Sound attenuation in dissipative expansion chambers

Recent improvements in fiber properties combined with their broadband acoustic dissipation characteristics make such materials potentially desirable for implementation in silencers. The acoustic modelling of simple silencer geometries, such as circular or rectangular cross-sections, has been established and classified depending on the assumption of locallyor bulk-reacting lining. Morse [1] used a locally-reacting model to investigate the sound transmission in pipes with absorbing material on the inner walls. By using bulk-reacting model for linings, Scott [2] studied the transmission of sound in infinite rectangular and circular ducts; Ko [3] investigated the characteristics of sound attenuation for rectangular, annular and circular ducts; Cummings and Chang [4] examined the effect of mean flow on the modal attenuation rates. The foregoing investigations [1–4] have targeted infinitely long ducts, thereby excluding the effects associated with the expansion/contraction of a finite chamber. In order to fully understand the acoustic performance of a finite-length dissipative silencer, it is essential to investigate the effect of discontinuities across the expansion/contraction of the expansion chamber. Cummings and Chang [5] analyzed the sound attenuation of a finite length dissipative silencer with mean flow by using a mode-matching technique. Axial acoustic particle velocity and pressure were matched across the expansion/contraction of the chamber, and a good agreement between predictions and measurements was observed. Peat [6] developed a transfer matrix formulation for the bulkreacting dissipative silencer from the match of average acoustic pressure/velocity across the silencer discontinuities. By using one-dimensional analytical approach, three-dimensional BEM, and experimental methods, the acoustic attenuation was discussed for both a single-pass dissipative expansion chamber [7] and a hybrid silencer [8] consisting of two single-pass dissipative expansion chambers and a Helmholtz resonator. A pod silencer, which is a lined circular duct with ARTICLE IN PRESS