Wind turbine noise measurements and abatement methods - Wind Power Generation and Wind Turbine Design - page 643

It is common sense that the extensive use of wind turbines is included in the
current and the future environmental plans of every country, fact which will
certainly place a strong demand for dealing noise issues in the near future.

2 Noise types and patterns

2.1 Sources of wind turbine sound

The sources of sounds emitted from operating wind turbines can be divided into
two categories: (1) mechanical sounds, from the interaction of turbine compo-
nents, and (2) aerodynamic sounds, produced by the fl ow of air over the blades.
A summary of each of these sound generation mechanisms follows, and a more
detailed review is included in [1].
More specifi cally, wind turbines produce energy by the rotational motion of the
blades due to the wind fl ow. Rotating blades are known to emit three different
types of acoustic signature:

￿ tonal noise

￿ broadband noise

￿ mechanical noise

Tonal noise is characterised by discrete frequencies and it is generated by the peri-
odical rotation of the turbine blades. An example of tonal noise history is shown in
Fig. 1. Tonal noise is caused by the unsteady air velocity due to the blade rotation which
disturbs the fl ow on the blade surface. It is directional as it is produced at the direction
the blades meet the airfl ow and therefore is dependent on the observer position.
As the blades rotate the load on the blade surface changes periodically causing
analogous changes in the unsteady pressure on the blade surface inducing sound
waves. Depending on the position of the observer in relation to the turbine while

Figure 1: Tonal noise time history.