Measuring the noise emissions of wind turbines during operation is no easy task. The turbines are 50 meters high and have a noisy background. The Acoustic Camera Star AC Pro is used to analyse wind turbine noise during operation under free-field conditions. For this purpose, a wind turbine was measured at different wind speed conditions between 7 and 10 m/s. Noise emissions occurring at the rear of the wind turbine were identified and localized using the NoiseImage software.
Acoustic analysis of wind turbines during operation.
Outdoor mapping of a wind turbine, localization of sound in different frequency bands at different wind speeds. For this purpose, a wind turbine has been measured at different wind speed situations between 7 and 10 m/s. The noise emissions created by the turbine have been identified and localized with the software NoiseImage.
Wind power turbine (3 rotor blades, performance below 1 MW, height of mast 50 m, blade length 20 m).
The measurements have been carried out with the microphone array Star 48 AC Pro and an 48-channels data recorder connected to a mobile power supply that provides up to 4 hours of autonomy.
During the measurement, a relatively loud background noise originating from wind and other environmental sources was recorded. This was minimized with the help of a filter.
This array is a 48 channel measurement system designed for outdoor applications. The lightweight aluminium array body is designed as slim as possible to ensure easy handling and accurate microphone positioning. The included high-end tripod allows setup in almost every measurement environment imaginable.
This data recording device has been developed especially for use with the Acoustic Camera. It allows for high scanning frequencies and high numbers of channels but is lightweight and compact at the same time.
Mobile power supply SilmMPS for independent operation of the Acoustic Camera for up to 4 hours.
The main target of the analysis was to form a conclusion about the focus of the noise emissions on a wind turbine. First, results of the A-weighted data show the flow noise at the blade tips which depends on wind direction and wind turbine location in conjunction with the position of the Acoustic Camera. For the measurement, the Acoustic Camera has been positioned slightly on the left behind the wind turbine. During the analysis of the blade noise, the acoustic focus was located to the left of the gear box which is the side with the shortest distances to the microphones.
For a comprehensive analysis, a whole rotation of one blade has been taken as calculation base. The software module Spectral Photo 2D has been employed for this purpose which offers a pre-calculated location of the emissions for any frequency. The analysis led to the insight that the point of emission strongly depends on the observed frequencies.
For the third octave band of 400 Hz the emission can be located clearly at the root part of the blade while this blade is turning downwards located at 140 degrees.
Looking at the frequency band from 706 to 891 Hz, it becomes obvious that the blade tips and also the gear box emit this flow noise.
It became clear that the higher frequencies are mainly emitted at the gear box while at the same time the hissing noises become visible as more quiet sources on the blade tips.
For further analysis a narrow band analysis was conducted – also the NoiseImage module Spectral Photo 2D. Here, the noise origin of the single peaks in the spectrum was analyzed. Evidently, the main source of this sine wave noise is the gear box.
Due to these results, focus has been put on the gear box itself. The examination of a specific part of the measurement object does not require a new measurement. The part of interest can easily be chosen during post-processing. This way, it shows that even at great distances it is possible to allocate certain frequencies to different emission points. To analyze the parts of the gear box themselves, we it is advisable to conduct a more detailed analysis from a shorter distance, possibly on a test bench.
