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.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at 

www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Acoustic Noise Test Report for 

the SWIFT Wind Turbine in 

Boulder, CO

Jason Roadman and Arlinda Huskey

National Renewable Energy Laboratory

Technical Report

NREL/TP-5000-58244

April 2013
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.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at 

www.nrel.gov/publications.

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Acoustic Noise Test Report for 

the SWIFT Wind Turbine in 

Boulder, CO

Jason Roadman and Arlinda Huskey

National Renewable Energy Laboratory

Prepared under Task No. WE11.0206

Technical Report

NREL/TP-5000-58244

April 2013
NOTICE

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Swift IEC Acoustic Noise Report 130215 Page 1 of 39

Acoustic Noise Test Report

for the

SWIFT Wind Turbine

in

Boulder, CO

Conducted for

National Renewable Energy Laboratory 

15013 Denver West Parkway 

Golden, Colorado 80401 

Conducted by

National Wind Technology Center 

National Renewable Energy Laboratory 

15013 Denver West Parkway 

Golden, Colorado 80401

Jason Roadman 

Arlinda Huskey

28 February 2013 
Swift IEC Acoustic Noise Report 130215 Page 2 of 39
Swift IEC Acoustic Noise Report 130215 Page 3 of 39

Table of Contents

1 Background........................................................................................................................................... 5

2 Test Summary....................................................................................................................................... 5

3 Test Turbine Configuration ................................................................................................................. 6

4 Test Site Description............................................................................................................................ 8

5 Test Equipment..................................................................................................................................... 9

5.1 Equipment Descriptions ................................................................................................................... 9

5.2 Instrument Locations ..................................................................................................................... 10

6 Results................................................................................................................................................. 10

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

6.2 Standardized Wind Speed Calculation ........................................................................................... 10

6.3 Apparent Sound Power Level ........................................................................................................ 11

6.4 One-Third Octave Analysis ........................................................................................................... 13

6.5 Tonality .......................................................................................................................................... 15

6.6 Uncertainty ..................................................................................................................................... 18

7 Exceptions........................................................................................................................................... 18

7.1 Exceptions to the Standard ............................................................................................................. 18

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

References................................................................................................................................................. 19

Appendix B. Pictures ................................................................................................................................ 21

Appendix C. Calibration Sheets .............................................................................................................. 24

List of Tables

Table 1. Test Results Summary................................................................................................................. 6

Table 2. SWIFT Wind Turbine General Data ............................................................................................. 6

Table 3. Sources of Noise Near the Turbine............................................................................................. 8

Table 4. Equipment Used for Acoustic Test............................................................................................. 9

Table 5. Reference Microphone Positions for Turbine and Background Measurements ................. 10

Table 6. Test Parameters Used in Wind Speed Calculations ............................................................... 11

Table 7. Sound Pressure and Power Levels for Standardized Integer Wind Speeds ........................ 12

(6 m/s Through 11 m/s)............................................................................................................................. 12

Table 8. One-Third Octave Analysis for Wind Speed Bins 6 Through 8 m/s ...................................... 13

Table 9. One-Third Octave Analysis for Wind Speed Bins 9 Through 11 m/s .................................... 14

Table 10. Tonality Results (In dBA)......................................................................................................... 16

Table 11. Type B Uncertainty Components for Sound Power Levels and Tonality ........................... 18

Table 12. AWEA Rated Sound Level ....................................................................................................... 20
Swift IEC Acoustic Noise Report 130215 Page 4 of 39

List of Figures

Figure 1. SWIFT test turbine at the NWTC................................................................................................ 7

Figure 2. Map of the test site ..................................................................................................................... 8

Figure 3. Measured and binned sound pressure levels as................................................................... 11

a function of the standardized wind speed ............................................................................................ 11

Figure 4. Sound power levels as a function of the standardized wind speed .................................... 12

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

Figure 6. Classification of spectral lines for the 3,849 Hz tone (typical in the 10 m/s bin) ............... 17

