IEC Acoustic Standard IEC 61400-11

Part I Acoustic Testing Stepping Through the 

IEC 61400-11 Standard Through Apparent 

Sound Power Level 

2011 Small Wind 

Turbine Testing 

Workshop 

Arlinda Huskey 

28 July 2011 

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Relevant Document 

IEC 61400-11 2002-12 Wind Turbine 

Generator Systems – Part 11: Acoustic 

Noise Measurement Techniques 2.1 

Edition 

• Sound power levels 

• One-third octave levels 

• Tonality 

Note: 

lowercase spl = sound pressure level 

uppercase SPL = sound power level 

NATIONAL RENEWABLE ENERGY LABORATORY

1 Scope 

General description and purpose for the standard 

“enable noise emissions of a wind turbine” 

“methods appropriate to noise emissions 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” 

“standarisation of measurement procedures will also 

facilitate comparisons between different wind turbines 

“characterise in a consistent and accurate manner” 

“If, in some cases, less comprehensive measurements are 

needed, such measurements are made according to the 

relevant parts of the standard” 

NATIONAL RENEWABLE ENERGY LABORATORY 

3

2 Normative References 

References to instrument standards for noise 

measurement equipment, defining accuracy 

requirements, filters, frequency, etc. 

Reference to power performance standard for wind speed 

derived from power 


4

3 Definitions and 4 Symbols and Units 

3 Definitions and explanation for more important variables 

4 Definitions all variables used in the standard along with 

units 


5

5 Outline of the Method 

Overview of the method 


6

6 Instrumentation 

Required acoustic measurements 

Sound pressure levels 

One-third octave levels 

Narrowband spectra 

Can be met by different types of instruments 

Signal analyzer 

Laptop and software 

Handheld instruments, sound level meter 

Requirements 

Must met IEC Type 1 accuracy requirements and filters 

Frequency range 


7


Microphone & preamplifier 

Size, diameter no larger than 13mm 

Measurement board 

Circular with diameter of at least 1m 

Made with acoustically hard material like plywood with 

thickness 12mm or metal with 2.5mm. A larger board is 

recommended for soft ground. 

Primary microphone windscreen 

One half of an open cell foam sphere with diameter of 90mm 

Secondary microphone windscreen 

Sold but expensive 

Example – wire frame hemispherical shape 450mm 

diameter covered with 13-25mm layer of open cell foam 


8


Acoustical calibrator 

Meet IEC 60942 class 1 

Example: 94dB at 1kHz calibrator 

Data recording/playback system 

Meet IEC 60651 for type 1, examples in Annex B 

Example: digital analog tape recorders, hard disk 


9


Anemometers 

End to end deviation of ±0.2m/s in range of 4-12m/s 

Capable of synchronizing with acoustic data 

Power transducer 

Meet IEC 60688, same as power performance 

Wind direction 

Accuracy of ±6 o 

Camera 

Distance measurement 

Pressure 

Accuracy of ±1kPa 

Temperature 

Accuracy of ±1 o C 


10


Traceable Calibration 

Acoustic calibrator 12 mo 

Microphone 24 mo 

Integrating sound level meter 24 mo 

Spectrum analyzer 36 mo 

Data recording/playback system 24 mo 

Anemometer 24 mo 

Power transducer 24 mo 

Exceptions 

Recalibrate after repair, damage, or suspect 


11

7 Measurements and Measurement Positions 

Acoustic measurement positions 

One required measurement position but 3 optional 

Downwind is the reference position 

Data ±15 o relative to the wind direction 

Horizontal distance for a horizontal axis turbine 

R o = H + D/2 

To minimize influence due to the edges of the measurement 

board, lay flat on ground and level edges or gaps with soil 

Inclination angle should be 25 o to 40 o 

Minimize reflections from nearby structures, less than 0.2dB 

Adjust the board to meet these requirements 


12


Wind speed and direction measurement positions 

Upwind at a height between 10m and rotor centre 

Horizontal distance of 2D-4D from rotor centre 

Allowable region for met tower for measured wind speed 

During test measurements, anemometer must not be in the 

wake of any structure including the turbine 

Anemometer and wind vane should not interfere with each 

other 

Other wordage about nacelle anno, not as important for 

small wind turbines 


13


Acoustic measurements 

The following information will be determined at integer wind 

speeds of 6, 7, 8, 9, and 10m/s 

- the apparent sound power levels 

- the one-third octave band levels 

- the tonality (not required for AWEA) 

