Developing an American (ANSI) standard for the prediction of wind turbine sound levels Mark Bastasch 1 Jacobs 2020 SW 4 th Avenue 3 rd Floor Portland, OR 97201 USA

ABSTRACT Common methods to model wind turbine sound in the United States yield relatively similar results as they rely on sound power levels from IEC 61400-11 and propagation algorithms of ISO 9613-2. Nonetheless, standardization of predictions is anticipated to result in a robust and repeatable process that increases regulatory confidence and understanding. American Clean Power (ACP) is recognized by the American National Standards Institute (ANSI) as an Accredited Standards Developer. A wind turbine sound modeling standard working group was established by ACP and reached consensus on a preferred method for predicting sound levels during the siting and permitting process. The goal of this standard is to establish a uniform method of predicting future project sound levels such that pre-development sound assessment results or predictions used in research can be readily compared.

1. INTRODUCTION

Permitting a new wind energy project often requires modeling of expected operational sound levels. In the United States, acoustical modeling of wind energy projects has historically relied on sound power levels based on IEC 61400-11 and the ISO 9613-2 propagation methodology. In 2021 ACP Standards established a working group to develop a standard for modeling sound levels from wind turbines. The working group held regular 1- to 2-hour virtual meetings over an approximately 10- month period and worked collaboratively to develop the standard. Consensus on a methodology for predicting sound levels was achieved. This methodology has been drafted into a new ANSI/ACP standard that has demonstrated consensus, complies with ANSI’s essential requirements, and was issued as a formal standard in the April 2022. A standardized calculation method ensures makes calculated results are repeatable, uniform, and more easily understood by all interested parties. This paper summarizes the outcome of the working group efforts and the agreed to methodology.

2. GOALS OF THE STANDARD

It is recognized that this new standard does not supersede permit requirements nor invalidate previous or future studies. Rather the goals of this standardization effort were to:

1 Mark.Bastasch@Jacobs.com

a Shea mar ce 21-24 AUGUST SCOTTISH BENT caso

• Serve as a guide for practitioners. • Establish a uniform method of predicting future projects sound levels at typical setback distances for utility-scale on-shore projects. • Reduce confusion and facilitate a uniform basis for comparing predicted results during permitting and in the peer-reviewed scientific literature. • Develop a robust and repeatable process that bolsters regulatory confidence in modeling results. • Provide reasonably conservative results for realistic worst-case conditions. • Recognize that other calculation methods have been used and that this standard does not indicate flaws or errors in other methods.

3. SUMMARY OF WORKING GROUP FINDINGS

The following discussions summarizes the standardized modeling parameters established in the standard. The apparent wind turbine model-specific sound power level should be determined in accordance with IEC 61400-11 [1] for the downwind location. Using this sound power level as the basis, propagation modeling should be conducted in accordance with the ISO 9613-2 [2] standard (Acoustics—Sound Attenuation During Propagation Outdoors Part 2: General Method of Calculation) using either of two sets of parameters, identified as Option 1 and Option 2 detailed below:

Option 1

A. Ground factor of G=0. B. While modeled receptor height does not influence model results with G=0, it can be stated as 1.5 meters for consistency with existing standards with respect to the microphone position for measurements or as 4 meters for consistency with Option 2. C. Turbine modeled at hub height using vendor’s apparent downwind sound power level specified consistent with IEC 61400-11. D. No other model adjustments.

Option 2

A. Ground factor of G=0.5. B. Receptor height of 4 meters. C. Turbine modeled at hub height using vendor’s apparent downwind sound power level specified consistent with IEC 61400-11. D. A model adjustment of + 2 decibels.

The difference between Options 1 and 2 is small, tenths of a decibel, a difference that is neither reliably measured nor discernable. Option 2 is provided as some jurisdictions have established a precedent of adjusting model inputs or results and requiring such adjustments to Option 1 is not supported. Using a receptor height of 4 meters in Option 2 should not influence the typical microphone measurement height.

Atmospheric absorption per ISO 9613-1 [3] for conditions of 10 degrees Celsius (50 degrees Fahrenheit) and 70 percent humidity are recommended for both options. 4. INFORMATIVE APPENDIX

As regulatory approaches in the United States vary from state to state and in some cases from county to county, the standard includes a brief appendix which provides additional commentary on a range of topics that the working group expected would assist those conducting the analysis. The topics addressed in the appendix are summarized in the following discussion.

