No, you should use categories and combinations. You can put the existing and new wind turbines into a single scenario, but create two different categories, one for existing turbines (say category 1) and one for new turbines (say category 2). Use on the on-line help or read the manual on how to create categories.
When you create the activities that represent the noise of the turbines, set the activities for existing turbines to category 1 and for new turbines to category 2.
And in the Categories dialogue, add three combinations - New Turbines just using Category 2, Existing Turbines, just using Category 1, and All Turbines, which is Category 1 + 2.
Then when you run the calculations, ensure you select to run all combinations (it is as quick as running only one).
When you download the results, you will see that you have results for each of the combinations, so you can display any of the above contours. Also, if you want to see where the new turbines are adding noise, use Compare Contour to plot the difference between All Turbines and New Turbines.
Be careful to ensure that when you do the calculations, you have set the cut-off distance sufficiently large and that you include enough tiles in the surround margin. A cut-off distance of 2500 m and a surround of 5 tiles might be appropriate. See other articles on this forum for more information on cut-off and surround.