Optional measurements 

- directivity 

- infrasound 

- low-frequency noise 

- impulsivity 


14


Acoustic measurement requirements 

Calibration of measurement chain at least before and after 

or if microphone is moved 

Record data for later analysis 

Omit data with interruptions in the background (cars, plane) 

Collect turbine and background data under similar conditions 

Cover as broad a range of wind speeds 


15


A-weighted sound pressure level 

At least 30 1-minute averaged measurements with wind speed 

when the turbine is operating (AWEA mod:10-sec) 

At least 3 measurements per wind speed bin. (For statistical 

purposes/good practice if using 10-second averages then 10 

points per wind speed bin) 

At least 30 minutes of background (turbine not operating) 

covering the same as turbine measurements 

A-weighted one-third octave levels 

At least 3 spectra measured over 1-minute for each integer 

wind speed and covering centre frequencies of 50Hz-10kHz 

(same as above modify for 10-second averages) 

Background covers the same requirements 

A-weighted narrowband 

At least 2 minutes closest to the integer wind speed 


16


Non-acoustic measurements: wind direction 

Method 1: determination of wind speed from the electric 

output and the power curve (not covering since not 

applicable to all small wind turbines) 

Method 2: determination of wind speed with an anemometer 

(AWEA mod) 

Adjust wind speed to 10m height and reference roughness 

length (covered in section 8) 

Measurement by anemometer at a height between 10m and 

hub height 

Wind speed data collected and arithmetically averaged over 

the same period as the acoustic measurements 


17


Non-acoustic measurements: wind direction 

Wind direction measurements must be within 15 degrees of 

nacelle azimuth position with respect to upwind 

Arithmetically averaged over the same period as acoustic 

measurements 

Non-acoustic measurements: Other 

Air temperature and pressure measured and recorded at 

least every 2 hours 

Optionally turbulence as discussed in Annex C 

Rotor speed and pitch angle are recommended 


18

8 Data Reduction Procedures 

Wind speed 

Wind speeds measured at height z shall be corrected to a 

wind speed at reference conditions by assuming wind 

profiles 


z Oref reference roughness length, 0.05m 

z O roughness length 

H rotor centre height 

z ref reference height, 10m 

z anemometer height 

Roughness length z O can be calculated or estimated using Table 1 

in the standard 

19


Correction for background noise 

Background correction is used for sound pressure levels, onethird 

octave levels, and tonality 


L s equivalent continuous sound pressure level of 

wind turbine operating alone 

L s+n equivalent continuous sound pressure level of 

wind turbine plus background noise 

L n background equivalent continuous sound 

pressure level 

If difference between Ls+n and Ln is greater than 6dBA use equation 

If between 3-6dBA then use correction of 1.3dBA and marked with 

an asterisk “*”. Cannot be used for sound power level calculation. 

If difference less than 3dBA, cannot report and report “turbine noise 

was less than background” 

20


Apparent sound power levels 

Preferred method from the standard is a 4 th order regression. 

AWEA mod is to bin data by wind speed into 1m/s bins and 

one point on both sides of the integer wind speed 

To get integer value, interpolate/extrapolate between bins 

This is done for total noise and background, then background 

correct integer value 

Calculate the apparent sound power levels for each integer 

wind speeds 


L aeq,c,k background corrected A-weighted sound pressure level at 

integer wind speed k and under reference conditions 

R 1 slant distance from rotor centre to microphone 

S O reference area, 1m 2 

21

A little breather before we move to looking at data 


Questions? 

22

Look at NoiseLab 

Setup 

Calibration 

Recording 

Clips 

Analysis 


23

Listen to turbine data 

EW50 background 

EW50 turbine, normal operation 

EW50 interruption 

VT10 turbine, normal operation 

VT10 turbine, unloaded 


24

Filtering for Valid Data 

– Listen for interrupting noises (cars, other turbines, airplanes) 

– Transitions 

– Wind direction or yaw direction outside of ±15 degrees from 

microphone board 

– Plot sound pressure vs. wind speed or power or rotor speed 

– Look for outliers, verify if they are “real” 

Rotor 

Wind 

Wind 

Turb or 

Time Power Speed Temperature Speed Pressure Direction Availability Leq WD ok? Back 

