The expansion chamber can be considered as a simple low pass filter. The transmission loss performance is determined by the cross-sectional area ratio between the inlet and outlet ducts and the volume. With the single expansion chamber the transmission loss falls to zero when a half wavelength or multiple thereof equates to the length of the chamber.
Extending the inlet and outlet pipes into the volume will give a more abrupt impedance change and hence a larger transmission loss.
P. O. A. L. Davies, "Plane wave reflection at flow intakes", Journal of Sound and Vibration, Volume 115, Issue 3, 22 June 1987, Pages 560-564
P.O.A.L. Davies, "Practical flow duct acoustics", Journal of Sound and Vibration, Volume 124, Issue 1, 8 July 1988, Pages 91-115
Subjects: Noise & Vibration