– 10 – 61400-11  IEC:2002(E)
The measurements are made at locations close to the turbine in order to minimise the
influence of terrain effects, atmospheric conditions or wind-induced noise. To account for the
size of the wind turbine under test, a reference distance R based on the wind turbine
o
dimensions is used.
Measurements are taken with a microphone positioned on a board placed on the ground to
reduce the wind noise generated at the microphone and to minimise the influence of different
ground types.
Measurements of sound pressure levels and wind speeds are made simultaneously over short
periods of time and over a wide range of wind speeds. The measured wind speeds are
converted to corresponding wind speeds at a reference height of 10 m and a reference
roughness length of 0,05 m. The sound levels at standardized wind speeds of 6, 7, 8, 9, and
10 m/s are determined and used for calculating the apparent A-weighted sound power levels.
The directivity is determined by comparing the A-weighted sound pressure levels at three
additional positions around the turbine with those measured at the reference position.
Informative annexes are included that cover
• other possible characteristics of wind turbine noise emission and their quantification
(Annex A);
• criteria for recording/playback equipment (Annex B);
• assessment of turbulence intensity (Annex C);
• assessment of measurement uncertainty (Annex D).
6 Instrumentation
6.1 Acoustic instruments
The following equipment is necessary to perform the acoustic measurements as set forth in
this standard.
6.1.1 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.
6.1.2 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.
6.1.3 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.

61400-11  IEC:2002(E) – 11 –
6.1.4 Microphone with measurement board and windscreen
The microphone shall be mounted at the centre on a flat hard board with the diaphragm of the
microphone in a plane normal to the board and with the axis of the microphone pointing
towards the wind turbine, as in Figures 1 and 2. The board shall be circular with a diameter of
at least 1,0 m and made from material that is acoustically hard, such as plywood or hard chip-
board with a thickness of at least 12,0 mm or metal with a thickness of at least 2,5 mm.
A larger board is recommended especially for soft ground. In the exceptional case that the
board is split (i.e. not in one piece) there are considerations; the pieces shall be level within
the same plane, the gap less than 1 mm, and the split must be off the centre line and parallel
with the microphone axis as shown in Figure 1a).
The windscreen to be used with the ground-mounted microphone shall consist of a primary
and, where necessary, a secondary windscreen. The primary windscreen shall consist of one
half of an open cell foam sphere with a diameter of approximately 90 mm, which is centred
around the diaphragm of the microphone, as in Figure 2.
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.
6.1.5 Acoustical calibrator
The complete sound measurement system, including any recording, data logging or
computing systems, shall be calibrated immediately before and after the measurement
session at one or more frequencies using an acoustical calibrator on the microphone. The
calibrator shall fulfil the requirements of IEC 60942 class 1, and shall be used within its
specified environmental conditions.
6.1.6 Data recording/playback systems
A data recording/playback system is a required part of the measurement instrumentation, and
the entire chain of measurement instruments shall fulfil the relevant requirements of
IEC 60651, for type 1 instrumentation. Examples are given in Annex B.
6.2 Non-acoustic Instruments
The following equipment is necessary to perform the non-acoustic measurements set forth in
this standard.
6.2.1 Anemometers
The anemometer and its signal processing equipment shall have a maximum deviation from
the calibration value of ±0,2 m/s in the wind speed range from 4 m/s to 12 m/s. It shall be
capable of measuring the average wind speed over time intervals synchronized with the
acoustic measurements.
6.2.2 Electric power transducer
The electric power transducer, including current and voltage transformers, shall meet the
accuracy requirements of IEC 60688 Class 1.

