Sound Propagation in Ducts Lined with Absorbing Materials

In ventilator and exhaust systems it is desirable to provide a high degree of attenuation for audiofrequency waves while offering low resistance to continuous or slowly pulsating air flow. For that purpose ducts lined with absorbing materials are sometimes used. This paper deals with sound propagation, particularly with its attenuation constant, in such ducts. Two types of lining are considered: (a) nonvibratile, i.e., a lining in which there is no wave motion propagated in the direction of the duct axis; (b) vibratile, i.e., a lining admitting of such motion. Methods for computing the propagation constants in terms of the acoustic constants of the lining, are given. Some experimental data are presented. Comparison of observed and computed values indicates that the computational procedure is substantially valid up to a frequency at which the sound wave-length is about twice the internal duct diameter.