Documents
8.0 Findings and Recommendations
The May 2007 operation sound measurements were in fact recorded "during wind conditions at or exceeding the predicted mean wind speeds with wind from predicted predominant directions". RSE recommends to the UPC Wind additional sound level measurements of wind farm operations during fall and winter periods using ground level anemometers at measurement positions.
Conclusion - (Peer Review)
This ambient and operating level sound study was to an extent compromised by the use of inadequate microphone wind protection and/or site-specific wind condition information.
Springtime operation noise level measurements alone do not represent climate factors, such as, snow-cover and blade icing which have been documented as acoustic factors enhancing windmill sound propagation and production which may contribute to significant seasonal operation sound level variations. It is the opinion of the reviewer that this initial assessment of the project indicates substantial compliance, but requires further measurement technique refinement with additional measurements to fully demonstrate results that are reasonable and technically correct according to standard engineering practices and the Department Regulations on Control of Noise(06-096 CMR 375.10) with operational noise limits set forth in the Control of Noise rules and the variance given in Department Order L-21635-26-A-N/L-21365-TG-B-N, dated June 1, 2004.
Recommendations
This reviewer concurs with RSE recommendations to:
• use ground level anemometers at monitoring locations, and
• complete future sound level measurements representing other seasons of the year (specifically, representative winter measurements).
A sound measurement assessment should occur when monitoring locations have snowcover and preferably during potential conditions for wind turbine blade icing. In addition to site-specific wind condition monitoring, should be the addition of secondary windscreens for recording ambient and operational sound levels when microphone wind interference exceeds levels less than 6 dBA below sound levels to be measured, utilizing recommendations prescribed by the equipment manufacturer.
Post Review Discussion & Recommendations
The following discussion and recommendations are directed toward the Maine Department of Environmental Protection Site Location of Development noise standards as specified in Chapter 375.10. and their application by the department.
Sounds emitted from an outdoor source are diminished by several factors including divergence, atmospheric absorption, ground surface factors, foliage, building construction (indoor noise concerns) and miscellaneous other factors. Although the usual greatest diminishing factor, divergence, is unaffected by sound frequency the remaining factors are frequency dependent. Increased low-frequency-content, sounds associated with windmill operation tend to propagate better and penetrate light weight building structures with much greater effectiveness than broadband sounds. In addition to wind mill low frequency sound emission, is the periodic "swooshing" or amplitude modulation produced as blades pass the support tower. These low-frequency modulations have been reported as penetrating and annoying.
Recommendation #1: Operation sound measurements should be required during periods or seasons consistent with predominant operating conditions and factors enhancing sound propagation toward nearby sensitive receptors. Measurement periods should include measurement site snow-cover conditions.
Recommendation #2: Sound level limits should be conservatively applied by the department given their periodic, low frequency modulating annoyance factor. Mountainous typography especially arising from plains or rolling hills, such as Mars Hill Ridge and its immediate surroundings, give rise to broadly varying atmospheric conditions over relatively short distances. For example, vigorous ridgeline winds may be consistent with up-wind low elevation surface conditions, but be contrasted downwind at surface levels by light or even calm conditions. Given these potential variations, upwind receptors would experience high level masking and "shadow" atmospheric refraction conditions minimizing ridgeline source sounds, whereas downwind receptors would experience minimal masking and atmospheric refraction lapse conditions that would enhance ridgeline source sounds. Ambient and operation sounds measured at high wind speeds (>12 mph) may produce non-noise artifact lessening the integrity of measured data. This confounding element can lead to false conclusions regarding ambient and operation sound levels. Manufacturer microphone and windscreen specifications must be appropriately applied.
Recommendation #3: Mountainous wind farm operation and ambient sound measurements at protected locations should be required with site-specific wind speed conditions.
Recommendation #4: Sound measurements required at wind speeds > 12 mph should be done in a manner consistent with manufacturer microphone/windscreen recommendations and/or appropriately specified secondary windscreens.
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