NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Wind Turbine Generator System 

Acoustic Noise Test Report for 

the Gaia Wind 11-kW Wind 

Turbine

Arlinda Huskey

Technical Report

NREL/TP-5000-51828

November 2011
NOTICE

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Page 1 of 44

Wind Turbine Generator System

Acoustic Noise Test Report

for the

Gaia Wind 11-kW Wind Turbine

in

Boulder, CO 

Conducted for

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401

Conducted by

Wind Energy Program

DOE / NREL

Arlinda Huskey

March 31, 2011
Notice 

This report was prepared by the National Renewable Energy Laboratory (NREL), 

operated for the United States Department of Energy (DOE) by the Alliance for 

Sustainable Energy, LLC (Alliance), as an account of work sponsored by the United 

States government. The test results documented in this report define the characteristics of 

the test article as configured and under the conditions tested .. 

THIS REPORT IS PROVIDED "AS IS" AND NEITHER THE GOVERNMENT, 

ALLIANCE, NREL NOR ANY OF THEIR EMPLOYEES, MAKES ANY 

WARRANTY, EXPRESS OR IMPLIED, INCLUDING THEW ARRANTIES OF 

MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR 

ASSUMES ANY LEGAL LIABILITY OR RESPONSIBILITY FOR THE 

ACCURACY, COMPLETENESS, OR USEFULNESS OF ANY SUCH 

INFORMATION DISCLOSED IN THE REPORT, OR OF ANY APPARATUS, 

PRODUCT, OR PROCESS DISCLOSED, OR REPRESENTS THAT ITS USE WOULD 

NOT INFRINGE PRIVATELY OWNED RIGHTS. 

Neither Alliance nor the U.S. Government shall be liable for special, consequential or 

incidental damages. Reference herein to any specific conunercial product, process, or 

service by trade name, trademark, manufacturer, or otherwise does not necessarily 

constitute or imply its endorsement, recommendation, or favoring by the United States 

government or any agency thereof. The views and opinions of the authors expressed 

herein do not necessarily state or reflect those of the United States government or any 

agency thereof or Alliance. 

NREL is a DOE Laboratory, and as an adjunct of the United States government, cannot 

certify wind turbines. The information in this report is limited to NREL's knowledge and 

understanding as of this date. 

NREL is accredited by the American Association for Laboratory Accreditation (A2LA) 

and the results shown in this test report have been determined in accordance with the 

NREL' s terms of accreditation unless stated otherwise in the report. 

This report shall not be reproduced, except in full, without the written approval of 

Alliance or successor operator of NREL. "' · .. -

Approval By: 

Date 

Review By: 

Jeroen van Dam, NREL Test Engineer Date 

Page 2 44 
Page 3 of 44

Table of Contents

1.0 Background............................................................................................................................................. 4

2.0 Test Summary......................................................................................................................................... 4

3.0 Test Turbine............................................................................................................................................ 4

4.0 Test Site ................................................................................................................................................... 7

5.0 Test Equipment....................................................................................................................................... 8

5.1 Equipment Descriptions....................................................................................................................... 8

5.2 Instrumentation Locations.................................................................................................................. 10

6.0 Results ................................................................................................................................................... 10

6.1 Test Conditions .................................................................................................................................. 10

6.2 Apparent Sound Power Level............................................................................................................. 10

6.3 One-Third Octave Analysis................................................................................................................ 12

6.4 Tonality .............................................................................................................................................. 15

6.5 Uncertainty......................................................................................................................................... 16

7.0 Exceptions ............................................................................................................................................. 17

7.1 Exceptions to the Standard................................................................................................................. 17

7.2 Exceptions to the Quality Assurance System..................................................................................... 18

8.0 References ............................................................................................................................................. 18

Appendix A. Picture of the soundboard ................................................................................................... 19

Appendix B. Equipment Calibration Sheets............................................................................................ 20

List of Tables

Table 1. Test turbine Configuration and Operational Data ............................................................................ 6

Table 2. Structures close to test turbine.......................................................................................................... 8

Table 3. Equipment list for acoustic test ........................................................................................................ 9

