The calculation efficiency of the conventional single-domain acoustic BEM (SBEM) is important for analyzing large scaled or complicated acoustic cavity systems. Although the multi-domain BEM (MBEM) has been developed to deal with such complex shaped and large size acoustic systems effectively, it is generally known that the MBEM requires more computation time and computer memory space than the SBEM. However, if the proper division were applied on a single cavity, it is thought that the MBEM could be the better method than the SBEM in the viewpoint of accuracy and efficiency. This might be possible when one reminds the fact that the effort in the MBEM calculations also depends strongly on the shape of the total acoustic cavity. In this article, the general computational characteristics of the MBEM for analyzing the interior acoustic fields are investigated to provide a guideline in the division of a single cavity into several subdomains for having better computational performance than using the SBEM. A two-dimensional long duct comprised of a number of linear elements is taken as a demonstration example. It is clearly shown that the modification of MBEM model through the present guidelines achieves more accurate and efficient computation than using the SBEM.