13:36:35 -156 0 10 8 80 228 0 41 -10 Back 

13:36:45 -156 0 10 6 80 240 0 40 -10 Back 

13:36:55 -156 0 10 8 80 226 0 39 -10 Back 

13:37:05 -156 0 10 9 80 225 0 40 -10 Back 

13:37:15 -156 0 10 8 80 220 0 40 -10 Back 

13:37:25 -156 0 10 9 80 221 0 39 -10 Back 

13:37:35 -156 0 10 10 80 228 0 41 -10 Back 

13:37:45 -156 0 10 10 80 221 0 41 -10 Back 


25

Outliers? 

sound pressure level (dB A) 

55 

50 

45 

40 

35 

30 

25 

20 


0 2 4 6 8 10 12 14 

wind speed (m/s) 

26

Binning and Averaging 

After binning by 10 meter wind speed, 

for each bin that has enough data in the 

bin 

– Arithmetically average wind speed, 

most likely will not be at the bin center 

but close 

– Energy average sound pressure 

levels 

– For each bin you have an average 

wind speed and sound pressure level 


wind 

Aweighted 

sound 

pressure 

count speed level 10 0.1*spl 

n spl 

1 11.9 50.59 114551 

2 11.6 50.88 122462 

3 11.3 48.13 65013 

4 10.4 46.49 44566 

5 10.0 45.95 39355 

6 9.1 45.57 36058 

7 9.6 45.73 37411 

8 8.3 44.36 27290 

82.2 Sum 486705 

10.3 Sum/n 60838 

log10 4.78 

10*log10 47.8

Bin Centers 

Bin wind speed averages likely will not 

be at the center so interpolate (or 

extrapolate at the ends) to get bin 

centers 

– Extrapolate 6 and 7 m/s bins to get 

the bin center at 6 m/s 

– Interpolate 6 and 7 m/s bins to get the 

bin center at 7 m/s 

– Interpolate 8 and 9 m/s bins to get the 

bin center at 8 and 9 m/s 

– Interpolate 9 and 10 m/s bins to get 

the bin center at 10 m/s 

Find turbine and background bin center 

values 


WS bin WS avg count spl avg 

6 6.02 16 48.0 

7 7.28 25 47.8 

8 7.95 47 50.4 

9 9.10 64 52.5 

10 10.20 55 55.7 

WS bin slope offset int spl 

6 -0.16 49.0 48.0 

7 -0.16 49.0 47.8 

8 1.83 35.9 50.5 

9 1.83 35.9 52.3 

10 2.91 26.0 55.1

Background Correction 

Calculate the background corrected 

sound pressure level 

spl 

corr 

� 

10 * log 

10 

Must have 6 dB separation between 

turbine and background noise to report 

sound power level 

– cannot report 7 m/s sound power level 


�0.1* splturb 

� �0.1* splback 

� 10 � 

�� 

�10 � 

�� 

WS 

bin 

Turb 

spl 

Back 

spl 

CORR 

spl 

Diff 

6 48.0 41.4 46.9 6.6 

7 46.7 42.8 45.4 3.8 

8 50.7 44.3 49.6 6.4 

9 52.3 45.0 51.4 7.4 

10 55.0 46.4 54.4 8.6

Sound Power Level 

Calculate the sound power level for each 

bin using the sound pressure levels, 

slant distance (distance from rotor center 

to microphone), and reference area (S o 

= 1 m 2 ) 

SPL 

� 

SPL = 82.8 dB A 

spl corr = 46.9 dB A 

slant distance, R = 35.1 m 

S o = 1 m 2 

spl corr 


� 

6 

� 

10lg 

� 

� 

� 

� 

2 

4� 

R 

S0 

� 

� 

� 

� 

WS 

bin 

CORR 

spl 

Diff SPL 

6 46.9 6.6 82.8 

7 45.4 3.8 * 

8 49.6 6.4 85.4 

9 51.4 7.4 87.3 

10 54.4 8.6 90.2


Questions? 

Before we move onto one-third octaves 

31

One-Third Octave Levels 

Concepts are the same but averaging and correcting on the onethird 

octave bands. 

– Take three minutes of one-third octave data closest to the integer 

wind speed for turbine and background 

– Arithmetically average wind speeds and energy average the onethird 

octave spectra by band (i.e. 20, 25, 31.5 Hz) 

– Extrapolate or interpolate to get integer values 

– Background correct by band if there is a 6 dB separation. There is 

a standard correction for differences between 3 and 6 dB. Cannot 

background correct for differences lower than 3 dB. Note: There 

can be negative numbers. There can be bands where the 

background levels greater than the turbine levels. 