Table 4. Microphone positions for turbine and background measurements................................................. 10

Table 5. Sound power levels for integer wind speeds 6 m/s through 12 m/s................................................ 11

Table 6. One-third octave analysis for wind speed bins 6 through 9 m/s..................................................... 13

Table 6 (continued). One-third octave analysis for wind speed bins 10 and 11 m/s .................................... 14

Table 7. Tonality results............................................................................................................................... 16

Table 8. Type B uncertainty components for sound power levels and tonality............................................ 17

List of Figures

Figure 1. Gaia-Wind 11-kW test turbine at the NWTC. PIX #15705. ........................................................... 5

Figure 2. Test turbine location........................................................................................................................ 7

Figure 3. Sound power levels as a function of the standardized wind speed................................................ 11

Figure 4. Measured sound pressure levels as a function of the standardized wind speed............................. 12

Figure 5. One-third octave levels ................................................................................................................. 15

Figure 6. Picture of sound board during the test........................................................................................... 19
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1.0 Background

This test is being conducted as part of the U.S. Department of Energy’s (DOE) 

Independent Testing project. This project was established to help reduce the barriers of 

wind energy expansion by providing independent testing results for small turbines. In 

total, four turbines are being tested at the National Wind Technology Center (NWTC) as 

a part of this project. Acoustic noise testing is one of up to five tests that may be 

performed on the turbines; other tests may include duration, safety and function, power 

performance, and power quality tests. 

2.0 Test Summary

The Gaia-Wind 11-kW wind turbine was installed at the National Wind Technology 

Center located south of Boulder, Colorado. The test turbine is a two-bladed, downwind 

wind turbine with a rated power of 11 kW. The test turbine was tested in accordance 

with the International Electrotechnical Commission standard, IEC 61400-11 Ed 2.1 2006-

11 Wind Turbine Generator Systems – Part 11Acoustic Noise Measurement Techniques,

hereafter referred to as the Standard. Noise and meteorological data were collected on 

two days, 28 February 2009 and 11 March 2010. Standardized wind speed (at 10 meters) 

was determined from wind speed measured at hub height (at 18.2 meters).

3.0 Test Turbine

Figure 1 shows a picture of the test turbine and Table 1 lists the turbine configuration.
Page 5 of 44

Figure 1. Gaia-Wind 11-kW test turbine at the NWTC. PIX #15705.
Page 6 of 44

Table 1. Test turbine Configuration and Operational Data

Test Turbine

Wind turbine details:

Manufacturer Gaia-Wind

Model number Gaia-Wind 11-k/w

Serial number 10711114

Configuration:

Vertical or horizontal axis Horizontal

Upwind or downwind rotor Downwind

Hub height 18.2 m

Horizontal distance from rotor center to tower 

axis

2.1 m

Diameter of rotor 13 m

Operating details:

Passive/active stall or pitch-controlled turbine Passive stall

Constant or variable speed Constant

Rotational speed at standardized integer wind 

speed from 6 to 10 m/s

60-61 rpm

Pitch angle at standardized integer wind 

speeds from 6 to 10 m/s

0 degrees

Rated power output 11 kW

Control software version Gaia-Wind IC-1000, Rev. 1:P00515\031020

Geometric Configuration:

Tower type (lattice or tube) Tubular

Rotor details:

Rotor control devices Tip brakes

Blade type Gaia-Wind T202, glass fiber, centrifugally 

activated tip brake

Number of blades 2

Presence of vortex generators, stall strips, 

serrated trailing edges, etc.

None
Page 7 of 44

4.0 Test Site

The test turbine is located at site 3.1 at the National Wind Technology Center, south of Boulder, 

Colorado. The topography surrounding the test site is flat with some small bushes. Figure 2 

shows the turbine and meterorological tower locations. This figure also shows nearby turbines 

and topographical features of the site.

