Open the approximation window either by click on "Filter"  "Approximation" or by the toolbar. 



 
Start/Stop: Define the start and stop frequency for a desired stopband. 
 
Goal: Define the absorbability for the chosen frequency range. 
 
Type: Choose the type of the goal. 
 
Add/Remove Goal: This two buttons allow to add and remove stopbands and passbands to and from the list on the right. 
 
Solve Approximation: With the Solve Approximation button the optimal solution for the specification will be found. 

Note: The defined stopbands and passbands are displayed in blue and red in the graph, respectively. 