Find it here:
Menu: Noise > Buildings > Dialog box
Select one building in the list
Dialog box: Set absorption
Closes all dialog boxes before the following question:
Enter absorption factor (0.0-1.0):
State a value between 0 and 1 and press <Enter> (or <Esc> to cancel the function). This is used as an absorption factor for the building(s).
Note: Absorption factor is not used in Nordic Calculation Method for
Road Traffic Noise of 1996.