– 12 – 61400-11  IEC:2002(E)
6.2.3 Wind direction transducer
The wind direction transducer shall be accurate to within ±6°.
6.2.4 Other instrumentation
A camera and instruments to measure distance are required. The temperature shall be
measured with an accuracy of ±1°C. The atmospheric pressure shall be measured with an
accuracy of ±1 kPa.
6.3 Traceable calibration
The following equipment shall be checked regularly and be calibrated with traceability to a
national or primary standards laboratory. The maximum time from the last calibration shall be
as stated for each item of equipment:
• acoustic calibrator (12 months);
• microphone (24 months);
• integrating sound level meter (24 months);
• spectrum analyzer (36 months);
• data recording/playback system (24 months);
• anemometer (24 months);
• electric power transducer (24 months).
If the acoustic calibrator is calibrated as a part of the integrating sound level meter, the
maximum calibration interval may be extended to 24 months.
An instrument shall always be recalibrated if it has been repaired or is suspected of fault or
damage.
7 Measurements and measurement procedures
7.1 Measurement positions
To fully characterize the noise emission of a wind turbine, the following measurement
positions are required.
7.1.1 Acoustic measurement positions
One, and optionally another three, microphone positions are to be used. The four positions
shall be laid out in a pattern around the vertical centreline of the wind turbine tower as
indicated in the plan view shown in Figure 3. The required downwind measurement position is
identified as the reference position, as shown in Figure 3. The direction of the positions shall
be accurate within ±15° relative to the wind direction at the time of measurement. The
horizontal distance R from the wind turbine tower vertical centreline to each microphone
position shall be as shown in Figure 3, with a tolerance of 20 % and shall be measured with
an accuracy of ±2 %.
As shown in Figure 4a), the reference distance R for horizontal axis turbines is given by:
D
R = H + (1)
where
H is the vertical distance from the ground to the rotor centre; and
D is the diameter of the rotor.

61400-11  IEC:2002(E) – 13 –
As shown in Figure 4b), the reference distance R for vertical axis wind turbines is given by:
R = H + D (2)
where
H is the vertical distance from the ground to the rotor equatorial plane; and
D is the equatorial diameter.
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 φ, as shown in Figure 4,
shall be between 25° and 40°. This may require adjustment of the measurement position
within the tolerances stated above.
The measurement position shall be chosen so that the calculated influence from any reflecting
structures, such as buildings or walls, shall be less than 0,2 dB.
7.1.2 Wind speed and direction measurement positions
The test anemometer and wind direction transducer shall be mounted in the upwind direction
of the wind turbine at a height between 10 m and rotor centre. The transducers shall be
placed at a distance between 2D and 4D from the rotor centre. If method 2 (see 7.3.1.2) is
used to determine the wind speed, the allowable region in which the anemometer and wind
direction transducer shall be located is given in Figure 5.
The angle β is given by:
z − z
ref
(3)
β = ()β − β + β
max min min
H − z
ref
where
z is the anemometer height, see Figure 6;
z is the reference height of 10 m;
ref
H is the height of the rotor centre or equatorial plane of the wind turbine, see Figure 4;
o
β is the maximum angle for β , β = 90 ;
max max
o
β is the minimum angle for β , β = 30 .
min
min
During the course of the measurements, the test anemometer shall not be within the wake of
any portion of any other wind turbine rotor or other structure. The wake of a wind turbine shall
be considered to extend 10 rotor diameters downwind of the wind turbine. The wind speed
and wind direction transducers shall be placed so that they do not interfere with each other.
7.2 Acoustic measurements
The acoustic measurements shall permit the following information to be determined about the
noise emission from the wind turbine at the integer wind speeds 6, 7, 8, 9 and 10 m/s (wind
speed at 10 m height and roughness length of 0,05 m):
– the apparent sound power level;
– the one-third octave band levels;
– the tonality.
Optional measurements may include directivity, infrasound, low-frequency noise and
impulsivity.
– 14 – 61400-11  IEC:2002(E)
7.2.1 Acoustic measurement requirements
For all acoustic measurements, the following requirements are valid:
• The complete measurement chain shall be calibrated at least at one frequency before and
after the measurements, or if the microphones are disconnected during repositioning.
• All acoustical signals must be recorded and stored for later analysis.
• Periods with intruding intermittent background noise (as from aircraft) shall be omitted.
• With the wind turbine stopped, and using the same measurement set-up, the background
noise shall be measured immediately before or after each measurement series of wind
turbine noise and during similar wind conditions. When measuring background noise,
every effort shall be made to ensure that the background sound measurements are
representative of the background noise that occurred during the wind turbine noise
emission measurements
• The measurements shall cover as broad a range of wind speeds as practically possible.
To obtain a sufficient range of wind speeds it may be necessary to take the measurements
in several measurement series.
Additionally, the following requirements are valid for the individual acoustic measurements.
7.2.2 Acoustic measurements at the reference position 1
7.2.2.1 A-weighted sound pressure level
The equivalent continuous A-weighted sound pressure level of the noise from the wind turbine
shall be measured at the reference position by a series of at least 30 measurements
concurrent with measurements of the wind speed. Each measurement shall be integrated over
a period of not less than 1 min. At least three measurements shall be within ±0,5 m/s at each
integer wind speed.
For the background noise at least 30 measurements in total shall be made, covering
corresponding ranges of wind speed as above.
7.2.2.2 One-third octave band measurements
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.
7.2.2.3 Narrow band measurements
For each integer wind speed, at least two minutes of wind turbine noise and background noise
are required. These two minutes shall be as close as possible to the integer wind speeds.
7.2.3 Optional acoustic measurements at positions 2, 3 and 4
The equivalent continuous A-weighted sound pressure level of the noise from the wind turbine
shall be measured in the non-reference position by one of the following two methods.
In the first (preferred) method, the measurements in the non-reference positions shall be
made simultaneously with corresponding measurements in the reference position. The
measurements in the three non-reference positions may be made individually, but each one
shall be made simultaneously with measurement in the reference position. The sound
pressure level at each position shall be determined as the energy average of five
measurements each integrated over at least 1 min. The five periods with an average wind
speed closest to 8 m/s shall be used.

