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Problems related to the use of the existing noise measurement standards when predicting noise from wind turbines and wind farms

Presented at the 2004 conference of the Australian Wind Energy Association (now “Clean Air Council”)

Erik Sloth, Vestas
Niels Christian Møller Nielsen, Vestas
Ejler Kristensen, Bonus Energy
Bo Søndergaard, Delta

Noise Measurements (IEC 61400-11:2002)

We correct for:
Air pressure
Air temperature
Standard terrain roughness z = 0.05 m
All recalculated to 10 m wind speed

We do not correct for:
Actual inflow angles
Actual air density, except in the power curve
Actual wind shear
Actual turbulence intensity
… All parameters that are known to influence the sound emission

The results are standardized noise levels, which are fairly comparable from measurement to measurement on a given turbine type.
The wind turbine is used as a wind speed meter through a power curve measured on an ideal site (IEC 61400-12).
Other parameters influence the noise level: relative humidity, turbulence, inflow angle, wind shear, turbine pitching are not accounted for.

The result is a fairly good tool for verification of warranties, but not a good tool for predicting noise at imission points where people actually can get annoyed.

Typical problems in using the measurement results

The wind turbines are almost always raised at sites where roughness differ from the standardized completely flat measurement site.

Further we see:
Different air density
Different wind shear
Different turbulence in inflow air
Different inflow angles

Finally we often see other hub heights than used during documentation.

Conclusion on measurement results

The differences in site conditions creates differences in emitted sound power level.
The differences could be both increased and decreased emitted sound power levels in real life applications.
The differences will transfer directly to the imitted sound power levels, and may thereby create increased annoyances in real life.

Therefore – site specific sound power levels should be used unless a good safety margin is present using standardized emission levels.

Solution to the outlined problems

Accept that different sound power levels should be used in predictions and warranties.
Avoid using sound power levels that include inaccuracy in predictions unless there is a good safety margin.
The inaccuracy should be included in the calculation – the higher the number of WTG’s the less the probability that all are in the high end of the uncertainty interval.
Use sound power levels that at least are corrected for: hub height, wind shear, air density, turbulence, inflow angle.
Be careful to make sure that the background noise measurements and wind conditions at the turbine positions uses the same reference position.

Most noise level calculation models are developed for noise from industry, wind speeds below 5 m/s and standard meteorological conditions and must be suspected to give poor results at larger distances. …

Noise Assessment

The noise level at the imission points are normally given as an A-weighted noise level at different wind speeds.
A tonality evaluation is normally included for the receiving points.

What we know of the annoyance of the noise:

We know that noise from wind turbines sometimes annoys people even if the noise is below the noise limits.
Often people complaints on low frequency noise which many investigations often show in not present.
The noise limits are usually adapted from industrial noise limits and are based on the principle that a given percentage of the population will feel annoyed when the limit is exactly fulfilled.
Evaluation of tonality in the turbine noise is based more on the reproducibility of the results than on pure knowledge on what is actually annoying.

Other descriptors need to be investigated to understand the annoyance caused by wind turbines:
Low frequency noise and Infrasound – we cannot see it in our measurements
Modulation – may be the parameter that is heard as low frequency noise
Masking – which noise can mask noise from wind turbines
Other characteristics …

Download original document: “Problems related to the use of the existing noise measurement standards when predicting noise from wind turbines and wind farms [1]