Ultrasound emissions from wind turbines as a potential attractant to bats: a preliminary investigation

Wind turbine ultrasound emission, a preliminary investigation 2 Summary Although audible acoustic emissions from wind turbines have been extensively measured (i.e., frequencies below 20 kHz), the ultrasound emissions remain uncharacterized for most wind turbines. We performed a basic characterization of ultrasound emissions from a variety of wind turbines to determine whether ultrasound emissions may contribute to attracting bats toward wind turbines with consequential fatalities from rotor strikes. We were particularly interested in characterizing ultrasound emissions from the 1.5 MW NEG Micon turbines because of the documented bat mortality from these turbines operating at the Mountaineer Wind Energy Center in West Virginia. All turbines sampled generated only minor ultrasound above ambient sound levels. The majority of acoustic energy was emitted at audible frequencies and trailed off rapidly above audible frequencies with a similar profile to that of ambient wind noise. Measured from ground level, 34 m directly below the 1.5 MW NEG Micon wind turbine rotors, these turbines emitted approximately 5, 3, and 2 dB above ambient at 20, 30, and 40 kHz respectively. Above 50 kHz there was no significant difference from ambient sound levels. We conclude that ultrasound emissions, as measured from the ground-level, from these wind turbines do not likely play a significant role in attracting bats. However, ultrasound could be emitted from other turbines we did not measure during this preliminary investigation, or from the nacelle of turbines, possibly warranting further investigation.