Abstract
The finite element approch for acoustic transmission systems was discussed by Kagawa, and Young and Crocker, and the usability of the technique has been verified experimentally for acoustic filters with arbitrary circular cross section. The treatment is based on the electrical four-terminal network analogy with regard to the input and output ports of the filter, which is considered tobe ideally reactive or free from dissipation. The present work is the extension of our previous work to a more general system with a wall of arbitrary acoustic imdedance. The load impedance at the output port can simply be regarded as a part of the wall. so that the transmission characterstic can be computed directly. The finite element formulation is developed on the basis of the true-adjoint system approach. For the discretization, the second order polynomial is used for the traial function of the trangular ring element. A computer program is developed, with which the transmission loss of the expansion-type acoustic filters is calculated. The wall of the chamber is partly treated with sound-absorbing felt. The calculation results coincide well with the experiment results (Figs. 8〜11). The program is also applied to the simulation of the acoustic horn problem, another example of an acoustic transmission sytem with variable cross section. A model is considered in which the radiation half sphere is assumed to be surrounded by a hypothetical wall with an air impedance ρc and the finete element calculation is applied inside. The driving-point impedance of the horn throat is calculated, the result of which again agrees reasonably with the measured result (Fig. 13). It is to be concluded that the simulation technique thus developed can provide a means of predicting and designing the behaviour and the characteristics of acousitc transmission systems of this kind.
