Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 455

acoustic standards are available for direct readout or for recording them. Either frequency-
modulated or direct-record tape systems are acceptable.
For cases where the frequency range of measurements must extend below 20 Hz, some
special items of equipment may be required. Although standard microphone systems can
be used, their frequency response is poor at the lowest frequencies. Special microphone
systems will increase signal fidelity, along with special procedures to minimize wind noise
problems.
Windscreen Applications
Noise measured in the presence of wind is contaminated by various types of wind-
related noises. These include natural wind noise , from atmospheric turbulence; microphone
noise , caused by the aerodynamic wake of the microphone or its windscreen; vegetation
noise , caused by nearby trees, bushes, and ground cover; and miscellaneous wake noise ,
from the aerodynamic wakes of accessories (such as a tripod) or a nearby structure.
Because of the deleterious effects of the wind, windscreens are recommended to reduce
microphone noise for all measurements of wind turbine and site background noise. Com-
mercial windscreens of open-cell polyurethane foam are usually adequate for routine
measurements at or near ground level, where wind speeds are relatively low. For
measurements above ground, larger windscreens of custom design may be necessary
[Sutherland, Mantley, and Brown 1987]. It is essential that the acoustic insertion loss of
any windscreen be either zero or known as a function of frequency, so that appropriate
corrections can be made to the data.
Wind noise is a particularly severe problem at the lowest frequencies. The ambient
( i.e. , wind-only) noise spectrum increases as frequency decreases and may submerge some
low-frequency wind turbine noise components in the ambient noise at the microphone
location. In such situations, customized windscreens may help reduce the low-frequency
wind noise. Some special cross-correlation analysis techniques have also been applied that
use measurements from pairs of microphones [Bendat and Piersol 1980].
Little can be done to reduce noise from vegetation, other than to locate microphones
away from significant sources. Noise generated by the aerodynamic wakes of accessories
such as tripods can often be reduced by streamlining.
Data Analysis
The data analysis required depends on the types of acoustic information desired. If A-
weighted data are needed, they can be obtained directly from a sound-level meter or from tape
recordings and A-weighting filters. Statistical data can also be obtained directly by means of
a community noise analyzer . Broadband data are routinely produced from one-third-octave
band analysis such as that illustrated in Figure 7-8. Narrow-band analysis can be obtained with
the aid of a wide range of filter bandwidths, the main requirement being that the bandwidth is
small compared to frequency intervals between discrete frequency components.