WS bin 20 25 31.5 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 

Turb 10.0 16.4 18.6 20.2 23.6 23.4 25.9 28.7 31.2 33.1 35.4 45.4 55.0 39.9 39.5 41.9 41.6 42.2 42.4 42.1 

Back 10.1 16.4 18.7 20.2 24.1 22.2 24.3 28.7 30.0 30.9 32.8 36.9 34.8 35.3 34.6 34.6 35.0 34.4 34.7 33.8 

Integer Turb 10 16.4 18.7 20.3 23.6 23.4 25.9 28.8 31.3 33.1 35.4 45.4 55.1 40.0 39.6 42.0 41.7 42.2 42.5 42.1 

Integer Back 10 16.2 18.4 20.0 24.0 22.2 24.3 28.6 30.0 30.9 32.7 36.7 34.7 35.2 34.5 34.5 34.8 34.3 34.6 33.6 

Difference 10 0.2 0.3 0.3 -0.4 1.3 1.6 0.2 1.3 2.2 2.7 8.7 20.4 4.8 5.1 7.5 6.8 7.9 7.9 8.5 

Back Corr 10 44.8 55.1 38.7 38.3 41.1 40.7 41.5 41.7 41.5 


One-Third Octave Levels 

Sound pressure level [dB(A)] 

60 

50 

40 

30 

20 

10 

0 


Frequency [Hz] 

5 m/s 

6 m/s 

7 m/s 

8 m/s 

9 m/s 

10 m/s

That was quick and easier than I thought… 

Right? 


Questions? 

Before we move onto tonality 

34

Tonality 

To begin 

– Two minutes of FFT spectra data closest to the integer wind speed 

for turbine and background 

– Turbine narrowband analysis use 10-second spectra 

– Background spectra are energy averaged into one spectrum for 

use later in the turbine analysis 

– For each turbine 10-second spectrum identify possible tones. For 

each possible tone: 

• Determine critical bandwidth 

• Classify spectral lines in the critical band 

• Determine tone level 

• Determine masking level and correct for background 

• Determine tonality 

• Determine if the tone is reportable or “audible” 


What Is Considered a Possible Tone? 


-10 


50 

40 

30 

20 

10 

0 

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 

frequency (Hz) 

Technically – it is a peak with adjacent lines with lower amplitudes 

Analyze possible tones

Determine Critical Bandwidth 

Calculate the critical bandwidth 

Critical 

Bandwidth 

For this possible tone at 249 Hz (f c = 249), the critical 

bandwidth is 104.43 Hz 

Center critical bandwidth around the peak. For this 

example 196.8 to 301.2 Hz. 

This will not symmetrically fit due to the resolution. How 

to specifically deal with this is not covered in the 

standard. 

For our purposes, the critical bandwidth is 102 Hz 

covering the lines from 198 to 300 Hz 


� 

� � f � 

c 

� 25 � 75�1� 

1, 

4� 

� 

� �1000� 

� � � 

2 

� 

� 

� 

� 

� 

0, 

69 

Freq (Hz) spl (dB A) 