Figure 7. Classification of spectral lines for the 3,921 Hz tone (typical in the 11 m/s bin) ............... 17

Figure B1. Picture of the sound board during the test ......................................................................... 21

Figure B2. The test turbine as viewed from the reference microphone position............................... 22

Figure B3. The test turbine as viewed from the meteorological mast................................................. 23

Figure C1. Calibration sheet for the microphone 2406811 ................................................................... 24

Figure C2. Calibration sheet for the microphone 2406809 ................................................................... 25

Figure C3. Calibration sheet for the sound level meter ........................................................................ 26

Figure C4. Calibration sheet for the acoustic calibrator....................................................................... 27

Figure C5. Calibration sheet for the primary anemometer ................................................................... 28

Figure C6. Post-test calibration sheet for the primary anemometer ................................................... 29

Figure C7. Page 1 of the power transducer calibration sheet.............................................................. 30

Figure C8. Calibration sheet for the pressure transducer.................................................................... 31

Figure C9. Calibration sheet for the temperature sensor ..................................................................... 32

Figure C10. Calibration sheet for the wind vane ................................................................................... 33

Figure C11. Calibration sheet for the signal conditioning module 1496266....................................... 34

Figure C12. Post-test calibration sheet for the signal conditioning module 1496266....................... 35

Figure C13. Calibration sheet for the signal conditioning module 14A34EE ..................................... 36

Figure C15. Calibration sheet for the signal conditioning module 1494F69....................................... 38

Figure C16. Post-test calibration sheet for the signal conditioning module 1494F69....................... 39
Swift IEC Acoustic Noise Report 130215 Page 5 of 39

1 Background

An acoustic noise test was conducted as part of the U.S. Department of Energy’s (DOE’s) 

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

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

turbines were selected for testing at the National Wind Technology Center (NWTC), as a part of 

the Small Wind Turbine Independent Testing project. Acoustic noise testing is one of up to five

tests that may be performed on turbines. Other tests include duration, safety and function, power 

performance, and power quality. Located in Grand Rapids, Michigan, Cascade Engineering is the 

North American distributor of the SWIFT turbine (manufactured by Renewable Devices). 

Cascade Engineering was the recipient of the DOE grant and provided the turbine for testing. 

The primary goal of this test was to characterize the acoustic emissions of the SWIFT wind 

turbine in accordance with the International Electrotechnical Commission’s (IEC) standard, 

Wind Turbine Generator Systems - Part 11: Acoustic Noise Measurement Techniques, IEC 

61400-11, Edition 2.1, 2006-11; hereafter referred to as the “Standard.” This test report 

documents the measurement techniques, turbine configuration, test site, test equipment, and 

results for the following quantities at integer wind speeds from 6 to 11 m/s:

• Apparent sound power level 

• Third octave band levels 

• Tonality. 

Engineers at the NWTC conducted the acoustic noise test in accordance with the National 

Renewable Energy Laboratory’s (NREL’s) quality system procedures to ensure that this final test 

report meets the full requirements accreditation by A2LA. NREL’s quality system procedures 

require that the test meet all applicable requirements specified by A2LA and ISO/IEC 17025 (or 

to note any exceptions in the test report).

2 Test Summary

The turbine was tested in accordance with the Standard. Turbine acoustic emissions and 

meteorological data were collected on 6 April 2012, 24 May 2012, and 28 May 2012. 

Standardized wind speed (at 10 m) was derived from wind speed measured at hub height (14.28 

m). Table 1 gives a summary of the test results.
Swift IEC Acoustic Noise Report 130215 Page 6 of 39

Table 1. Test Results Summary

Standardized wind speed at 10 m height, Vs [m/s] 6 7 8 9 10 11

Electrical power output calculated from power curve [kilowatts (kW)] 0.05 0.13 0.25 0.43 0.65 0.90

Measured pitch angle [°] 6 6 6 6 6 6 

Measured rotor speed [1/min] - - - - - - 

Apparent sound power level [dBA] NR * 82.6 84.4 85.0 85.6

Combined uncertainty in the sound power level, UC [dBA] - 2.9 2.6 1.8 1.7 1.5

Frequency of the most prevalent tone [hertz (Hz)] - - - - 3849 3921

Tonality, ΔLk [dBA] - - - - -1.45 -2.53

Tonal audibility, ΔLa,k [dBA] - - - - 2.77 1.71

* The difference between total and background noise was less than 6 dB, but greater than 3 dB. Per Section 8.2 of the Standard, these data points were not used in the 

determination of the apparent sound power level, but were used for one-third octave and tonality. A standard background correction of 1.3 dB was applied. 

