Fiberglass internal duct fining is often used to attenu ate sound in ducts. Data for sound attenuation of unlined rectangular ducts and for insertion loss of acoustically lined rectangular ducts, obtained by several investigators, were examined. The data were for no airflow in the ducts. Multivariable regression analyses were performed on the data to develop equations that can be used to predict sound attenuation and insertion loss values for rectangul ar ducts. The results of these analyses are presented in this paper.