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An axially-short expansion chamber in a duct acts as a resonator muffler. The radial size of the chamber must be altered in order to change the target frequency for noise reduction because the resonance frequency depends on the depth of the chamber. The present study reveals that the resonance frequency can be altered by changing the shape of the expansion to an 'L' shape, even if the radial size of the expansion is fixed. The structures of the acoustic field in the duct are obtained numerically based on a finite difference method for several L-shaped expansions. The computed results show that the resonance frequency decreases with increasing axial length of the expansion, whereas the minimum transmission coefficient is approximately constant. In addition, increasing the inlet length of the expansion enables the resonance frequency to be increased and the minimum transmission coefficient to be reduced. These results are in good agreement with previously reported experimental data.

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