NR The difference between total and background noise was less than 3 dB. According to Section 8.2 of the Standard, the wind turbine noise was less than the background 

noise. 

3 Test Turbine Configuration 

Table 2 lists the configuration of the SWIFT turbine that was tested at the NWTC.

Table 2. SWIFT Wind Turbine General Data

Turbine manufacturer and address

Renewable Devices Ltd

AeroMarine House, Turnhouse Aerodrome

Turnhouse Road, Edinburgh

EH12 9DN 

Scotland, UK

Turbine distributor and address

Cascade Engineering

4855 Thirty-Seven St. SE 

Grand Rapids, MI 49512

Model SWIFT

Rated power (kW) 1

Rated wind speed (m/s) 11

Serial number N000780-N

Blade make, type, serial number Injection-molded nano-fiber reinforced polymer, F000648

Generator make, type, serial number Ginlong, Permanent Magnet, N000780-N

Gearbox make, type, serial number N/A, direct drive

Control software Kaco Blueplanet 1502x, Software V2.05

Wind turbine type Horizontal-axis, upwind

Tower type Trunk – conical tubular

Number of blades 5

Hub height (m) 14.28*

Rotor diameter (m) 2.134 with outer ring*

Horizontal distance from rotor center to tower axis (m) 0.27

Speed control Passive stall with furling

Constant or variable speed Variable

Rotational speed at standardized integer wind speed 

from 6 to 10 m/s (rpm) 210-364

Pitch angle at standardized integer wind speeds from 6 

to 10 m/s Fixed at 6° at the tip

Rotor control devices Diffuser ring

*Measurements verified the rotor diameter and hub height.
Swift IEC Acoustic Noise Report 130215 Page 7 of 39

Figure 1. SWIFT test turbine at the NWTC 

(Photo by Ismael Mendoza, NREL 22083) 
Swift IEC Acoustic Noise Report 130215 Page 8 of 39

4 Test Site Description

The SWIFT wind turbine was located at test site 3.1 of the NWTC, approximately 8 km south of 

Boulder, Colorado. The site consists of mostly flat terrain with short vegetation (see Appendix A 

for photos) and has prevailing winds bearing 292° relative to true north. Figure 2 shows the 

turbine and meteorological tower locations, as well as nearby obstructions. NREL limited 

assessments of power and energy production to data obtained when winds were within the 178° 

to 311° measurement sector. In this measurement sector, the influence of terrain and obstructions 

on the anemometer is small and meets the requirements of IEC 61400-12-1(without conducting a 

site calibration test). Table 3 lists the nearby turbines and whether or not they were operating 

during data collection.

Figure 2. Map of the test site

(Source: NREL 2011)

Table 3. Sources of Noise Near the Turbine

Source Location Shutdown for noise test

E 1.5 4.0 Yes

Alstom ECO100 3.0 MW 4.1 Yes

Controls Advanced Research Turbine (CART)-3 4.3 Yes

CART-2 4.3 Yes

Siemens 2.3 MW 4.4 No

Southwest Windpower Skystream (Two turbines) 3.2 Yes

Viryd CS8 3.3 Yes

NW100 1.2 No

Test shed heating, ventilating, and air conditioning (HVACs) 3.1, 3.2, 3.3 Yes

Sound Boards
Swift IEC Acoustic Noise Report 130215 Page 9 of 39

5 Test Equipment 

5.1 Equipment Descriptions

Table 4 shows the equipment used for the test. All instruments meet the requirements defined by 

the Standard. 