Figure 2. Test turbine location
Page 8 of 44

Table 2. Structures close to test turbine

Source Location Shutdown for noise test

NW100b 3.4 Yes

ARE 442 3.3a Yes

Southwest

Windpower

Skystream (2*)

3.2 Yes

Endurance 3.1 Yes

CART 4.2 No

EW50 1E1 Yes

Bergey Excel 1.4 No

5.0 Test Equipment

5.1 Equipment Descriptions

Table 3 shows the list of equipment used for the test. All instruments meet the 

requirements defined by the Standard.
Page 9 of 44

Table 3. Equipment list for acoustic test

Instrument Manufacturer Model Number Serial Number Calibration Due Date

Digital Recorder and

Signal Analyzer

Delta Acoustics NoiseLab 1258E43 November 24, 2010

Microphone Bruel & Kjaer 4189-A-021 2395206 November 21, 2010

Preamplifier Bruel & Kjaer 4012 2373719 November 21, 2010

Calibrator Bruel & Kjaer 4231 2326144 November 11, 2009

January 14, 2011

Anemometer 

(replaced during test)

Thies First Class 0707890

0707892

April 7, 2009 

February 4, 2010

Anemometer post-Test 

calibrated and was in 

compliance

Wind Vane Met One 020C X4357 April 7, 2009

March 18, 2010

Pressure Sensor

(replacing during test)

Vaisala PTB101B T5030003 August 26, 2009

Pressure sensor 

Post-test calibrated 

and was in compliance

Temperature Sensor

(replaced during test)

Met One T-200 RTD 0789021 October 10, 2009

Temperature sensor 

post-test calibrated 

and was in compliance

Data acquisition

(replaced during test)

National 

Instruments

Compact 

DAQ

w/LabView

cDAQ

backplane

NI 9229

NI 9217

NI 9205

NI 9229

NI 9217

NI 9205

12E4DA3

12CBC7A

12BFEE2

12E9C99

140A596

140DCB9

140E2BD

August 14, 2008

July 20, 2008

October 8, 2008

Modules post-test

Calibrated and were

In compliance

February 10, 2010

February 12, 2010

February 10, 2010

Modules post-test 

calibrated and were in 

compliance
Page 10 of 44

5.2 Instrumentation Locations

The anemometer and wind vane were on a meteorological tower located 32.7 meters from 

the test turbine at a bearing of 290o true. The anemometer was at hub height 18.3 m. 

The meteorological tower distance is 2.5 rotor diameters from the test turbine and within 

the range of between 2 and 4 rotor diameters specified in the IEC standard.

Table 4 gives the location of the microphone for the measurement sessions.

Table 4. Microphone positions for turbine and background measurements

Microphone Distance

to Turbine

[m]

Slant

Distance

[m]

Position

relative to

turbine

[degrees]

Reference 24.7 30.7 295

6.0 Results

6.1 Test Conditions

The analysis was done using the measured wind speed and 10-second averages of the 

data. NREL has found that noise from small wind turbines correlates better with 

measured wind speed than it does with derived wind speed. For the 24 February 2009 

data, the range of standardized wind speeds and wind directions used for the analysis 

were 1.4 to 16.3 m/s and 118 to 355 degrees, respectively. The range of temperature and 

pressure were 14.6 to 16.3 C and 80.4 to 80.7 kPa, respectively. For the 11 March 2010 

data, the range of standardized wind speeds and wind directions used for the analysis 

were 3.1 to 12.2 m/s and 256 to 309 degrees, respectively. The range of temperature and 

pressure were 3.9 to 6.2 C and 80.3 to 80.4 kPa, respectively.

6.2 Apparent Sound Power Level

The apparent sound power level at standardized wind speeds if 6, 7, 8, 9, 10, 11, and 12 

m/s are shown in Table 5 and Figure 3. In this test, wind speed was measured at a hub 

height of 18.2 m and then standardized to a 10 m height, assuming an idealized wind 

profile based on a terrain roughness of 0.05 m.
Page 11 of 44

Table 5. Sound power levels for integer wind speeds 6 m/s through 12 m/s

Figure 3 shows sound power levels at the standard integer wind speeds. The sound 

pressure levels were binned by wind speed. The integer wind speed values were 

calculated by interpolation between bins. 

