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TECHNICAL PAPERS

Mode Interactions and Sound Power Transmission Loss of Expansion Chambers

[+] Author and Article Information
C. K. Lau

Department of Building Services Engineering,  The Hong Kong Polytechnic University, Hong Kong, P.R.C.

S. K. Tang1

Department of Building Services Engineering,  The Hong Kong Polytechnic University, Hong Kong, P.R.C.besktang@polyu.edu.hk

1

Corresponding author.

J. Vib. Acoust 129(2), 141-147 (Feb 14, 2006) (7 pages) doi:10.1115/1.2202357 History: Received June 07, 2005; Revised February 14, 2006

The mode interactions and the sound transmission loss across the expansion chambers with and without tapered sections are studied by the finite element method in the present investigation. Results from chambers with symmetrical inlet and outlet suggest lower sound power transmission loss at frequencies below that of the first symmetrical transverse chamber mode when the tapered section angle is reduced. Weak sound power transmission loss is also observed for this chamber type at frequency higher than that of the first symmetrical duct mode. Numerous high and low sound power transmission loss regions are observed between these two eigenfrequencies. Higher plane wave power transmission loss can be found at smaller tapered section angle only if one of the chamber endings is not tapered. Such chamber bears important industrial application.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the computational model. —–: chamber boundaries, – ∙ –: anechoic termination boundaries.

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Figure 2

Variation of overall sound power transmission loss with wave number for abrupt expansion and contraction chamber. w∕d=4, L∕d=12. – – –: one-dimensional theory, —–: finite element method.

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Figure 3

Sound fields inside abrupt expansion and contraction chamber. (a)kd=0.5π; (b)kd=0.76π; (c)kd=1.12π. w∕d=4, L∕d=12.

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Figure 4

Effects of tapered inlet and outlet on overall sound power transmission loss across expansion chambers under fixed width condition. (a)ϕi=ϕo=tan−1(3∕2); (b)ϕi=ϕo=tan−1(3∕4). w∕d=4.

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Figure 5

Sound field patterns inside expansion chamber with varying cross section inlet and outlet under fixed width condition. (a)kd=0.52π, (b)kd=0.68π, (c)kd=0.82π, (d)kd=0.88π, (e)kd=1.01π. w∕d=4, ϕi=ϕo=tan−1(3∕2).

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Figure 6

Overall sound power transmission loss of expansion chamber with varying cross section inlet and outlet at a fixed chamber length. (a)ϕi=ϕo=tan−1(9∕10); (b)ϕi=ϕo=tan−1(25∕48). L∕d=12.

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Figure 7

Sound power transmission loss across chamber with divergent inlet and abrupt contraction. (a)ϕi=tan−1(3∕2); (b)ϕi=tan−1(3∕8); (c)ϕi=tan−1(3∕16). w∕d=4, ϕo=π∕2.

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Figure 8

Sound field patterns of the expansion chamber with divergent inlet. (a)kd=π∕2, ϕi=tan−1(3∕8); (b)kd=2π, ϕi=tan−1(3∕8); (c)kd=2π, ϕi=tan−1(3∕16). w∕d=4, ϕo=π∕2.

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Figure 9

Sound transmission across chamber with abrupt expansion and convergent outlet for kd>2π. (a)ϕo=tan−1(3∕2); (b)ϕo=tan−1(3∕8); (c)ϕo=tan−1(3∕16). w∕d=4, ϕi=π∕2. —–: Overall sound power transmission loss; 엯: plane wave mode power transmission coefficient; ◻: first higher symmetric duct mode power transmission coefficient.

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Figure 10

Sound field patterns of chamber with abrupt expansion and convergent outlet. (a)ϕo=tan−1(3∕2); (b)ϕo=tan−1(3∕8); (c)ϕo=tan−1(3∕16). w∕d=4, ϕi=π∕2, kd slightly greater than 2π.

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