Table 4. Equipment Used for Acoustic Test

Instrument Manufacturer Model 

Number

Serial 

Number

Calibration

Due Date

Digital recorder 

and signal 

analyzer

Delta Acoustics NoiseLab 1258E43 9 Nov 2012

Microphone Bruel & Kjaer 4189-A-021 2395209

2395206

8 Nov 2012

8 Nov 2012

Preamplifier Bruel & Kjaer 4012 2373721

2373719

8 Nov 2012

8 Nov 2012

Calibrator Bruel & Kjaer 4231 2388951 19 Dec 

2012

Anemometer Thies First Class 0609006 7 Apr 2012

Wind vane Met One SS 201 W5515 25 Oct 2012

Nacelle 

anemometer

NA NA NA NA

Pressure sensor Vaisala PTB101B C1040008 25 Oct 2012

Temperature 

sensor

Met One T-200 0603-1 20 Oct 2012

Power transducer Secondwind Phaser

5FM-4A20

04607 8 Nov 2012

Data acquisition National 

Instruments 

CompactDAQ

w/ LabView

cDAQ-9172 

NI 9229 

NI 9217 

NI 9205 

13AB4F9 

14A34EE 

1494F69 

1496266 

NA

22 Mar 

2012 

22 Mar 

2012 

22 Mar 

2012

The calibration on the primary anemometer and the data acquisition modules expired during the 

test. Post-test calibration sheets are included in Appendix C. The anemometer and modules were 

found to be within tolerance. 
Swift IEC Acoustic Noise Report 130215 Page 10 of 39

5.2 Instrument Locations

The primary anemometer on the meteorological tower was used to derive the standardized wind 

speed. This tower was located 5.20 m from the test turbine, at a bearing of 302o true north, with 

the anemometer at a height of 14.33 m. The wind vane was mounted at a height of 11.35 m on 

the meteorological tower. The turbine was 2.4 rotor diameters from the meteorological tower, 

within the range of 2 and 4 rotor diameters specified in the Standard. 

Table 5 provides the location of the microphone for the measurement sessions. 

Table 5. Reference Microphone Positions for Turbine and Background Measurements

Microphone

Distance

Turbine

[m]

Slant

Distance

[m]

Position Relative

to Turbine

[deg true]

6 Apr 2012 16.54 22.05 80

24 May 2012 16.34 21.90 110

28 May 2012 16.34 21.90 110

6 Results

6.1 Test Conditions

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

NREL engineers have found that using 10-second averages instead of 1-minute averages better 

characterizes the dynamic nature of small turbines. The range of standardized wind speeds and 

wind directions used for the analysis were 2.1 to 18.1 m/s and 245.8 to 304.8 degrees, 

respectively. The range of temperature and pressure were 9.7°C to 18.1°C and 80.4 kPa to 81.3 

kPa, respectively.

6.2 Standardized Wind Speed Calculation

Standardized wind speed, Vs, was calculated using Equation 1 and the values in Table 6, where 

Vz is the measured wind speed. 





























































































=

0 0

0 0

lnln

lnln

z

z

z

H

z

H

z

z

VV

ref

ref

ref

zs (1)
Swift IEC Acoustic Noise Report 130215 Page 11 of 39

Table 6. Test Parameters Used in Wind Speed Calculations

Parameter Name Value

Hub Height, (m) H 14.28

Roughness length, (m) Zo 0.05

Anemometer height, (m) Z 14.33

Reference roughness length, (m) Z0ref 0.05

Reference height, (m) Zref 10.00

6.3 Apparent Sound Power Level

Sound pressure levels were binned by wind speed. Integer wind speeds values were calculated 

using interpolation between bins and extrapolation at the ends. The sound pressure levels were 

then background corrected according to the Standard. Figure 3 shows the scatter plot of the 

sound pressure levels of the validated total (operating plus background) and background noise, 

along with the binned sound pressure levels. At low wind speeds, background levels were close 

to turbine levels, resulting in the 6 and 7 m/s bins being unreportable. The measured and 

background corrected apparent sound pressure level at standardized wind speeds of 6 through 11 

m/s are shown in Table 7, along with the calculated sound power levels. Figure 4 shows the 

sound power levels graphed against the standardized wind speed. 

