In this paper, an application process is studied at which the insertion loss (IL) test data of sound insulating parts or noise control treatments are utilized for the sound transmission loss (STL) simulation of the trimmed dash structure. The considered sound barrier assemblies were composed of a felt layer, a mass layer, and a decoupler layer. Flat samples of sound barrier assemblies with several different thicknesses were prepared, and ILs of them were measured by using a sound transmission loss facility. Flat samples were assumed to have mass-spring-mass resonance frequencies. The mass was set as the area mass of the sound barrier layer of the felt layer and the mass layer. The spring constant of the decoupler layer was assumed as the multiplication of that of an air spring and a spring correction factor. At the beginning, the spring correction factor was assumed to be 1, and the measured IL data vs frequencies are converted to IL data vs frequency ratios, frequency f over the resonance frequency fres. With the converted IL data, a trend line was analyzed below the frequency ratio of 1, and another trend line also analyzed above the frequency ratio of 1. It was assumed that the two trend lines had to be similarly matched at the frequency ratio of 1, and the spring correction factor was found by varying the spring correction factor and observing the values of the two trend lines. After the spring correction factor was decided, the high frequency drop curve of the barrier layer were decided. By using the estimated spring correction factor and the high frequency drop curve, IL data were calculated at each thickness, which were necessary for sound transmission loss simulation. The calculated IL data were applied to the STL simulation of a trimmed dash structure, and the simulated STL of the trimmed structure and IL of dash isolation pad (dashmat) are fairly acceptable compared to the test values.