198 25.30 

201 25.50 

204 26.02 

207 29.15 

210 32.15 

213 37.62 

216 39.88 

219 39.75 

222 39.86 

225 41.20 

228 40.31 

231 42.48 

234 43.87 

237 42.31 

240 43.43 

243 43.21 

246 43.50 

249 45.44 

252 41.22 

255 35.09 

258 30.45 

261 29.38 

264 30.78 

267 27.78 

270 26.37 

273 26.79 

276 27.61 

279 28.68 

282 27.74 

285 27.67 

288 27.38 

291 27.10 

294 27.68 

297 27.91 

300 27.46

Calculate L 70% for Classification 

Within each critical band every spectral line is classified 

as tone, masking, or neither 

Calculate L 70% level 

The energy average of the 70% of spectral lines in the 

critical band with the lowest spl levels 

L 70% = 33.13 dB A 

Calculate the criterion level, L 70% + 6 dB A = 39.13 dB A 



50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

198 

201 

204 

207 

210 

213 

216 

219 

222 

225 

228 

231 

234 

237 

240 

243 

246 

249 

252 

255 

258 

261 

264 

267 

270 

273 

276 

279 

282 

285 

288 

291 

294 

297 

300 


Freq (Hz) spl (dB A) 10^0.1*spl 

198 25.30 338.67 

201 25.50 354.59 

204 26.02 399.88 

270 26.37 433.42 

273 26.79 477.90 

291 27.10 512.81 

288 27.38 546.63 

300 27.46 557.78 

276 27.61 576.47 

285 27.67 584.37 

294 27.68 586.63 

282 27.74 594.64 

267 27.78 599.23 

297 27.91 617.72 

279 28.68 737.67 

207 29.15 822.10 

261 29.38 866.01 

258 30.45 1108.94 

264 30.78 1197.30 

210 32.15 1642.39 

255 35.09 3231.56 

213 37.62 5781.61 

219 39.75 9447.28 

222 39.86 9678.80 

216 39.88 9733.16 

228 40.31 

225 41.20 

252 41.22 

237 42.31 

231 42.48 

243 43.21 

240 43.43 

246 43.50 

234 43.87 

249 45.44 

sum/n 2057.10 

10*log10 33.13

Classify ‘Masking’ 

A line is classified as ‘masking’ if its level is less 

than the criterion level L 70% + 6 dB. 

L pn,avg is then the energy average of all ‘masking’ 

lines. 


50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

L pn,avg = 30.11 dB A 

198 

201 

204 

207 

210 

213 

216 

219 

222 

225 

228 

231 

234 

237 

240 

243 

246 

249 

252 

255 

258 

261 

264 

267 

270 

273 

276 

279 

282 

285 

288 

291 

294 

297 

300 



masking 

L pn,avg 

Freq (Hz) spl (dB A) Classification 10^0.1*spl 

198 25.30 masking 338.7 

201 25.50 masking 354.6 

204 26.02 masking 399.9 

207 29.15 masking 822.1 

210 32.15 masking 1642.4 

213 37.62 masking 5781.6 

216 39.88 0 

219 39.75 0 

222 39.86 0 

225 41.20 0 

228 40.31 0 

231 42.48 0 

234 43.87 0 

237 42.31 0 

240 43.43 0 

243 43.21 0 

246 43.50 0 

249 45.44 0 

252 41.22 0 

255 35.09 masking 3231.56 

258 30.45 masking 1108.94 

261 29.38 masking 866.01 

264 30.78 masking 1197.30 

267 27.78 masking 599.23 

270 26.37 masking 433.42 

273 26.79 masking 477.90 

276 27.61 masking 576.47 

279 28.68 masking 737.67 

282 27.74 masking 594.64 

285 27.67 masking 584.37 

288 27.38 masking 546.63 

291 27.10 masking 512.81 

294 27.68 masking 586.63 

297 27.91 masking 617.72 

300 27.46 masking 557.78 

sum/n 1025.83 

10*log10 30.11

Classify ‘Tone’ 

A line is classified as ‘tone’ if its level exceeds L pn,avg + 6 dB A = 36.11 dB A 

Where there are several lines classified as ‘tone’, the line having the greatest 

level is identified. Adjacent lines are then only classified as ‘tone’ if their levels 

are within 10 dB A of this level. 



50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

198 

201 

204 

207 

210 

213 

216 

219 

222 

225 

228 

231 

234 

237 

240 

243 

246 

249 

252 

255 

258 

261 

264 

267 

270 

273 

276 

279 

282 

285 

288 

291 

294 

297 

300 


masking 

tone 

L pn,avg + 6

Determination of Tone Level 

The sound pressure level of the tone L pt, is determined 

by energy summing all spectral lines classified as ‘tone’ 

spl 

sum 

� 10 * log 

If there are more than 2 lines then a correction must be 

applied for using the Hanning window. Divide the 

energy sum by 1.5. 

L pt = 51.88 dB A 

n 

10 � 

i�1 


10 

0. 