Figure 3. Measured and binned sound pressure levels as a function of the standardized 

wind speed

40

42

44

46

48

50

52

54

56

58

60

62

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Sound Pressure Level [dBA]

Standardized Wind Speed, Vs [m/s]

Measured Sound Pressure Level vs. Wind Speed

06 Apr 2012 Total

24 May 2012 Total

28 May 2012 Total

06 May 2012 Background

24 May 2012 Background

28 May 2012 Background

Binned Total

Binned, BG Corrected Total Noise

Binned Background
Swift IEC Acoustic Noise Report 130215 Page 12 of 39

Table 7. Sound Pressure and Power Levels for Standardized Integer Wind Speeds 

(6 m/s Through 11 m/s)

* The difference between total and background noise was less than 6 dB but greater than 3 dB. According to Section 8.2 of the Standard, 

these data points were not used to determine the apparent sound power level.

NR The difference between total and background noise was less than 3 dB. According to Section 8.2 of the Standard, the wind turbine noise 

was less than the background noise.

Figure 4. Sound power levels as a function of the standardized wind speed

78

80

82

84

86

88

5 6 7 8 9 10 11 12

Sound Power Level, LWA [dBA]

Standardized Wind Speed, Vs [m/s]

Wind

Speed 

Bin

[m/s]

Total 

Sound 

Pressure 

Level

[dBA)]

Background 

Sound 

Pressure 

Level

[dBA]

Background

Corrected 

Sound 

Pressure 

Level

[dBA]

Sound

Power

Level

[dBA]

Type A

Uncert.

[dBA]

Type B

Uncert.

[dBA]

Combined

Uncert.

[dBA]

6 47.7 44.8 NR NR NR NR NR

7 49.6 45.4 48.3 * 2.7 1.1 2.9

8 52.0 45.9 50.8 82.6 2.4 1.1 2.6

9 53.6 46.5 52.6 84.4 1.5 0.9 1.8

10 54.1 47.2 53.2 85.0 1.5 0.9 1.7

11 54.8 47.8 53.8 85.6 1.2 0.9 1.5
Swift IEC Acoustic Noise Report 130215 Page 13 of 39

6.4 One-Third Octave Analysis

One-third octave levels were analyzed at standardized wind speeds of 6, 7, 8, 9, 10, and 11 m/s. 

The results (with uncertainty) are provided in Table 8, Table 9, and Figure 5. 

Table 8. One-Third Octave Analysis for Wind Speed Bins 6 Through 8 m/s

Center 

Frequency

6 m/s OneThird Octave 

Levels

7 m/s OneThird Octave 

Levels

8 m/s OneThird Octave 

Levels

[Hz] [dBA] [dBA] [dBA]

50 NR NR NR

63 NR NR NR

80 29.6* ± 2.3 NR NR

100 NR NR NR

125 NR NR NR

160 NR NR NR

200 38.8* ± 2.1 NR NR

250 NR 39.8 ± 2.2 42.3 ± 1.9

315 NR NR 38.7* ± 2.1

400 NR NR NR

500 35.4* ± 2.1 40.2 ± 2.1 40.4 ± 1.9

630 34.9* ± 2.0 37.7* ± 2.1 40.6 ± 2.0

800 NR 37.6* ± 2.2 41.5 ± 2.0

1000 34.6* ± 2.1 38.7 ± 2.1 42.5 ± 2.0

1250 36.3 ± 2.0 38.9 ± 2.1 42.6 ± 2.0

1600 32.0 ± 2.1 36.3 ± 2.2 40.4 ± 2.0

2000 29.5 ± 2.1 32.6 ± 2.1 36.4 ± 2.0

2500 NR 27.4* ± 2.2 31.9 ± 2.1

3150 NR NR 27.7* ± 2.1

4000 28.4 ± 2.4 NR 25.3* ± 2.0

5000 NR NR NR

6300 NR NR NR

8000 NR NR NR

10000 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 according to Section 8.2 of the Standard.