61400-11  IEC:2002(E) – 15 –
The background noise measurements shall be energy averaged over five periods of at least
1 min.
In the second method, simultaneous measurements are not required. The equivalent
continuous A-weighted sound pressure level of the noise from the wind turbine in each of the
three non-reference positions shall be measured as a series of at least 10 measurements,
each energy averaged over at least 1 min concurrent with wind speed measurements. During
the measurements, the wind speed V shall differ by less than 2 m/s from 8 m/s, and at least
s
25 % of the measurements shall be above, and 25 % below, 8 m/s.
With the wind turbine stopped, at least 10 background measurements, each energy averaged
over at least 1 min, shall be obtained.
7.2.4 Other optional measurements
It is recommended that additional measurements be taken to quantify noise emissions that
have definite character that is not described by the measurement procedures detailed in this
standard.
Such character might be the emission of infrasound, low-frequency noise, low-frequency
modulation of broadband noise, impulses, or unusual sounds (such as a whine, hiss, screech
or hum), distinct impulses in the noise (for example bangs, clatters, clicks, or thumps), or
noise that is irregular enough in character to attract attention. These areas are discussed, and
possible quantitative measures outlined in Annex A. These measures are not universally
accepted and are given for guidance only.
7.3 Non-acoustic measurements
The following non-acoustic measurements shall be made.
7.3.1 Wind speed measurements
The wind speed shall be determined according to one of the following two methods. Method 1
is the preferred method and is mandatory for certification and declaration measurements.
7.3.1.1 Method 1: determination of the wind speed from
the electric output and the power curve
The power curve relates the power to the wind speed at hub height. For most wind turbines,
the wind speed can be determined from the measured electric power. Correlation between
measured sound level and measured electric power is very high up to the point of maximum
power.
The wind speed shall be obtained from measurements of the produced electric power using a
traceable power versus wind speed curve, preferably measured according to IEC 61400-12,
and preferably for the same turbine or, otherwise, for the same type of wind turbine with the
same components and adjustments. The power curve shall give the relation between the wind
speed at hub height and the electric power that the turbine produces for standard atmospheric
conditions of 15 °C and 101,3 kPa.
Electric power shall be averaged over the same period as the noise measurements.
The use of power measurements and the wind turbine power curve is the preferred method of
wind speed determination, provided the wind turbine operates below the maximum power
point during the noise measurement series. However, note that during background noise
measurements, the wind speed must be measured with an anemometer at a height of
at least 10 m.
– 16 – 61400-11  IEC:2002(E)
Record the power produced by the wind turbine and confirm that for each noise sampling
period, the power did not exceed 95 % of maximum power. Note that power values below
95 % of the maximum power may originate from high wind speeds above the wind speed
where the wind turbine reaches rated power. This may be controlled by checking the
measured wind speed.
For turbines with passive stall control, the electric power measured during the noise
measurements shall be converted to standard atmospheric conditions, using the following
equation:
 
