The propagation of sound between walls of porous material

Deficiencies of existing theories relating to the propagation of sound along ducts lined with porous sound-absorbing materials are discussed. Attention is drawn to the wave-nature of the propagation of acoustic disturbances in the lining material, and a new theory is put forward in which the propagation of acoustic waves in the composite system of air space and lining is investigated for two simple shapes of duct. The results of the theory are in agreement with those of an earlier theory by Morse as long as the attenuation in the duct is not large, but wide divergences appear, in particular where the attenuation in the duct begins to approach that characteristic of the propagation of sound through bulk lining material. In the latter case it is shown by a particular experimental example that the attenuation found in practice is in agreement with the theory described in this paper.