NR The difference between total and background noise was less than 3 dB. According to Section 8.2 of the Standard, the wind 

turbine noise was less than the background noise.
Swift IEC Acoustic Noise Report 130215 Page 14 of 39

Table 9. One-Third Octave Analysis for Wind Speed Bins 9 Through 11 m/s

Center 

Frequency

9 m/s OneThird Octave 

Levels

10 m/s OneThird Octave 

Levels

11 m/s OneThird Octave 

Levels

[Hz] [dBA] [dBA] [dBA]

50 NR NR NR

63 NR NR NR

80 NR NR NR

100 NR NR NR

125 NR NR NR

160 NR NR NR

200 NR NR 39.3* ± 2.1

250 41.8 ± 2.0 38.8* ± 2.0 38.3* ± 2.0

315 41.7* ± 2.1 45.8 ± 2.0 47.8 ± 1.9

400 NR NR 38.6* ± 2.0

500 39.9* ± 2.0 38.9* ± 2.0 39.1* ± 2.0

630 42.1 ± 1.9 41.4 ± 1.9 40.8* ± 2.0

800 43.2 ± 1.8 42.9 ± 1.9 42.0 ± 1.9

1000 44.8 ± 1.8 45.0 ± 1.8 44.8 ± 1.8

1250 45.1 ± 1.9 45.7 ± 1.8 45.5 ± 1.8

1600 43.5 ± 1.9 44.4 ± 1.8 44.6 ± 1.8

2000 39.7 ± 1.9 41.1 ± 1.9 42.9 ± 1.9

2500 35.9 ± 1.9 37.3 ± 1.9 38.7 ± 1.9

3150 31.3 ± 2.0 32.9 ± 2.0 34.7 ± 1.9

4000 27.4* ± 2.0 30.6 ± 2.2 35.3 ± 2.3

5000 NR NR NR

6300 NR NR NR

8000 NR NR NR

10000 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 according to Section 8.2 of the Standard.

NR The difference between total and background noise was less than 3 dB. According to Section 8.2 of the Standard, the wind 

turbine noise was less than the background noise.
Swift IEC Acoustic Noise Report 130215 Page 15 of 39

Figure 5. One-third octave levels

6.5 Tonality

The tonality analysis resulted in reportable tones for 10 and 11 m/s, as shown in Table 10. NREL 

engineers believe that these tones originated from the furling mechanism. The furling mechanism 

activated at Vs ~ 10 m/s and generated enough noise that it was impossible to collect valid 

background data above 10 m/s. On 6 Apr 2012, the mechanism was lubricated and the audible 

squeak was no longer noticeable. As a result, no audible tones existed for the 24 May 2012 or 28 

May 2012 measurement sessions. 

Additionally, engineers heard a rattle emanating from the nose cone in low wind speeds. As the 

turbine speed increased, the rattle disappeared. However, no quantitative comments can be made 

concerning this rattle, as the background levels at these wind speeds were within 3 dBA of the 

turbine levels. 

Cascade Engineering was made aware of both of these noises.

0

10

20

30

40

50

506380

100125160200250315400500630800

1000125016002000250031504000500063008000

10000

Sound Pressure Level [dBA]

Frequency [Hz]

6 m/s

7 m/s

8 m/s

9 m/s

10 m/s

11 m/s
Swift IEC Acoustic Noise Report 130215 Page 16 of 39

Table 10. Tonality Results (In dBA)