Figure 3. Sound power levels as a function of the standardized wind speed

82

83

84

85

86

87

88

89

90

91

92

2 4 6 8 10 12 14 16

sound power level (dB A)

standardized wind speed (m/s)

Wind

Speed Bin

[m/s]

Sound

Power

Level

[dB (A)]

Combined

Uncertainty

[dB (A)]

6 84.2 1.4

7 85.5 1.3

8 86.5 1.4

9 88.0 1.4

10 88.9 1.3

11 90.0 1.3
Page 12 of 44

Figure 4 shows the scatter plot of the sound pressure levels of the turbine and background 

noise.

.

Figure 4. Measured sound pressure levels as a function of the standardized wind speed.

6.3 One-Third Octave Analysis

One-third octave levels were analyzed at standardized wind speed of 6, 7, 8, 9, 10, and 11

m/s. The results are given in Table 6 and Figure 5.

40

42

44

46

48

50

52

54

56

58

60

62

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

sound pressure level (dB A)

standardized wind speed (m/s)

Turbine

Background

Binned background

Binned turbine
Page 13 of 44

Table 6. One-third octave analysis for wind speed bins 6 through 9 m/s

Center 

Frequency

6 m/s onethird octave 

levels

7 m/s onethird octave 

levels

8 m/s onethird octave 

levels

9 m/s onethird octave 

levels

[Hz] [dB A] [dB A] [dB A] [dB A]

20 NR NR NR NR

25 NR NR NR NR

31.5 NR NR NR NR

40 NR NR NR NR

50 NR NR NR NR

63 NR NR NR NR

80 NR NR NR NR

100 NR 32.2* ± 2.4 32.8* ± 2.2 NR

125 NR NR NR NR

160 33.8* ± 2.3 35.0* ± 2.3 36.6* ± 2.3 NR

200 NR NR NR NR

250 39.2* ± 2.3 40.1* ± 2.3 41.0* ± 2.2 NR

315 38.6* ± 2.3 39.2* ± 2.3 40.6* ± 2.3 41.4* ± 2.3

400 37.9* ± 2.3 38.6* ± 2.2 40.0* ± 2.3 40.9* ± 2.3

500 38.7* ± 2.2 39.7 ± 2.0 40.6* ± 2.3 41.4* ± 2.3

630 40.2 ± 2.1 41.4 ± 2.0 42.4 ± 2.2 43.4* ± 2.3

800 39.5 ± 2.0 40.4 ± 1.9 41.4 ± 2.2 42.3* ± 2.3

1000 39.3 ± 1.9 39.4 ± 1.9 40.2* ± 2.3 41.5* ± 2.4

1250 37.8 ± 1.9 38.6 ± 1.9 39.7 ± 2.3 40.9* ± 2.4

1600 39.4 ± 1.9 40.6 ± 1.9 42.0 ± 2.0 42.5 ± 2.2

2000 37.0 ± 2.0 38.6 ± 2.0 40.3 ± 2.1 41.5 ± 2.3

2500 36.1 ± 2.1 38.1 ± 2.1 40.3 ± 2.1 41.9 ± 2.2

3150 34.6 ± 2.1 36.7 ± 2.1 39.1 ± 2.2 40.8 ± 2.3

4000 32.1 ± 2.0 33.9 ± 2.0 36.1 ± 2.2 37.6 ± 2.4

5000 31.3 ± 2.1 33.0 ± 2.0 34.4 ± 2.0 35.2 ± 2.2

6300 30.8 ± 2.0 32.1 ± 2.0 32.9 ± 2.0 33.0 ± 2.1

8000 25.2* ± 2.2 25.9* ± 2.2 26.8* ± 2.3 NR

10000 NR NR NR NR

* The difference between total and background noise was less than 6 dB but greater than 3 dB. A standard 

background correction of 1.3 dB was applied.