For jurisdictions that require Leq durations to be evaluated which are outside of 10 minutes to 1- hour timeframe additional adjustments to the predicted value are noted to likely be warranted.

Averaging times longer than 1 hour are noted to yield sound levels less than those predicted given the standard relies on the full-acoustic output and conservative meteorological conditions.

Metrics such as the L10 are noted to present many measurement challenges (i.e., high variability given influence of non-project sources yields low repeatability) and thus inappropriate for use in modeling or measuring noise from wind turbines. The working group comments that the relevance of L10 style limits for wind energy facilities has not been established; rather, the L10 limits are often a carryover from existing regulations for other sources. The working group also noted in the appendix that:

“Averaging times less than 10 minutes have not been found to be appropriate for regulatory use on wind turbines, given the inherent inaccuracies in subtracting background sound levels from short-duration events. The Lmax has not been found to be a repeatable metric in that it is inherently an outlier and for wind turbines the measured Lmax is often related to non- project sources. This standard therefore does not address nor recommends that instantaneous maxima, such as LFmax, LSmax, or LImax, be relied on in regulatory settings.” Reference is made to the forthcoming IEC 61400-11-2 (Wind energy generation systems – Part 11- 2: Measurement of wind turbine noise characteristics in receptor postion [4]) which is expected to provide more thorough guidance on field measurements at far field locations when released. As such, measurements were considered outside the scope of this standard at this time. Though it is clarified that the standard identifies conservative parameters which cover a wide range of operating conditions and advises that if on/off measurement methods are used, it is appropriate for the analysis period to focus on typical operations rather than those that occur during the forced start and stop transitional periods. Additionally, it is stated that when operational measurements are required, collecting and analyzing the L90 and L50 metrics may be helpful.

Apparent sound power levels based on IEC 61400-11 is typically readily available from the turbine vendors for windspeeds up to 15 m/s. This data is stated to be appropriate as the highest sound power levels are generally achieved between 8 to 10 m/s. When a project utilizes more than one type of turbine whose highest sound power levels are achieved at different wind speeds, one may wish to consider what may occur simultaneously rather than utilize the highest sound power level of each turbine model simultaneously.

C-weighted and octave band sound levels are also briefly discussed. Several studies are cited which found that C- and A-weighted levels were highly correlated and there was no substantial advantage of one over another.

Caution is advised when considering potential additional reductions afforded by vegetation or terrain given the typical height of utility scale wind turbines may reduce the attenuation experienced at more conventional heights. When considering potential influence of topography, moving the source height from hub height to tip height as well as comparisons to flat ground predictions are noted to potentially assist in evaluating reasonableness of predicted shielding.

5. CONCLUSIONS

A new ANSI standard has been established to standardize the predictions of wind turbine sound levels. It relies on existing standard methods including ISO 9613-1 [3], ISO 9613-2 [2] and IEC 61400-11 [1]. It is available from https://cleanpower.org/standards-development and ACP Standards can be contacted via email: standards@cleanpower.org.

The standardization of predictions is anticipated to ensure a robust and repeatable process that bolsters regulatory confidence in the results and reduces potential confusion. 6. ACKNOWLEDGEMENTS

This effort would not have been possible without the collaboration, dedication and insight of the following working group members (listed alphabetically):

• Payam Ashtiani • Mark Bastasch • Kaveh Habibi • Michael Hankard • Kenneth Kaliski • Richard Lampeter • Krispian Lowe • Marcel Mibus • Chris Ollson • Justin Puggioni • Erik Sloth Special thanks is also owed to the American Clean Power Standards team for their assistance throughout the working group and standard development process.

7. REFERENCES

1. International Electrotechnical Commission (IEC), IEC 614000 Wind turbines – Part 11: Acoustic noise measurement techniques, Edition 3 2012-11 2. International Organization for Standardization (ISO), ISO 9613-2:1996 Acoustics – Attenuation of sound during propagation outdoors – Part 2: General method of calculation, Edition 1 1996- 12 3. International Organization for Standardization (ISO), ISO 9613-1:1993 Acoustics – Attenuation of sound during propagation outdoors – Part 1: Calculation of the absorption of sound by the atmosphere, Edition 1 1993-06 4. International Electrotechnical Commission (IEC), IEC 61400 Wind energy generation systems - Part 11-2: Measurement of wind turbine noise characteristics in receptor position , https://www.iec.ch/ , Unpublished as of preparation of this manuscript.