Limitations of acoustic monitoring at wind turbines to evaluate fatality risk of bats

Attenuation of sound pressure level (SPL) of echolocation calls as they propagate through air. Estimates were developed for two bats calling at 20 kHz and 110 dB peSPL and 40 kHz and 104 dB peSPL (source SPLs are referenced to 20 µPa at 1 m in front of the bat). The received SPLs were calculated from source SPL by subtracting geometric attenuation (-6 dB per doubling of distance) and frequency- and weather-dependent atmospheric attenuation (see insets on the right). The received SPLs are calculated for the full range of atmospheric attenuation, Percentage of the rotor-swept area of wind turbines covered by automated ultrasonic detectors (AUDs) with increasing blade length, for bats calling at 20 kHz and 40 kHz. For this estimate, we assumed the risk zone to equal the circular rotor-swept area with the blade length as radius. AUDs usually do not cover the upper half of this zone, because AUDs are installed at the nacelle bottom (between tower and blades) with the microphone pointing downwards. We further used the maximum distance at which bats can be detected by AUDs (trigger threshold: 60 dB SPL), which only applies for bat calls that are directed towards the AUD.