The paper presents general method to analyse acoustic mufflers or other duct-like systems assuming propagation of a multimode wave and applying the multiport method. Apart from the main propagating tube and expansion chambers the analysed device may contain segments of annular duct (concentric or not with the central tube) branch resonators, cavities and cavity diaphragms. Systems of such complicated geometrical structure have been widely analysed under the assumption that the wave propagating through any element is the plane wave. Thus the problem might have been analysed by means of the two-dimensional transfer matrices (or others equivalents) derived from conditions of conservation of the acoustic pressure and the volume velocity across each discontinuity. However, the assumption of the pure plane propagation across every element of the acoustic system imposes strong limitations on the excitation frequency or the radius of the duct segments as the Helmholtz number should not exceed the value 1.93 which represents the non-dimensional cut-on frequency of the first circumferential mode. If the excitation and the system are axially symmetric the limited value of the Helmholtz number raises to 3.83. The analysis is carried out by means of the scattering matrix which is finally transformed into transmit matrix.