The acoustic consulting engineering firm Hessler Associates, Inc., Haymarket, Virginia produced two sound level assessment reports for two wind projects proposed for Cape Vincent, New York: the first report in 2007 for BP and the second report in 2009 for AES-Acciona. Because there were concerns early on among local citizens that the BP report was misleading, the Wind Power Ethics Group (WPEG) contracted with Schomer and Associates, Champaign, Illinois to conduct an independent background sound survey of Cape Vincent.
Hessler’s BP study for the Cape Vincent Wind Power Facility appears to have selected the noisiest sites, the noisiest time of year, and the noisiest positions at each measurement site. Collectively, these choices resulted in a substantial overestimate of the a-weighted ambient sound level, 45-50 dB according to Hessler.
The present study was designed to address a number of flaws noted in Hessler’s BP study.
A summer survey was planned so it would not coincide with the emergence of vocal adult insects (e.g., fall crickets and cicadas on August 1). Two monitoring sites were selected within the Town of Cape Vincent. One site was a rural residence and the other a small dairy farm. At each of these sites, two sound level meters and a single small weather station were run for one week of continuous data collection. At each site one meter was set up close to the house or farm building and a road. This site was called the “Hessler” position, because it was typical of sites selected by Hessler for his studies in Cape Vincent. The other position was called the Community position and it was located back away from the noise influences of roads, houses and farm operations. The Community position also reflected guidelines adopted by the Cape Vincent Planning Board whereby sound levels were to be measured at the property lines, not residences.
The analysis of the spectral (frequency) content of the sound showed that much of the difference in sound levels between Hessler’s study and the present study was attributable to insect noise, sounds near 5000 Hz. Hessler failed to remove insect sound from his data and recalculate A-weighted sound levels, even though he previously (2006) recommended this procedure to other scientists and engineers in a professional journal publication. Had he followed his own advice, ambient sound levels would have been more comparable to the results in this study.
Furthermore, and more importantly, wind turbine sound spectra are low-frequency and mid-frequency phenomena; therefore, higher-frequency insect noise will not mask wind turbine sounds. So even if insect noise was present year round instead of for a few weeks it should still not be included in the ambient because it provides little or no masking of the wind turbine sound.
Other examples of Hessler’s misleading choices include arbitrarily discarding sound data from one of his sites because the levels were too low. Remarkably, the levels at that site were more similar to those in this study. Also, Hessler described position 3 in the BP study as “representative of a typical residence along NYS Rte 12E.” However, he failed to show that the trailer in the photograph was a field office for a construction company installing a new Town of Cape Vincent water district. Furthermore, at the back of the trailer, out of view, was a marshalling yard for trucks, supplies and heavy equipment. The choice of this site and suggesting it as a typical residence were very misleading.
The accurate measurement of spectrally relevant ambient sound is important because these levels are used by wind developers to assess wind turbine noise impacts on nearby, non-participating residents. Local Cape Vincent Planning Board guidelines suggest that these impacts should not exceed 5 dB above the A-weighted ambient at the property lines of non-participating residents. New York State noise assessment policy states that any new sound that exceeds 6 dB above the A-weighted ambient should undergo a detailed assessment and that the developer is required to mitigate any excessive noise. Therefore, using an inaccurate, elevated A-weighted ambient level, such as 47 dB, allows wind developers to place wind turbines much closer to non-participating residents in such a way that the A-weighted wind turbine noise level will be 52 dB (e.g., 5 dB above Hessler’s elevated ambient level). A much more accurate and typical ambient level is 30 dB, which is an average of both “Hessler” and Community positions during daytime, evening and nighttime periods from the present study. Using 30 dB as a typical A-weighted ambient level would then require wind developers to plan a wind farm where predicted noise at non-participating property lines would not exceed 35 dB, or 5 dB above this study’s A-weighted ambient level. Again, to adequately protect rural residents that are not participants in proposed wind farms it is essential to have accurate, unbiased assessments of ambient sounds.
- The Hessler position at a measurement site systematically and significantly yields higher sound levels than does the Community position.
- The sound levels measured in the present study show Cape Vincent to be a quiet rural area, much as depicted by the data for Hessler’s position 4.
- Measurements, such as those conducted at Hessler’s position 3, are not indicative of the noise environment of typical residences in the Cape Vincent area.
- Failure to remove insect noise in Hessler’s study violated his own recommended survey and analytical techniques and substantially misrepresented typical ambient sound levels.
- In assessing potential noise impacts from wind turbine development, rather than using 45-50 dB A-weighted levels as suggested by Hessler, a more accurate level would be 30 dB, which is the average value for the daytime, evening and nighttime L90 sound levels observed at both the “Hessler” and Community positions for sites A and B in this study. Arguably, the level should be 20 to 25 dB, since an A-weighted L90 of 20 dB occurs during the quietest nighttime hours, and the A-weighted L90 for the whole 9-hour night is 25 dB.
May 11, 2009
Paul Schomer, Ph.D., P.E.
Schomer and Associates, Inc.
This article is the work of the author(s) indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.
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