Wind Speed: 10 m/s 11m/s

f [Hz] 3,849 3,921

∆Ltn,1 -21.5 -21.5

∆Ltn,2 -3.7 -21.5

∆Ltn,3 -21.5 -21.5

∆Ltn,4 -21.5 -21.5

∆Ltn,5 -21.5 -21.5

∆Ltn,6 -21.5 -21.5

∆Ltn,7 -21.5 -21.5

∆Ltn,8 -21.5 -21.5

∆Ltn,9 -21.5 -21.5

∆Ltn,10 -21.5 -21.5

∆Ltn,11 -21.5 -21.5

∆Ltn,12 9.1 8.1

∆Lk -1.5 -2.5

∆La,k 2.77 1.7

UA N/A1 N/A1

UB 2.1 2.1

UC N/A1 N/A1

Figure 6 and Figure 7 show a 10-second energy averaged spectrum indicating the classification 

of spectral lines for each of the identified tones.

 

1 Tone was intermittent. Type A uncertainty was not calculated for the bipolar sample distribution. 
Swift IEC Acoustic Noise Report 130215 Page 17 of 39

Figure 6. Classification of spectral lines for the 3,849 Hz tone (typical in the 10 m/s bin) 

Figure 7. Classification of spectral lines for the 3,921 Hz tone (typical in the 11 m/s bin)

0

5

10

15

20

25

30

35

40

3522354635703594361836423666

36903714373837623786

3810383438583882390639303954397840024026405040744098412241464170

Sound Pressure Level [dB]

Frequency [Hz]

Masking

Tone

Neither

0

5

10

15

20

25

30

35

3588361236363660368437083732375637803804382838523876390039243948397239964020404440684092411641404164418842124236

Sound Pressure Level [dB]

Frequency [Hz]

Masking

Tone

Neither
Swift IEC Acoustic Noise Report 130215 Page 18 of 39

6.6 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 11. 

Table 11. Type B Uncertainty Components for Sound Power Levels and Tonality

Var Description

Type B 

Uncertainty for 

Sound Power 

Level (SPL)

dBA

Type B 

Uncertainty for 

1/3 Octave 

Levels (TOB)

dBA

Type B 

Uncertainty 

for Tonality

dBA

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 the typical value

UB4 Distance 0.1 0.1 0.05 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

Varies with wind

speed

Varies with wind 

speed and onethird octave center 

frequency bin

0.6

Calculated per IEC 61400-12-1 

Ed. 1.0, 205-12, and converted 

to dBA for SPL and TOB. 

Typical value for tonality

UB8 Direction 0.3 0.3 0.3 Assumption, used typical value

UB9 Background Varies with wind 

speed

Varies with wind 

speed and onethird octave center 

frequency bin

Varies by tone Standard deviation of the 

applied correction

7 Exceptions

7.1 Exceptions to the Standard

The analysis prescribed in the standard was altered for the small wind turbine by using 10-

second averages instead of 1-minute averages to better characterize the dynamic nature of this 

turbine. In addition, binning by wind speed was used instead of regression analysis, and the 

integer values were calculated by interpolating between bins and extrapolating at the ends.

7.2 Exceptions to the Quality Assurance System

The primary anemometer and data acquisition modules were used past the calibration due dates. 

The instruments and modules were post-test calibrated. The anemometer and modules were 

found to be within tolerances. 
Swift IEC Acoustic Noise Report 130215 Page 19 of 39

References

International Electrotechnical Commission (IEC). (2006). Wind Turbine Generator Systems – 

Part 11 Acoustic Noise Measurement Techniques, IEC 61400-11, Ed 2.1, 2006-11, Geneva, 

Switzerland.
Swift IEC Acoustic Noise Report 130215 Page 20 of 39

Appendix A. American Wind Energy Association 

Standard Acoustic Analysis

The American Wind Energy Association (AWEA) standard requires that the wind turbine sound 

levels be measured and reported in accordance with the IEC 61400-11 standard, and includes the 

following modifications:

• Using a 10-second averaging period 

• Using the measured wind speed 

• Using the method of bins 

• Covering a wide wind speed range as possible 

• Describing any obvious changes in sound at high wind speeds 

• Reporting the AWEA Rated Sound Level. 

