Wind turbine noise is a combination of audible acoustic and infrasonic components. Low infrasonic background noise is a prerequisite for infrasound detections of conceivable nuclear explosions in the atmosphere in the context of monitoring compliance with the Comprehensive Nuclear Test-Ban Treaty (CTBT). Aerodynamic noise emitted from the endlessly growing number of wind turbines in Germany is increasingly creating problems for the infrasound recording systems. During each revolution, the wind turbine blades encounter much variation in the air flow generated by changes in intensity and wind direction when flowing around the structural tower. As the wind turbine blade revolution process is repetitive, impulsive sound signals consisting of pure tones which are integer multiples of the fundamental blade-passing harmonic are generated. Consequently, studies on the intensity and frequency of the aerodynamic infrasonic noise signals produced have gained much interest.
In a recent paper published in Journal of Sound and Vibration Christoph Pilger and Lars Ceranna from The Federal Institute for Geosciences and Natural Resources (BGR) in Germany studied the influence of periodic wind turbine noise on infrasound array measurements. Their aim was solely focused on the effects of the infrasonic component of wind turbine noise on infrasound measurements by microbarometer arrays.
For the researchers to determine the emission of the infrasound signals by the wind turbines, a field campaign was carried out near a single 200kW horizontal axis turbine north of Hanover. Additionally, ten years of infrasound data recorded at the German infrasound array IGADE, were analyzed with respect to influences of nearby wind turbines. Theoretical models were then derived and validated by the field measurements using mobile microbarometer stations. Model computations on the influence of single versus multiple wind turbines and the effect of ducting and propagation on the sound pressure levels of infrasound observations were included and presented. Discussions and conclusions on least distances between wind turbines and infrasound arrays were derived from the model calculations of sound pressure levels in the infrasonic frequency range and verified by infrasound observations.
It was observed that at certain frequencies and during nearly all years and seasons, the aerodynamic sound waves of the BPH increased the SPL detected and quantified by the sensors. Blade-passing harmonics with multiples of 1 to 1.4Hz frequency were observed to occur during most of the observations and had the highest spectral increase compared to neighboring frequencies. From the model the researchers were able to estimate the generated sound pressure level of wind turbines and thus making it possible for specification of the minimum allowable distance between wind turbines and infrasound stations for undisturbed recording.
study demonstrated that a minimum distance of 20 km should be maintained between an infrasound station and a single wind turbine so as to guarantee unhindered recording and detection conditions. In the case of multi element wind farm, the distance would need to be increased to 50 km. However, if only occasional tropospheric ducts increasing the surface-near sound levels are considered, 5-10 km and 10-15 km would be adequate and sufficient to allow unhindered recording and detection conditions for a single turbine and a wind farm respectively.
Christoph Pilger, Lars Ceranna. The influence of periodic wind turbine noise on infrasound array measurements. Journal of Sound and Vibration. volume 388 (2017) pages 188–200
Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany.Go To Journal of Sound and Vibration