Bahtiarian, Beaudry, Infrasound Measurements, Falmouth Wind Turbines
Infrasound Measurements of
Falmouth Wind Turbines Wind #1 and Wind #2
Michael Bahtiarian, INCE Bd. Cert.
Allan Beaudry
TECHNICAL MEMO 2015-004 February 27, 2015
CONTENTS
Executive Summary
Introduction
Background
Test Overview
Instrumentation
Results
Conclusion
References
(Excerpts)
EXECUTIVE SUMMARY
Noise Control Engineering, LLC (NCE) was retained by Senie & Associates P.C. to evaluate the acoustic impact at the home of Neil and Betsy Andersen at 211 Blacksmith Shop Road, East Falmouth, Massachusetts. The goal of the evaluation was to determine if the three nearby wind turbines were detectable within the interior of the home. These wind turbines are all Vestas, model V82 at 1.65 megawatts. Two wind turbines are owned by the Town of Falmouth; known as “Wind #1” and “Wind #2”. The third turbine is privately owned by Notus Clean Energy and referred to as the “Notus” turbine. Wind #1 is the closest to the Andersen home at a nominal distance of 1,385 feet. The other two wind turbines are more than double that distance.
Soon after the first wind turbine was operational, complaints were filed by the Andersens and other neighbors. In the following years, evaluations of audible sound were performed by various organizations including NCE, consultants for the Town, consultants for Notus, and even the Massachusetts Department of Environmental Protection (MADEP). Various results were reported with some evaluations showing compliance and some showing non-compliance.
The study reported herein differed in a number of ways from previous evaluations performed by NCE and others. The major difference is that the primary measurements reported here is infrasound. Briefly, infrasound is sound pressure levels with frequency below 20 hertz which is generally considered an inaudible frequency range. Another difference is that measurements were taken both inside and outside the home. All previous tests were performed at exterior locations due to the fact that State regulations and local ordinance were only applicable at outdoor locations.
The methods used herein allowed for the collection of infrasonic sound pressure levels within the inside of the Andersen residence. Inspection of this data shows that there is a readily identifiable acoustic signature that is attributable to the Wind #1 Turbine, and to slightly lessor extent the Wind #2 turbine both inside and outside the Andersen home. These results are similar to results from other international researchers with references given in the report.
Based on our experience, NCE can unequivocally state that the infrasonic signature captured inside the Andersen residence with the wind turbines operational is 100% attributable to one or both of the Town’s Wind Turbines. To put the conclusions more commonly, this study finds that the wind turbine(s) produce acoustic emissions which are “acoustically trespassing” into the Andersen home.
1.0 INTRODUCTION
Noise Control Engineering, LLC (NCE) was retained by Senie & Associates P.C. of Westborough, Massachusetts to evaluate the acoustic impact at the home of Neil and Betsy Andersen at 211 Blacksmith Shop Road, Falmouth, Massachusetts. The goal of the evaluation was to determine if the sound from the nearby wind turbines is detectable within the interior of the home. This evaluation was conducted by measuring infrasound.
2.0 BACKGROUND
In 2010 the Town of Falmouth erected the first of two Vestas V82, 1.65 megawatt wind turbines, known as “Wind #1” and in 2012 the second turbine known as “Wind #2” was installed. Also in 2010, Notus Clean Energy erected the same Vestas V82 wind turbine known as the “Notus” wind turbine. Appendix A provides a copy of the equipment data sheet for information only. Figure 1 shows the locations of the three wind turbines in relation to the Andersen Home at 211 Blacksmith Shop Road. As shown in Figure 1, Wind #1 is the closest to the residence with a dstance of 1,385 feet. Wind #2 is 2,600 feet and Notus is 3,900 feet from the residence.