During the test, NREL engineers analyzed the data to calculate the 10-second averages. To 

ensure that the acoustic data was downwind from the turbine, the wind direction was filtered to 

assure that the measurement board was within 15 degrees of the downwind position. The data 

were also filtered by the provided status to determine the total (operating plus background), 

background, and interrupted/excluded data. The data were binned by the standardized wind 

speed into 1m/s wind speed bins centered on the integer wind speed. The bin centers were 

calculated by interpolation (and extrapolation at the ends). 

The AWEA Rated Sound Level is defined as: the sound level that will not be exceeded 95% of 

the time (assuming an average wind speed of 5 m/s); a Rayleigh wind speed distribution; 100% 

availability; and an observer location that is 60 m from the rotor center. This requirement defines 

the AWEA wind speed to be 9.8 m/s at hub height. The total and background noise for 9.8 m/s 

were obtained by interpolation between the 9 and 10 m/s binned values. The two values are used 

to obtain the background corrected sound pressure level. Next, the sound power level is 

calculated. The AWEA Rated Sound Level is then calculated using this value.

Table 12. AWEA Rated Sound Level

AWEA Rated Sound Level

dBA 

Combined 

Uncertainty

dBA 

38.3 1.9
Swift IEC Acoustic Noise Report 130215 Page 21 of 39

Appendix B. Pictures

Figure B1. Picture of the sound board during the test 

(Photo by Jason Roadman, NREL) 
Swift IEC Acoustic Noise Report 130215 Page 22 of 39

Figure B2. The test turbine as viewed from the reference microphone position 

(Photo by Jason Roadman, NREL) 
Swift IEC Acoustic Noise Report 130215 Page 23 of 39

Figure B3. The test turbine as viewed from the meteorological mast 

(Photo by Ismael Mendoza, NREL 22083) 
Swift IEC Acoustic Noise Report 130215 Page 24 of 39

Appendix C. Calibration Sheets

Figure C1. Calibration sheet for the microphone 2406811
Swift IEC Acoustic Noise Report 130215 Page 25 of 39

Figure C2. Calibration sheet for the microphone 2406809
Swift IEC Acoustic Noise Report 130215 Page 26 of 39

Figure C3. Calibration sheet for the sound level meter
Swift IEC Acoustic Noise Report 130215 Page 27 of 39

Figure C4. Calibration sheet for the acoustic calibrator
Swift IEC Acoustic Noise Report 130215 Page 28 of 39

Figure C5. Calibration sheet for the primary anemometer
Swift IEC Acoustic Noise Report 130215 Page 29 of 39

Figure C6. Post-test calibration sheet for the primary anemometer
Swift IEC Acoustic Noise Report 130215 Page 30 of 39

Figure C7. Page 1 of the power transducer calibration sheet
Swift IEC Acoustic Noise Report 130215 Page 31 of 39

Figure C8. Calibration sheet for the pressure transducer
Swift IEC Acoustic Noise Report 130215 Page 32 of 39

Figure C9. Calibration sheet for the temperature sensor
Swift IEC Acoustic Noise Report 130215 Page 33 of 39

Figure C10. Calibration sheet for the wind vane
Swift IEC Acoustic Noise Report 130215 Page 34 of 39

Figure C11. Calibration sheet for the signal conditioning module 1496266
Swift IEC Acoustic Noise Report 130215 Page 35 of 39

Figure C12. Post-test calibration sheet for the signal conditioning module 1496266
Swift IEC Acoustic Noise Report 130215 Page 36 of 39

Figure C13. Calibration sheet for the signal conditioning module 14A34EE
Swift IEC Acoustic Noise Report 130215 Page 37 of 39

Figure C14. Post-test calibration sheet for the signal conditioning module 14A34EE
Swift IEC Acoustic Noise Report 130215 Page 38 of 39

Figure C15. Calibration sheet for the signal conditioning module 1494F69
Swift IEC Acoustic Noise Report 130215 Page 39 of 39

Figure C16. Post-test calibration sheet for the signal conditioning module 1494F69