NR The difference between total and background noise was less than 3 dB so no results are reported.
Page 14 of 44

Table 6 (continued). One-third octave analysis for wind speed bins 10 and 11 m/s

Center 

Frequency

10 m/s one-third 

octave levels

11 m/s one-third 

octave levels

[Hz] [dB A] [dB A]

20 NR 16.8* ± 2.6

25 NR 19.2* ± 2.5

31.5 NR 22.0* ± 2.6

40 NR NR

50 NR 25.6 ± 2.3

63 NR 28.3* ± 2.4

80 33.8 ± 2.5 31.8 ± 2.4

100 33.5* ± 2.4 35.8 ± 2.2

125 NR 39.1* ± 2.2

160 38.9* ± 2.3 38.8* ± 2.3

200 NR NR

250 40.9* ± 2.3 41.7* ± 2.3

315 41.9* ± 2.3 43.2 ± 2.2

400 41.6* ± 2.3 43.0 ± 2.1

500 42.4 ± 2.1 43.6 ± 1.9

630 44.7 ± 1.9 45.7 ± 1.9

800 43.9 ± 1.9 44.8 ± 1.9

1000 43.4 ± 1.9 44.4 ± 1.9

1250 42.8 ± 2.0 43.9 ± 1.9

1600 43.8 ± 1.9 44.7 ± 1.9

2000 42.9 ± 1.9 43.8 ± 1.9

2500 42.8 ± 1.9 43.7 ± 1.9

3150 42.0 ± 1.9 42.9 ± 1.9

4000 39.1 ± 1.9 39.9 ± 1.9

5000 36.1 ± 1.9 36.7 ± 1.9

6300 33.5 ± 1.9 33.9 ± 1.9

8000 27.4* ± 2.3 28.3* ± 2.3

10000 NR NR

* The difference between total and background noise was less than 6 dB but greater than 3 dB. A standard 

background correction of 1.3 dB was applied.

NR The difference between total and background noise was less than 3 dB so no results are reported.
Page 15 of 44

Figure 5. One-third octave levels

6.4 Tonality

The tonality analysis resulted in reportable tones for 6, 7, 8, and 9 m/s. The tonality 

analysis was not conducted for the 10 m/s wind speed bin because the data available was 

not suitable for assessing tonality. Table 7 shows the results for the tonality analysis.

0

5

10

15

20

25

30

35

40

45

50

2025

31.5

40

5063

80

100125

160200250315400500630800

1000125016002000250031504000500063008000

10000

sound pressure level (dB A)

one-third octave center frequency (Hz)

6 m/s

7 m/s

8 m/s

9 m/s

10 m/s

11 m/s
Page 16 of 44

Table 7. Tonality results

6.5 Uncertainty

The Type A uncertainties for sound power levels, one-third octave levels, and tonality 

were calculated using the methods prescribed in the Standard.

The type B uncertainty components are shown in Table 8.

k (m/s)

Freq (Hz) 807 1569 1614 810 1569 1620 648 810 1575 1623 651 813 1578 1623

∆Ltn1,k -6.1 -8.8 -17.3 -0.7 -17.2 -4.3 -14.5 -1.9 -17.2 -17.3 -4.6 1.2 -17.2 -17.3

