Strutt|Directivity|Louvre/Opening Directivity inserts louvre directivity values obtained using the SRL method into the active row of the spreadsheet.
The calculation performs simultaneous double interpolations in Fig 11.2 of the SRL Noise Control in Mechanical Services book to determine the directivity of the louvre in the horizontal and vertical directions. The horizontal and vertical directivity components are arithmetically added to give the total louvre directivity.
Note that the louvre directivity doesn’t include any “surface directivity”, i.e. +3dB for hemispherical radiation for a louvre in a baffle/wall (as a typical scenario); it is simply a source directivity.
Reference: SRL Mech Services Book