Soon after the first wind turbine was operational, complaints were filed by the Andersens and other neighbors. In the following three years, evaluations of audible sound (20 to 20,000 hertz) were performed by many different organizations. NCE conducted some of the first sound measurements and reported these results to the Town of Falmouth during a meeting with the Board of Selectman (reference 1). NCE identified a characteristic time domain pattern known as “Amplitude Modulation” which demonstrated excess to the Town of Falmouth 40 dB(A) wind turbine sound ordinance (reference 2).
Following this work a series of evaluations were performed by another consultant, Harris Miller Miller & Hanson (HMMH) under contract to the Town’s engineering firm that supervised the installation of the wind turbines. The purpose of this evaluation was to compare acoustic performance to the Massachusetts Department of Environmental Protection (MADEP) noise regulation2 (reference 3). The wind turbines were found to be somewhat in compliance in both assessment reports which evaluated the data using two different approaches, (references 4, 5). However, the results showed that 4 dB to 15 dB increases in broadband sound over the background sound occurred depending on the measurement location (reference 4, 5).
Another consulting firm, Epsilon Associates, Inc. evaluated the Notus wind turbine and reported results in reference 6. This study evaluated the wind turbine sound with respect to the Falmouth Special Permit conditions, reference 7. The special permit conditions required no more than a 6 dB increase in A-weighted sound pressure level, no pure tones and no more than 6 dB increase in infrasound. The Town of Falmouth Zoning Board of Appeals applied a 6 dB allowance over background noise for Notus and in connection with one other privately owned turbine. In 2013 the Falmouth Town Meeting adopted the 6 dB limitation as a Town-wide zoning provision applicable to all wind turbines. The Epsilon report found that the wind turbine was compliant for all three conditions. However, the infrasound condition was found to have an increase of as much as 5.7 dB.
Lastly, in 2012, the MADEP conducted their own set of measurements using only MADEP staff from the Lakeville office. Attended measurements were performed on multiple days during both the nighttime (reference 8) and daytime (reference 9). The nighttime report found that Wind #1 exceeded the 10 dB regulation while the daytime report found no excess to the 10 dB regulation.
In summation, the purpose of this section is to indicate the variety of acoustical evaluations that were performed of the Falmouth turbines (Wind #1, Wind #2 and Notus). Three different acoustical consulting groups conducted surveys for three different clients (Town of Falmouth, Notus Clean Energy, and residence groups) and compared results to three different sets of requirements (Falmouth Wind Turbine ordinance, Notus, special permit, and MADEP regulations). Within all these evaluations, various degrees of compliance and non-compliance were declared.
5.0 RESULTS
Infrasonic measurements were performed during 4 visits to the Andersen residence between July 2014 and February 2015. Table 1 provides a summary for each visit including date, time of day, and wind conditions.
Table 1: Site Visit Date, Time, and Wind Conditions
|
Measurement |
Approximate |
Wind |
|
|
Date |
Start Time |
Direction |
Speed |
|
July 5, 2014 |
1:30 pm |
Northwest |
17 mph |
|
November 21, 2014 |
6:30 pm |
Southwest |
26 mph |
|
December 13, 2014 |
6:30 pm |
Northwest |
8 mph* |
|
February 5, 2015 |
6:30 pm |
Northwest |
18 mph |
*Notus Turbine was not operating on this day
With the exception of the initial visit in July 2014, each visit occurred during the nightly shutdown of the Wind #1 and Wind #2 at 7:00pm. This allowed for a direct comparison of turbine operation and ambient conditions within a 1 hour period. In general, for data presented herein, operational measurements were taken between 6:30pm and 7:00pm while ambient measurements were taken from 7:00pm to 7:30pm, immediately following the shutdown of the turbines. As the July 2014 site visit occurred earlier in the afternoon, ambient measurements were not taken. For the November, December, and February visits, asynchronous infrasonic measurements were taken both within the interior of the Andersen residence and right outside the home. Indoor measurements were taken within the living room while outdoor measurements were taken on the front lawn.
