Sound Propagation in Ducts Lined with Absorbing Materials

In ventilator and exhaust systems it is desirable to provide a high degree of attenuation for audio frequency sound waves while offering low resistance to continuous or slowly pulsating air flow. For that purpose ducts lined with absorbing materials frequently are used. This paper deals with sound propagation, particularly with its attenuation constant, in such ducts. A method for computing the propagation constant in terms of the acoustic constants of the lining and of the duct dimensions, is given. Two types of lining are considered, nonvibratile and vibratile, the former admitting of no wave motion propagated in the direction of the duct axis within the lining itself. Experimental data are presented for a duct lined with rockwool, and are compared with computed attenuations. It is concluded that the computational procedure is fairly valid up to a frequency at which the sound wave-length is about twice the internal duct diameter.