In a complex vibroacoustic system the overall noise or vibration in a given location is the sum of multiple subsystem contributions. From an experimental perspective, the total noise can be directly measured but not the contributions. Methods based in transmissivity measurements, as ATPA, allow to find these contributions experimentally and understand the system behaviour through the path concept. Two different contributions to the ATPA method are included here. On the one hand, a numerical model that simulates a simple vibroacoustic problem is shown. This is a closed cuboid-shaped box with air cavity inside. The ATPA experimental procedure is reproduced numerically in order to gain knowledge on some aspects of the method. On the other hand, a technique for the auto matic identification of the subsystems which is based on the path concept and transfer matrices is applied to the acoustic problem of coupled rooms. The proper definition of subsystems influences very much the reliability of ATPA results.