NORD2000 is an advanced calculation engine for noise calculation. It was developed for traffic noise and has in later years been validated for calculation of wind turbine noise.
A calculation with NORD2000 is an attempt to calculate the real noise experienced at a receptor. There are many factors influencing the noise impact and therefore there are many parameters in a NORD2000 calculation. Importantly many of these parameters are variable in time and so a standard NORD2000 calculation calculates the noise for a specific situation that may be present for only a short while.
Most official noise codes define specific conditions during which the noise from the wind turbines must comply with a critical noise impact level. Often these are hard-coded into specific noise propagation models, which must be used. Such models are implemented in the DECIBEL module. At other times specific conditions are required, but not a specific noise propagation model. In those situations the NORD2000 model can be used to simulate those situations.
The basic NORD2000 calculation calculates the noise propagation from a wind turbine to a receptor (neighbor) given the specific terrain, wind and climatic condition.
A not unimportant issue is to find the source noise level of the turbine. This is a function of wind speed at the turbine and the turbine specifications. The source noise level is divided into eight octaves or 24 third octaves.
This task may be complicated by the fact that turbines in a wind farm rarely experience the same wind speed at the same time and that the reference wind speed location is often undefined.
The second task is to define the terrain. The terrain itself causes terrain attenuation. This is calculated as a function of the terrain shape (height contours) and the terrain acoustic hardness. Hard terrain like water or a parking lot may attenuate very little noise, while soft ground like natural or agricultural land my attenuate the noise well.
The terrain will also influence the wind profile in the form of a terrain roughness (roughness length), which affect the atmospheric attenuation.
This will also depend on a number of climatic parameters. These are:
- Wind direction
- Wind speed
- Turbulence strength (wind)
- Turbulence strength (temperature)
- Standard deviation of wind fluctuations
- Inverse Monin Obukov length
- Temperature scale T*
The last five of these parameters are rather exotic and have for ease of calculation been reduced to standard settings for day and night, clear sky and clouded.
With these settings for source noise level, terrain and atmosphere the data is sent to the NORD2000 calculation engine, which return the resulting noise level at the receptor from the turbine calculated.
Typical calculations would be a Point Calculation that calculates a very specific set of conditions.
Another would be the range or Speed/direction analysis where a range of wind speeds and directions are calculated to find the worst case noise.