Frequently Asked Questions

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Measurements are made with an Array Star48.  It is a 48-channel measurement system designed for outdoor applications. It consists of three arms with 16 microphones each. The diameter of the array aperture is 3.4 meters. The array is suitable for the localization of sound sources in the frequency range between 66 Hz and 13 kHz. For data acquisition sampling rate up to 192 kS/s and a resolution of 32 bit are available.

For a typical wind turbine inspection setup, the system can make high-resolution diagnostics of the structural health of high value wind turbine components. Our degree of spatial resolution allows us to measure low sound pressure levels (SPLs) and small, slight differences in SPLs on order of 3dB at multiple discrete blade positions and provide an accurate characterization of subtle differences of noise from blade to blade.  Our patent pending process can be used to provide a ready, reliable standard catalog that checks on critical turbine operating conditions.

The beamformed signal at the TDC position is on average 18 dB quieter than the single microphone signal.  This allows us to locate and find the origin of small and subtle SPLs that are no louder than a whisper at the location of the listener to be accurately found and seen on the turbine.  Differences in SPLs in as little as a few dB’s are easily discernible.  These slight and subtle differences from blade to blade provide a quick and readily available accurate baseline of the turbine operating conditions and a standard checklist of wind turbine operating conditions available for rapid review and integration with other data streams.  

The quality of sound source localization is not influenced by the measurement position.  In most cases it seems advisable to choose the downwind position in accordance with IEC 61400-11 which enables a better comparability to acoustic data obtained by conventional measurements.  In the upwind case the signal noise caused by the wind flow is possibly a little smaller.

The entire setup of the Acoustic Camera takes about 15 minutes. For the power supply a transportable, high-capacity battery has been used which offers a continuous operation of more than 4 hours.  This compact, lightweight portable array can be quickly set up and installed by a small two-man crew and allow multiple turbines to be monitored at multiple locations over the course of day. 

The more variable the wind conditions the generally longer the acquisition.  For the identification of the rotor position with the loudest sound emission regarding the measurement position, an integration time of about one revolution is suitable. To compare the effectiveness of constructive measures such as the installation of serrations, a short integration time of less than a twelfth of a revolution or 30 degrees can be used.  These short integration times correspond to a specific position of each blade on the rotor such as a blade at top dead center or crossing the tower.  Other blade positions that are sensitive to well-known types of damage such as trailing edge, pitch control errors or blade warping can also be viewed.  

These high-resolution acoustic measurements can be made over multiple turns of the rotor for a variety of blade positions.  Averages of sound pressure levels over multiple turns of the rotor tend to remove the variations related to changing atmospheric conditions, and environmental and cultural noises.   The critical elements of turbine performance and structural health can be determined in as few as six or seven turns of the rotor.  Our confidence in the data further increases as more turns of the rotor are measured.  Overall, most turbine measurements are expected to take about an hour per turbine and that is what enables us to monitor multiple turbines at multiple positions in a day.  

The noise emitted by an operating wind turbine can be divided into mechanical and aerodynamic noise. Mechanical noise originates from different machinery components, such as the generator and the gearbox. This noise is typically narrow band and propagates as structure-borne sound and is emitted via the structure as airborne sound. Aerodynamic noise is often broadband.  It is caused by the interaction of turbulence with the blade surface and is radiated from the leading and trailing edges of the blades. The turbulence can be originated either from atmospheric turbulence present in the incoming flow or from the viscous flow in the boundary layer around the blades.  

Changes in blade operating conditions tend to affect the noise levels from blade to blade. So-called serrations at the end edge of rotor blades break the turbulence into smaller structures, reducing the strength of the end edge sound. Simple differences in the sound pressure level (SPL) measurements can be performed to check the effectiveness of these serrations. Other cases may involve the detection of blade damage or pitch control error. SPL measurements with single microphones provide only integral values about the overall sound emission.  In contrast, beamforming methods allow one to determine the location of sound sources on the blade and an estimation of the amplitude of these sources from blade to blade as the rotor turns.   

The beamforming makes us images of the SPLs and differences in SPLs at these positions provide the inspection criterion with a standard checklist of turbine health measurements and a consistent catalog of what and where inefficient operating conditions are likely to be found, and guide and inform the structural health monitoring strategies and wind turbine generator optimization programs.  

The acoustic camera is a cost-effective tool that allows us to visualize the structural health of wind turbine generators.  

The noise in and from wind turbines during operation is complex because there are multiple potential sources of noise from the turbine: Blades, gearbox, generator, brakes, tower, etc., that are all making various noises at the same time.  In addition, cultural and aerodynamic noise can be present in the external part like blades or nacelle. Being able to identify these sources separately is the key for analyzing the contribution of each one in the global level and to take the right decisions about how to improve the noise reduction in a wind turbine. In addition, the noise localization provides a new point of view for maintenance purposes by visualizing where the specific noise like chirps, squeaks or squeaks are coming from.  The Acoustic Camera system can provide this kind of results, using a special multichannel microphone array, a data recorder and a computer running the software, all this analysis in time and frequency domain can be done on site.   

The Acoustic Camera can be used in research & development for new wind turbine prototypes, the control and monitoring for installed ones or comparison between different models or different workflows. One of the main features of this system is that it is not necessary to install any sensor on the wind turbine.  

With the Acoustic Camera it now becomes routine to detect sound sources in different parts on wind turbines, identify inefficient turbine operating conditions and provide a ready, reliable check list of standard turbine operating conditions. In addition, the results can be shown in the wind turbine 3D model, acquired using a laser-scanner. 

Yes! Understanding asymmetric load imbalances measured under environmental conditions can help in understanding the effects of partial wake effects due to leading turbine-trailing turbine relationships, and topographic effects. By recording these data in-situ during normal and stressed operating conditions, one can not only design a more strategic maintenance program, but also inform multi-physics modeling of wind farms. Developing more comprehensive physical models that simulate the partial wake effects utilizing real data can lead to more efficient re-powering designs and inform future wind farm layouts.