Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 153

CHAPTER 5
Wind turbine acoustics
Robert Z. Szasz 1 & Laszlo Fuchs 1,2
1 Lund University, Lund, Sweden.
2 Royal Institute of Technology, Stockholm, Sweden.
This chapter presents some of the most important issues related to wind turbine
acoustics. After a short background, an introduction to acoustics follows where we
defi ne the most important parameters that are met in the chapter. Next, the noise
generation mechanisms are discussed from a physical point of view. Some aspects
related to noise propagation and their perception are presented. A major part of the
chapter consists of a review of the prediction models with different level of com-
plexity together with experimental methods that are often used for wind turbine
aeroacoustics. Finally, a short list of noise reduction strategies is also given.
1 What is noise?
Sound is the propagation of low-amplitude pressure waves traveling with the speed
of sound . These pressure waves might be generated in several ways: for example
the vibration of the vocal chords or the membrane of a loudspeaker, periodical vor-
tex shedding in a cavity gives rise to a pure tone (i.e. having a single frequency). In
contrast, turbulent fl ows may generate a broad spectrum sound, containing a wide
range of modes with different amplitudes. Thus, the mechanisms for generating
sound may be quite different resulting in sounds with widely different characters.
Hence, a simple modal characterization of the sound might not be enough to evalu-
ate its perception. When the generated sound is not wanted it is considered to be a
noise . Since the perception of sound as pleasant or annoying is highly individual,
there are no strict limits delimiting noise from acceptable sound.
2 Are wind turbines really noisy?
With the increasing demand of alternative energy sources the number of installed
wind turbines increased exponentially in recent years. Due to the limited amount