∆Ltn2,k -5.2 -8.3 -17.3 -0.7 -17.2 -4.7 -3.3 1.8 -17.2 -5.8 -2.8 1.5 -17.2 -17.3

∆Ltn3,k -6.7 -0.1 -17.3 -3.9 -8.8 -17.3 0.0 0.6 -17.2 -3.4 -7.8 -2.0 -17.2 -17.3

∆Ltn4,k -5.8 -1.1 -17.3 -3.7 -8.5 -17.3 -4.4 -4.7 1.7 -17.3 -2.8 2.8 -17.2 -10.4

∆Ltn5,k -2.8 1.8 2.2 -3.7 0.3 -17.3 -5.9 -3.1 -0.5 -17.3 -3.6 3.3 -17.2 -8.5

∆Ltn6,k -4.2 -0.4 0.2 -5.7 -0.4 0.0 -5.3 -3.9 0.2 0.5 -3.2 3.1 -17.2 -8.6

∆Ltn7,k -6.1 1.3 1.7 -2.2 -0.2 -17.3 -5.8 -5.0 -0.4 -17.3 -14.5 -6.0 -2.3 -1.6

∆Ltn8,k -7.9 -2.3 -1.8 -5.9 -3.7 -3.3 -5.7 -5.6 -1.0 -17.3 -14.5 -7.1 -4.0 -2.7

∆Ltn9,k -4.2 -1.3 -0.5 -4.3 0.0 0.2 -7.3 -5.0 -1.6 -1.0 -2.5 -2.0 0.6 0.7

∆Ltn10,k -4.3 1.2 1.9 -5.1 -0.5 0.3 -5.9 -6.2 1.8 2.2 -3.2 -4.9 -1.0 -0.9

∆Ltn11,k -14.9 -5.5 -17.3 -7.5 -1.0 -0.7 -5.1 -4.2 1.1 1.5 -2.4 -4.2 -2.5 -2.5

∆Ltn12,k -4.1 -2.7 -1.8 -5.9 -1.7 -1.2 -14.5 -4.7 -1.3 -17.3 -5.2 -5.3 -1.4 -1.3

∆Lk dB(A) -5.3 -1.1 -1.7 -3.6 -2.3 -3.0 -5.1 -2.7 -1.1 -3.2 -4.3 -0.2 -4.5 -3.8

∆La,k dB(A) -2.7 2.2 1.5 -1.0 0.9 0.3 -2.6 -0.1 2.2 0.1 -1.8 2.5 -1.2 -0.5

UA dB(A) 6.3 2.1 3.4 4.5 3.4 4.4 6.1 3.6 2.4 5.1 5.3 1.5 6.3 5.0

UB dB(A) 2.0 1.9 1.9 2.0 1.9 1.9 2.1 2.1 1.9 1.9 1.9 2.0 1.9 1.9

UC dB(A) 6.6 2.8 3.9 4.9 3.9 4.7 6.4 4.1 3.0 5.4 5.6 2.5 6.6 5.3

6 7 8 9
Page 17 of 44

Table 8. Type B uncertainty components for sound power levels and tonality

Variable Description

Type B 

uncertainty for 

sound power 

level

dB (A)

Type B 

uncertainty for 

one-third 

octave levels

dB (A)

Type B 

uncertainty for 

tonality

dB (A)

Comment

UB1 Calibration 0.2 0.2 0.1 Assumption, used 

typical value

UB2 Instrument 0.2 0.2 0.2 Assumption, used 

typical value

UB3 Board 0.3 1.7 1.7 The board was 

placed well and used 

typical value.

UB4 Distance 0.1 0.1 0.1 Assumption, used 

typical value.

UB5 Impedance 0.1 0.1 0.1 Assumption, used 

typical value.

UB6 Turbulence 0.4 0.4 0.2 Assumption, used 

typical value.

UB7 Wind speed, 

measured

0.5 x slope 0.5 x slope 0.6 Calculated using an 

estimated 0.5 m/s 

uncertainty on the 

wind speed

UB8 Direction 0.3 0.3 0.3 Assumption, used 

typical value.

UB9 Background Varies by bin Varies by bin 

and one-third 

octave center 

frequency bin

Varies by 

tone.

Difference between 

the regression for 

turbine and 

background for 

sound power level. 

7.0 Exceptions

7.1 Exceptions to the Standard

1. The tonality analysis for the 10m/s bin was not performed because the available 

data was not suitable. There were other turbines operating in the background and 

this would have caused unreliable tonality results.

2. Ten-second averages were used in the analysis instead of one-minute averages to 

better characterize the dynamic nature of this small wind turbine.
Page 18 of 44

3. Pictures were not taken of the soundboard, turbine, and meteorological tower 

during the test. A picture of a soundboard is included that is representative of 

conditions during the test.

7.2 Exceptions to the Quality Assurance System

Meteorological instruments were used past the calibration due dates. The instruments 

were post-test calibrated to assure the instruments were within their tolerances.

8.0 References

IEC 61400-11 Ed 2.1 2006-11 Wind Turbine Generator Systems – Part 11 Acoustic 

Noise Measurement Techniques
Page 19 of 44

Appendix A. Picture of the soundboard

Figure 6. Picture of sound board during the test. PIX #19420.
Page 20 of 44

Appendix B. Equipment Calibration Sheets
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