2007 年 2 巻 3 号 p. 558-566
A short expansion chamber in a duct acts as a resonator type muffler. The properties of this type of chamber have already been studied in detail by the authors for circular duct-chambers. In this paper, short chambers are applied to rectangular ducts. These chambers consist of two rectangular branches located symmetrically on opposite sides of a rectangular duct. The transmission properties of plane waves through the chambers were examined experimentally. For a very short axial chamber length, the resonant frequency was determined in the same manner as for a side-branch muffler with open end correction. Though considered proportional to depth in circular chambers, it becomes constant in rectangular chambers with fixed axial length, regardless of chamber depth. When the chamber length is close to half of the chamber height (corresponding to the upper limit of resonator-type mufflers), the resonant frequency is close to, but in excess of, the cut-off frequency of the chamber's second resonant mode. The reason for this discrepancy becomes clearer subsequent to examination of the sound field contours calculated using the finite difference method.
