Strutt|Mechanical Services|Duct Elements|End/Grille inserts the attenuation due to duct end reflection into the active row of the worksheet.
Attenuation due to duct end reflection can be calculated using the referenced values in the Arup Sound and Vibration in Mechanical Services Design (Blue Book) or the ASHRAE (2011) method.
Blue Book method references values in Table 17, where the duct dimension referred to is either the diameter of circular ducts or `1.13 * A^2`, where `A` is the section area of rectangular ducts. For grilles this is the core area of the grille and for ceiling diffusers it is `1.25 * D`, where `D` is the neck diameter.
The ASHRAE 2011 method is based on more-recent research from 2008 and is considered more-accurate.
Strutt calculates the attenuation associated with duct-end reflection losses from sheet-metal ductwork as follows:
Note that no significant end reflection will occur for flexible ductwork; if flexible ductwork is selected then the end reflection is set to zero.
Reference: Michaud and Cunefare (2008) Experimental Investigation of Reflection of Airborne Noise at Duct Terminations, ASHRAE Research Project RP-1314