Classical Transfer Path Analysis (TPA) is a well-established method for tackling NVH problems. It is proven to be reliable, but because of its complexity and time-consuming procedure, the industry is constantly seeking for simpler and faster methods. One such method which has quickly gained attention in the NVH field is Operational Path Analysis (OPA). OPA is a fully-operational method requiring only operational measurements of the path references (e.g. body-side mount accelerations, pressures close to vibrating surfaces, nozzles, apertures) and target response(s). A transmissibility matrix between references and target(s) is estimated from operational data under different test conditions (e.g. run up, run down). The claim for its accuracy is based on being able to reproduce the original target signal(s) by summing the calculated path contributions. However, this provides no evidence that the individual path contributions are correct. In this paper, the OPA method is critically examined and compared to a classical TPA measurement. The results of this examination reveal three significant limitations:
The first critical element is the cross-coupling between the path references. Due to the system’s modal behaviour, a single force in one of the mounts causes vibrations at all path references. This cross-coupling easily leads to a false identification of significant paths and to wrong engineering decisions.
The second limitation of OPA are the numerical conditioning problems related to transmissibility estimations from operational data. These problems lead to unreliable transmissibility estimates in many cases.
The third critical element are the potential errors of missing transfer paths in the analysis. The contributions of missing paths are distributed over the other ones, introducing errors that are hard to recognize as the summed contribution is not affected in most cases.