Figures 2-5 present the indoor infrasonic sound pressure levels measured from 0 to 10 Hz for each visit. The graphs for the latter three visits also include the measured outdoor operational and indoor ambient infrasonic sound pressure levels. In each figure, regular discernable tones can be identified to varying degrees between 0.7 and 5 Hz. It was determined that the lowest of these tones, occurring at 0.72 Hz, coincides with the blade pass frequency (BPF) of the Vestas V82 turbine at full rotation speed (as given in the Vestas data sheet, Appendix A). The blade pass frequency is seen in all rotating machinery with blades including fans and propellers and is a function of the machinery rotation speed in revolutions per minute (rpm) and the number of blades. The BPF in hertz is calculated using the following formula (see downloadable report for formula):
In addition to the blade pass frequency, rotating bladed machinery produces harmonics of the BPF which occur at integer multiples of the BPF. Table 2 shows the turbine blade pass frequency (1x BPF) and the first seven harmonics (2x – 8x BPF). Each of the frequencies shown in Table 2 was identified during at least one visit and many were found during all operational measurements.
Table 2: Calculated Blade Pass Frequency Harmonics
| 1x BPF | 2x BPF | 3x BPF | 4x BPF | 5x BPF | 6x BPF | 7x BPF | 8x BPF | |
| Freq. (Hz) | 0.72 | 1.44 | 2.16 | 2.88 | 3.60 | 4.32 | 5.04 | 5.76 |
Of note in Figures 3-5, while these tones are clearly identified in the operational indoor measurements, they are completely absent from the ambient indoor measurements following the shutdown of the turbines. Clear identification of these tones is less consistent in the outdoor measurements due to higher overall broadband infrasonic noise, likely due to wind which is not found for measurements taken indoors.
Examination of the data with the two Town wind turbines shut down shows no indication of any residual infrasound inside the home. This would be the case if the Notus Wind turbine had any impact at the Andersen residence. It should be noted, that the differences between the infrasonic measurements with the wind turbines secured and with the Wind #1 and Wind #2 operating are much greater than 6 dB.
Figure 6 is a compilation of the measured indoor infrasound from the four visits. This graph shows that the tones associated with the BPF and its harmonics occur at consistent frequencies over the span of the four visits. Further, with this figure, the substantial variations in amplitude between the visits can be more easily seen and explanations for this variation can be theorized. Note that the highest measured levels for these tones were taken during the July visit during a moderate (17 mph) downwind condition while the lowest levels were taken during the December measurements during a low (8 mph) downwind condition. While substantially lower in absolute amplitude, the December measurements have a similar peak-to-trough difference (10+ dB) from the tones to the frequencies between the tones suggesting, even within the house, the wind controls the ambient broadband infrasonic sound level. Finally, measurements performed in November show both high broadband levels and lower peak-to-trough differences suggesting high wind speed and/or an upwind wind direction partially obscure the clearly identifiable wind turbine infrasonic signature.
Historically, when the wind turbine sound is particularly bothersome, Mrs. Andersen has reportedly sought refuse in the dining room which is located in the back of the home. NCE understands that at times she has used this room as a second “bedroom”. NCE tested this room and found a lower level of infrasound in the 4 to 7 Hertz range as shown in Figure 7. NCE does not have any explanation why this room has lower infrasound only at these frequencies, but her actions are consistent with these test results.
6.0 CONCLUSIONS
The methods used herein allowed for the collection of infrasonic sound pressure levels within the inside of the Andersen residence. As shown in Figure 6, there is a readily identifiable acoustic signature that can be definitively attributable to Wind #1 and possibly Wind #2 located outside the Andersen home. To NCE’s knowledge, this is the first time such measurements have been performed and reported with respect to the Falmouth wind turbines. However, this is not the first time such measurements have been performed, and other researchers have collected low frequency infrasonic acoustic signatures at other wind turbine sites in Wisconsin and Australia (references 11, 12). As reported in these other studies, the same blade passage rate infrasound and harmonic shown inside the Andersen home have been identified.