Commenced in January 2007
Frequency: Monthly
Edition: International
Paper Count: 30135
Effects of Duct Geometry, Thickness and Types of Liners on Transmission Loss for Absorptive Silencers

Authors: M. Kashfi, K. Jahani


Sound attenuation in absorptive silencers has been analyzed in this paper. The structure of such devices is as follows. When the rigid duct of an expansion chamber has been lined by a packed absorptive material under a perforated membrane, incident sound waves will be dissipated by the absorptive liners. This kind of silencer, usually are applicable for medium to high frequency ranges. Several conditions for different absorptive materials, variety in their thicknesses, and different shapes of the expansion chambers have been studied in this paper. Also, graphs of sound attenuation have been compared between empty expansion chamber and duct of silencer with applying liner. Plane waves have been assumed in inlet and outlet regions of the silencer. Presented results that have been achieved by applying finite element method (FEM), have shown the dependence of the sound attenuation spectrum to flow resistivity and the thicknesses of the absorptive materials, and geometries of the cross section (configuration of the silencer). As flow resistivity and thickness of absorptive materials increase, sound attenuation improves. In this paper, diagrams of the transmission loss (TL) for absorptive silencers in five different cross sections (rectangle, circle, ellipse, square, and rounded rectangle as the main geometry) have been presented. Also, TL graphs for silencers using different absorptive material (glass wool, wood fiber, and kind of spongy materials) as liner with three different thicknesses of 5 mm, 15 mm, and 30 mm for glass wool liner have been exhibited. At first, the effect of substances of the absorptive materials with the specific flow resistivity and densities on the TL spectrum, then the effect of the thicknesses of the glass wool, and at last the efficacy of the shape of the cross section of the silencer have been investigated.

Keywords: Transmission loss, absorptive material, flow resistivity, thickness, frequency.

Digital Object Identifier (DOI):

Procedia APA BibTeX Chicago EndNote Harvard JSON MLA RIS XML ISO 690 PDF Downloads 611


[1] R. Glav, “The transfer matrix for a dissipative silencer of arbitrary cross-section,” J. Sound Vib. vol. 236, no.4, pp. 575-594, 2000.
[2] R.A. Scott, “The propagation of sound between walls of porous material”, Proc. Phys. Soc., vol. 58, pp. 358-368, 1946.
[3] V. Tarnow, C. Pommer, “Attenuation of sound muffler with absorption and lateral resonances”, J. Acoust. Soc. Am., vol. 83, pp. 2240-2245, 1988.
[4] A. Cummings, I.- J. Chang, “Internal mean flow effects on the characteristics of bulk-reacting liners in circular ducts”, Acoustica. vol. 64, pp.170-178, 1987.
[5] K.S. Peat, “A transfer matrix for an absorption silencer element”, J. Sound Vib. vol. 146, pp.353-360, 1991.
[6] R. Kirby, “Simplified techniques for predicting the transmission loss of a circular dissipative silencer”, J. Sound Vib., vol. 243, pp.403-426, 2001.
[7] F.D. Denia, A. Selamet, F.J. Fuenmayor, R. Kirby, “Acoustic attenuation performance of perforated dissipative mufflers with empty inlet/outlet extensions”, J. Sound Vib., vol.302, pp.1000-1017,2007.
[8] A. Selamet, Z.L. Ji, “Acoustic attenuation performance of circular expansion chambers with extended inlet/outlet”, J. Sound Vib., vol. 223,197-212, 1999.
[9] M.A. Delany, E.N. Bazely, “Acoustic properties of fibrous absorbent materials”, Appl. Acoust., vol. 3, pp., 105-116, 1970.
[10] Tutorial of COMSOL, Introduction to the Acoustics Module. COMSOL Multiphysics 2012 Version 4.3.
[11] D.A. Bies, C.H. Hansen, “Flow resistance information for acoustical design”, Appl. Acoust. vol.14, pp. 357-391, 1980.