T p
k ref
 
P = P
n m
 
T p
 ref
(4)
where
P is the normalised electric power (kW);
n
P is the measured electric power (kW);
m
T is air temperature in K, T = T + 273 ;
k k c
T is the air temperature (°C);
c
T is the reference temperature, T = 288 K;
ref ref
p is the atmospheric pressure (kPa);
p is the reference atmospheric pressure, p = 101,3 kPa.
ref ref
The wind speed at rotor centre height obtained from the power curve at P shall be converted
n
to a height of 10 m and the reference roughness length, as described in Equation (7).
For turbines with active power control, the wind speed at hub height shall be corrected
according to:
 p T  3
k
ref
 
(5)
V =V
H D
 
p T
ref
 
where
V is the corrected wind speed at hub height (m/s);
H
V is the derived wind speed from the power curve (m/s).
D
The corrected wind speed at hub height shall be converted to standardised wind speed at
height of 10 m and the reference roughness length, as described in Equation (7).
If the standardised wind speed corresponding to 95 % of rated power is below 10 m/s, the
following method shall be used. For all data points with power levels below 95 % of rated
power, the ratio of standardised wind speed and measured wind speed, κ, shall be derived.
This ratio shall then be applied to the measured wind speed of the data points with power
levels above 95 % of rated power to estimate the standardised wind speed using Equation (6).
V = κ V (6)
s z
where
V is the standardised wind speed;
s
V is the wind speed measured at anemometer height z.
z
61400-11  IEC:2002(E) – 17 –
7.3.1.2 Method 2: determination of wind speed with an anemometer
If an anemometer is used to measure wind speeds, the wind speed measurement results
shall be adjusted to a height of 10 m and the reference roughness length as described in
Equation (7).
Measurement by an anemometer at a height between 10 m and hub height will also be
appropriate during background noise measurements, when the wind turbine is parked, and the
turbine has been used as an anemometer during the turbine noise measurements.
Wind speed data shall be collected and arithmetically averaged over the same period as the
acoustic measurements.
7.3.2 Wind direction
Wind direction will be observed from a wind direction transducer to ensure that measurement
locations are kept within 15° of nacelle azimuth positions with respect to upwind, and to
measure the position of the anemometer. Wind direction shall be averaged over the same
period as the noise measurements.
7.3.3 Other atmospheric conditions
Air temperature and pressure shall be measured and recorded at least every 2 h.
Turbulence in the wind incident to a wind turbine can affect its aerodynamic noise emission.
A discussion of assessment of turbulence is contained in Annex C.
8 Data reduction procedures
8.1 Wind speed
The wind speeds measured at height z or determined at rotor centre height H from
measurements of electrical power shall be corrected to the wind speed V at reference
s
conditions by assuming wind profiles in the following equation:
 
   
z
H
ref
   
 ln ln 
   
z z
 
 0ref  0
(7)
=
V V  
s z
   
 H z 
   
ln ln
 
   
z z
 0ref 0 
   
 
where
z is the reference roughness length of 0,05 m;
0ref
z is the roughness length;
H is the rotor centre height;
z is the reference height, 10 m;
ref
z is the anemometer height.
Equation (7) uses the following principles:
– the correction for the measured height z to the rotor centre height H uses a logarithmic
wind profile with the site roughness length z to account for the actual site conditions.
– the correction from rotor centre height H to reference conditions uses a logarithmic wind
profile with a reference roughness length z . This describes the noise characteristic
0ref
independent of the terrain.
– 18 – 61400-11  IEC:2002(E)
The roughness length z can be calculated from wind speed measurements of several heights
or estimated according to Table 1. If the preferred method (Method 1) is used to determine
wind speed, κ may also be used to calculate the standardised wind speed for backgroun
...