Titel: Noise Immission Measurement from Wind Turbines – Investigating Different Signal Analysis Techniques
Författare/Author: Patrik Rynell
Utgivare/Publisher: Chalmers Tekniska Högskola
Årtal/Year: 2009
Ämnesord/Subject: ljud
Sökord/Keywords: noise immission, DEMON (Detection of Envelope Modulation On Noise), Aerodynamical noise, Mechanical noise, measuring
Examensarbete/Master of Science Thesis via www.soundandvibration.se
Sammanfattning/Abstract: Measuring noise immission from wind turbines is a tricky science. By nature, there is always a lot of background noise induced by the wind because one is forced to measure at high wind speeds. Usually when measuring outdoor noise from for example industries one is measuring at low wind speeds to avoid background noise. This is not possible when measuring noise from wind turbines, so one is forced to compensate for the background noise. This is done by performing a separate measurement without the wind turbine running. The wind speed is measured at the same time and the measurements with and without the wind turbine running is compared at the same wind speed. If there is a lot of background noise then it is not even possible to measure the wind turbine noise at all; it has to be calculated by using the specification for the emitted sound power by the wind turbine together with a model for the sound emission.
This master’s thesis presents different ways of using signal processing to filter out the wind turbine noise from the background noise. The idea is that wind turbine noise is periodic and that this periodicity cannot be found in the background noise. A method called DEMON (Detection of Envelope Modulation On Noise) is examined. This method finds the envelope of the periodic signal and by finding the maximum amplitude for the envelope it should be possible to estimate the amount of noise from the wind turbine. Also other approaches are examined and the common factor is that they all assume that the wind turbine noise is periodic.
It was more difficult than expected to filter out the wind turbine noise part from a noisy signal. After applying DEMON analysis or any other of the described methods used here, it was still not possible to find the periodicity in the signal. Even the fundamental frequency for the wind turbine noise was hard to determine, not only by using signal processing but also by listening to the wind turbine noise. This indicates that it is not possible to estimate the noise immission from wind turbines by examination of the periodic part of the signal.