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Research Study on Wind Turbine Acoustics DRAFT
March 7, 2014
Interim Results II for WNTAG
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203.07.2014 RSG
Interim Report II
Interim Report II for WNTAG focuses on a comparison of sound metrics with sound modeling to help inform and synchronize pre-construction estimates with post-construction monitoring.
CONTENTS
• New terms
• Review of data collection
• Sound monitoring metrics
• Pre-construction sound predictions
• Attended sound monitoring
• Statistical confidence
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303.07.2014 RSG
Review of data collection
• Four sites to date – - all in Massachusetts - all 1.5 MW or greater
• Five sound monitoring locations at each site- 1/3 octave bands + other metrics at 100 ms to 1 s intervals-Type I sound monitors
• Infrasound monitoring at one location (inside and outside) at one site• One LIDAR location at each site• One 10-meter met tower at each site• Turbine operating conditions collected by operator• Over 120,000,000 data records logged• Over 150 sound level, meteorological, operational, and observational variables
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Sound monitoring metrics
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503.07.2014 RSG
New terms
• Site
• Location
• Background sound level vs ambient
• L90 LAf max (1-sec)
• L90 of the L90
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603.07.2014 RSG
Consideration of new sound monitoring metric for Turbine sound – L90 of Lafmax(1-sec)
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703.07.2014 RSG
Consideration of new sound monitoring metric for Turbine sound – L90 of Lafmax(1-sec)
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803.07.2014 RSG
Consideration of new sound monitoring metric for Turbine sound – L90 of Lafmax(1-sec)
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903.07.2014 RSG
Sound monitoring metrics – Background sound
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1003.07.2014 RSG
Sound monitoring metrics – Background sound
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1103.07.2014 RSG
Sound monitoring metrics – Background sound
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1203.07.2014 RSG
Background L90 - Variability
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1303.07.2014 RSG
Effect of wind speed on L90 – wind shearWind Shear
-0.2 0.3
0.6
Wind shear
exponents
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1403.07.2014 RSG
Background L90 and Wind Speed are significantly correlated
Slopes of 80-meter wind speed vs sound level for various methodologies
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1503.07.2014 RSG
Wind speeds vary during any measurement period
Example of a 10-minute period at one site, showing the frequency of occurrence of 0.5 m/s bins for 9 m/s average wind speed
ws90
1.3 standard deviations
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Pre-construction sound predictions
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1703.07.2014 RSG
Example of pre-construction modeling methodology for one site 370 meters downwind
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1803.07.2014 RSG
Example of pre-construction modeling methodology for one site 370 meters downwind
Location
L90 of 5-minute
L90s
L90 of 5-minute
Leqs
L90 of 5-minute
L50s
L50 of 5-minute
L90s1 1.7 1.2 1.6 1.82 1.6 1.3 1.4 1.53 1.6 1.4 1.3 1.84 1.2 1.2 1.2 1.25 1.3 1.2 1.2 0.6
Average 1.5 1.3 1.3 1.4
Slope of brown line in db/meter per second
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1903.07.2014 RSG
Example of pre-construction modeling methodology for one site 370 meters downwind
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2003.07.2014 RSG
Example of pre-construction modeling methodology for one site 370 meters downwind
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2103.07.2014 RSG
Example of pre-construction modeling methodology for one site 370 meters downwind
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2203.07.2014 RSG
Measured L90s of turbine sound levels
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2303.07.2014 RSG
Perfect modeling of wind turbine sound
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Attended sound monitoring
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2503.07.2014 RSG
Filtering background sound
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2603.07.2014 RSG
Filtering background sound
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2703.07.2014 RSG
Filtering background sound
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Statistical Confidence in Measurements
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2903.07.2014 RSG
New terms
• Statistical Bias
• Accuracy
• Precision
• Confidence Interval
• Standard Deviation
• Standard Error
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3003.07.2014 RSG
Comparing background to turbine-on measurements
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3103.07.2014 RSG
Estimate means and confidence intervals
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3203.07.2014 RSG
Estimate means and confidence intervals
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3303.07.2014 RSG
Suggested strategy for using different metrics for background and turbine-on measurements
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Conclusions
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3503.07.2014 RSG
Some specific conclusions from the report
• Background sound levels vary by time of year, time of day, and day of week.
• Natural short-term variation is partly a function wind speed and wind shear
• Sound levels measured on the ground increase when 80 meter wind speed increases
• Wind shear variation is highest at night and at low wind speeds
• Background sound will contaminate measurements of wind turbine sound
-Wind alone can have a significant effect-By definition, 90% of the turbine-on measurements have
background levels that are higher than the L90
• When measuring over five or 10 minutes, the wind speed exceeded 90 percent of the time is likely to be at a lower integer wind speed than the mean wind speed
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3603.07.2014 RSG
More specific conclusions from the report
• Since L90 and wind speed are correlated, this means that the L90 is also likely to occur at a lower wind speed relative to the mean. Adjustments can be made to account for this.
• The 10th percentile wind speed is a function of the mean and standard deviation of the measured wind speed over a period
• Considerations of sound metrics
-Using L90 of Lafmax (1-sec) for both background and turbine-on measurements
- Improving predictability by establishing a turbine-only sound limit based on background measurements during pre-construction
- Incorporating some type of statistical analysis to improve confidence in compliance measurement
-Adjust turbine-on sound metric (if different from background metric) to account for higher background sound.
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3703.07.2014 RSG
General conclusions
Overall, real-world systems are dynamic. Methods developed should take into account likelihood that •Conditions change during the measurement •Conditions change over time•Measurements including everything that produces sound in the environment•Methods to measure and model sound will have biases•Methods to measure and model sound will have variability
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www.rsginc.com
Contacts
www.rsginc.com
Contact
Kenneth Kaliski, P.E., INCE Bd. Cert.Senior Director
802-295-4999