The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook provides tabulated information that can be used to determine the insertion loss of a variety of lined and unlined ducts. However, the tabulated cross-sections are primarily for square ducts. Not surprisingly, there is a need for information on the insertion loss of ducts having different aspect ratio cross-sections. Hence, the insertion loss for large aspect ratio cross-sections are investigated using a previously validated finite element simulation approach. A coupled structural-acoustic finite element analysis is performed, and data is compared to measurement results from the literature for a few configurations. An analysis campaign is then performed which better explains the effect of aspect ratio on duct insertion loss.