The experimental modal synthesis method has been very popular for analyzing the dynamic behavior of structures. Recently, the demand for reducing not only vibration but also noise has been increasing in the development of industrial machines. In order to deal with noise and vibration simultaneously, the structural–acoustic coupling problem must be solved. The equations for the coupling system are asymmetric (there is not a reciprocity between displacement and pressure). Experimental modal analysis has been applied to solve the asymmetric equations instead of using numerical methods such as NASTRAN, and its usefulness has been shown. This paper describes its theory and its application to the reduction of low-frequency car interior noise, which is attained with a few refinements to the present modal analysis software.