Can ANSYS 10.0 model the sound attenuation of a muffler? I am not matching
the attached benchmark exactly.

Yes, ANSYS 10.0 can model the essential behavior of a muffler which produces
attenuation due to cancellation with the reflection from the opening to the large
diameter chamber. You are performing the benchmark as a harmonic analysis
when the benchmark was done as a transient with just a pulse loading. This does
not produce the resonance that you are seeing.

"The velocity inlet has a time varying velocity imposed
at the start of the solution in the form of a full period
sinusoid with an amplitude of .05 m/s. After the sinusoid
is complete, the velocity remains zero at the inlet for
the remainder of the solution."

To model that you should have done a transient with just a
single period of applied flow and coupled pressure. Performing
a harmonic analysis with an applied pressure could explain why
you are getting a negative transmission loss for some frequencies
(resonance in the inlet pipe.)

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