Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 453

Measuring Points
Most noise measurements (other than those for research purposes) are made to charac-
terize the radiated noise of a particular machine. This infers that all data should be obtained
far enough from the machine to be in the acoustic far-field. For practical applications, the
reference distance d 0 illustrated in Figure 7-40 should be approximately equal to the total
height of a HAWT or, in the case of a VAWT, the total height plus the rotor equatorial
radius [IEA-WECS 1988]. The choice of a much greater distance may not be acceptable
because of the reduced signal-to-noise ratio and because atmospheric attenuation and
refraction effects can complicate interpretation of the data. To ensure the best possible
signal-to-noise ratio, measurement points should be as close to the source as possible
without being in the acoustic near-field.
Figure 7-40. Recommended patterns of measuring points for acoustic surveys of wind
turbines. [IEA-WECS 1988]
The number of measurement points needed can be determined by inspecting the polar
diagrams in Figures 7-6 and 7-7. The aerodynamic noise sources in wind turbines are not
highly directional, but the highest levels are usually in the upwind and downwind quadrants.
A rather coarse azimuthal spacing seems adequate to define these aerodynamic radiation
patterns around a HAWT, because they are generally symmetrical about its axis of rotation.
If a particular turbine produces significant mechanical noise, however, that radiation pattern
may be asymmetrical and highly directional.
Microphone Positions
An important consideration in laying out a measurement program is defining micro-
phone height above the ground. Placing the microphone at ear level is conceptually attrac-
tive because it should record what people hear. The disadvantage of this height is that the
data are more difficult to interpret. Figure 7-41 illustrates how sound pressure level data
can be affected by microphone height, and compares sound pressure levels near the ground