Abstract Passive silencers with acoustic fill such as glass fiber, rock wool or foam are commonly used in conventional heating, ventilation and air conditioning systems. Acoustic performance can be estimated for a few basic silencers through the use of design curves available in the literature. Recently, a large microcomputer data base using design curves generated to cover the entire range of manufactured rectangular silencers was made available. Details of the design curves and the mathematical model are presented. Insertion loss of the silencers is estimated from attenuation rates calculated from a finite element method. Unlike existing models, the present method considers multimodal acoustic propagation and can be extended to account for the effects of shearing airflows in the airway. The sound-absorbing material is considered to be bulk reacting. Wave propagation in the material is thus included. Design curves are grouped by using three nondimensional parameters, thereby covering the entire line of conventional rectangular duct silencers. Results from the model are compared to least attenuated mode predictions and to actual test data. The results show that the present model gives better predictions than least attenuated mode models. Good comparison between the current model and the test data was also observed. Development of a computer program for a quick estimation of the insertion loss is also described.