research study on wind turbine acoustics draft

Research Study on Wind Turbine Acoustics DRAFT

March 7, 2014

Interim Results II for WNTAG

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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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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

Sound monitoring metrics

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New terms

• Site

• Location

• Background sound level vs ambient

• L90 LAf max (1-sec)

• L90 of the L90

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Consideration of new sound monitoring metric for Turbine sound – L90 of Lafmax(1-sec)

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Consideration of new sound monitoring metric for Turbine sound – L90 of Lafmax(1-sec)

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Consideration of new sound monitoring metric for Turbine sound – L90 of Lafmax(1-sec)

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Sound monitoring metrics – Background sound

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Sound monitoring metrics – Background sound

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Sound monitoring metrics – Background sound

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Background L90 - Variability

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Effect of wind speed on L90 – wind shearWind Shear

-0.2 0.3

0.6

Wind shear

exponents

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Background L90 and Wind Speed are significantly correlated

Slopes of 80-meter wind speed vs sound level for various methodologies

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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

Pre-construction sound predictions

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Example of pre-construction modeling methodology for one site 370 meters downwind

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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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Example of pre-construction modeling methodology for one site 370 meters downwind

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Example of pre-construction modeling methodology for one site 370 meters downwind

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Example of pre-construction modeling methodology for one site 370 meters downwind

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Measured L90s of turbine sound levels

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Perfect modeling of wind turbine sound

Attended sound monitoring

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Filtering background sound

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Filtering background sound

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Filtering background sound

Statistical Confidence in Measurements

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New terms

• Statistical Bias

• Accuracy

• Precision

• Confidence Interval

• Standard Deviation

• Standard Error

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Comparing background to turbine-on measurements

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Estimate means and confidence intervals

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Estimate means and confidence intervals

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Suggested strategy for using different metrics for background and turbine-on measurements

Conclusions

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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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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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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

www.rsginc.com

Contacts

www.rsginc.com

Contact

Kenneth Kaliski, P.E., INCE Bd. Cert.Senior Director

Ken.kaliski@rsginc.com

802-295-4999