1* 

spl 

i 

Freq (Hz) spl (dB A) Classification 10^0.1*spl 

198 25.30 0.0 

201 25.50 0.0 

204 26.02 0.0 

207 29.15 0.0 

210 32.15 0.0 

213 37.62 tone 5781.6 

216 39.88 tone 9733.2 

219 39.75 tone 9447.3 

222 39.86 tone 9678.8 

225 41.20 tone 13176.0 

228 40.31 tone 10750.3 

231 42.48 tone 17696.5 

234 43.87 tone 24360.8 

237 42.31 tone 17038.8 

240 43.43 tone 22031.6 

243 43.21 tone 20962.0 

246 43.50 tone 22364.2 

249 45.44 tone 34958.8 

252 41.22 tone 13247.8 

255 35.09 0.0 

258 30.45 0.0 

261 29.38 0.0 

264 30.78 0.0 

267 27.78 0.0 

270 26.37 0.0 

273 26.79 0.0 

276 27.61 0.0 

279 28.68 0.0 

282 27.74 0.0 

285 27.67 0.0 

288 27.38 0.0 

291 27.10 0.0 

294 27.68 0.0 

297 27.91 0.0 

300 27.46 0.0 

sum/1.5 154151.80 

Lpt 51.88

Correction for Background 

Remember the 2-minute background spectrum? 

Look at the same critical band (198 to 300 Hz) and 

ensure the tone does not originate from background 

Calculate the background noise level by taking the 

energy sum of the same critical band if the difference is 

more than 6 dB A. Otherwise, “influenced by 

background” 

Correct masking level L pn,ave with background level 

spl 

corr 

Background level = 23.03 dB A 

L pn,ave = 30.11 

� 

10 * log 

10 

L pn, corr = 23.03 dB A 


�0.1* splturb 

� �0.1* splback 

� 10 � 

�� 

�10 � 

�� 

Freq (Hz) spl (dB A) 10^0.1*spl 

198 6.651 4.62 

201 7.221 5.27 

204 7.710 5.90 

207 8.941 7.84 

210 9.224 8.36 

213 7.170 5.21 

216 6.567 4.54 

219 6.220 4.19 

222 6.058 4.03 

225 6.011 3.99 

228 6.661 4.64 

231 7.248 5.31 

234 7.962 6.26 

237 8.537 7.14 

240 8.857 7.69 

243 9.650 9.23 

246 9.775 9.50 

249 7.935 6.22 

252 7.621 5.78 

255 8.037 6.36 

258 8.231 6.65 

261 8.132 6.50 

264 7.866 6.12 

267 6.957 4.96 

270 6.399 4.36 

273 6.146 4.12 

276 5.996 3.98 

279 6.050 4.03 

282 5.735 3.75 

285 6.019 4.00 

288 6.291 4.26 

291 7.187 5.23 

294 7.742 5.95 

297 8.222 6.64 

300 9.165 8.25 

sum 200.86 

10*log10 23.03

Determination of Masking Level 

Calculate the masking level, L pn 

� critical bandwidth � 

Lpn � Lpn, 

corr �10 

* log10 

� 

� 

�effective 

noisebandwidth 

� 

Where for our case 

critical bandwidth = 300 – 198 = 102 Hz 

effective noise bandwidth = 1.5 * resolution = 4.5 Hz 

Masking level 

L pn = 36.58 dB A 


Determination of Tonality 

The difference between the masking level and tone level is the tonality ∆L tn 

For the one critical band analyzed 


�L 

� L �L 

tn 

pn 

∆L tn = 51.88 - 36.58 = 15.30 

pt

Determination of Tonality 

The difference between the masking level and tone level is the tonality ∆L tn 

For the one critical band analyzed 

∆L tn = 15.30 

There will be 12 spectra to analyze the same peak and get a ∆L tn value for 

each spectrum 

For spectra where there are no tones use the following value 

The 12 resulting ∆L tn are energy averaged to one ∆L k 


�L 

� L �L 

tn 

pt 

� critical bandwidth � 

�L 

tn 

� �10 

* log10 

� 

� 

�effective 

noisebandwidth 

� 

pn

Determination of Audibility 

For each value ∆L k, a frequency dependent correction must be applied to 

compensate for the response of the human ear to tones of different frequencies 

The tonal audibility, ∆L a,k 

Where 

For tonal audibilities meeting the condition 

The values of ∆L a,k are reported 


L 

ΔL 

� �L 

�L 

a 

� 

a,k 

k 

a 

� � f � 

�2 

� lg�1� 

� � 

�� 

� 502� 

Δ 

L a,k 

� 

�3. 

0 

2. 

5 

� 

� 

��

Tonality can be complicated and there is a 

flowchart in the standard to help 


Questions? 

We are done! 

Do you think you could do a noise analysis now? 

For questions beyond the workshop: 

Arlinda Huskey 

arlinda.huskey@nrel.gov 

303-384-6987 

47

IEC Acoustic Standard IEC 61400-11

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