The assessment of the Transmission Loss (TL) of vehicle components at Low-Mid Frequencies generally raises difficulties associated to the physical mechanisms of the noise transmission through the automotive panel. As far as testing is concerned, it is common in the automotive industry to perform double room TL measurements of component baffled cut-outs, while numerical methods are rather applied when prototype or hardware variants are not available. Indeed, in the context of recent efforts for reduction of vehicle prototypes, the use of simulation is constantly challenged to deliver reliable means of decision during virtual design phase.While the Transfer matrix method is commonly and conveniently used at Mid-High frequencies for the calculation of a trimmed panel, the simulation of energy transfer at low frequencies must take into account modal interactions between the vehicle component and the acoustic environment. After providing a brief review of the established approaches for TL simulation at LF, the article will present a new FE methodology for TL simulation and introduce the advantages of “in-situ” TL simulations by means of fluid-structure FE calculation. In a first step, results are given on bare flat plate as well as bare vehicle structures, in comparison to the results of mass law representations. In a second step, it is shown how the simulation of insulation trims by means of poro-elastic FE can be used to assess the TL of in-situ trimmed vehicle components. On one hand, the FE simulation of the in-situ TLs highlights the limitations of the Transfer matrix method at low frequencies. On the other hand, it is shown that, as far as only the Insertion Loss (IL) of the insulation treatment is concerned, the transfer matrix method can be used in a broader range toward lower frequencies.