Dynamic measurements of wind turbine acoustic signals, employing sound quality engineering methods considering the time and frequency sensitivities of human perception
A set of binaural time-recordings and analyses of wind turbine noise outside and inside a residence in Huron County, Michigan, made over two days and the intervening night in December 2009, is presented, centering on analysis at the time-frequency resolutions of human hearing according to the well-established practices of sound quality engineering and soundscaping. The purpose of this paper is to present wind turbine acoustic measurements at these time-frequency scalings and to suggest that such consideration, frequently neglected in favor of frequency resolution and long-term level averages, could augment the perceptually inappropriate averages (often A-weighted) typically taken over much longer intervals. The authors maintain that most measurements of wind turbines up to now have not considered, or not adequately considered, these signals’ very complex and varying behaviors at the time-bandwidth scalings of human perception. Although treating wind turbine noise aspects at all relevant frequencies, this paper will concentrate on low-frequency information.
What this paper adds to the discussion of wind turbine noise and potential adverse health effects
The reason the wind industry experts could claim that wind turbines produced insignificant levels of infra and low frequency sound is not because there isn’t any, but instead, because the instruments/methods they used could not detect it. They went hunting for a needle in the haystack using a magnet when the needle was made out of plastic.
When analyzed using a tool that can detect it, we find that it is there and at SPL’s much higher than previously considered likely. The infrasound from wind turbines rises and falls in sound pressure level (amplitude modulation) at a very rapid rate (approximately 60 msec peak to peak or so) and with a high dynamic range, phenomena too fast to be ‘noticed’ when standard acoustical filters are used to isolate this region of acoustic energy (a 1 Hz ANSI S1.11 1/3-octave filter has an impulse response of about 5 seconds). The understatement of the true peaks that occurs during analysis using standard acoustical instruments/methods flattened and stretched out the dynamic modulation (crest factor) leading to a misconception that the levels were insignificant.
This study shows that when analyzed according to the time response of the human transducer, the peaks of the energy waves can be above 90 dB SPL. Combined with the findings of Dr. Salt’s research this analysis shows that the dynamically modulated infrasound can be perceived by the auditory system at levels that are below the conventionally-determined threshold of audibility.
It is the short duration and extent of the change in sound pressure that is stimulating the vestibular system, not the overall energy level. This is not about the average energy but instead about the short duration, peak values and extent of change in energy assuming that some lower threshold like Dr. Salt’s 60 dBG for OHC activity has been reached.
Presented at NOISE-CON 2011, Portland, Oregon, July 25-27, 2011
HEAD Acoustics, Brighton, Mich.
E-Coustic Solutions, Okemos, Mich.
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