Data for the insertion loss of spiral dual-wall circular ducts and circular radiused elbows were obtained by Bodely (1988). The acoustical lining of the ducts and elbows was a 0.75 lb/ft³ density fiberglass blanket that ranged in thickness from one to three inches. The fiberglass was covered with an internal liner of perforated galvanized steel, which had an open area of 25%. The data were for no airflow in the ducts. Multivariable regression analyses were conducted on the data to develop equations that can be used to predict insertion loss value& The results of these analyses are presented in this paper.