This study examined the effect of the acoustical parameters on the subjective judgment of balance between the vocal on the stage and the instruments in the orchestra pit inside an opera house. First, the virtual sound fields of the different receiver positions of different theatres were developed in a testing room using a stereo-dipole technique to obtain widely variable ranges of acoustical parameters. Second, the virtual sound fields of the different receiver positions of a single theatre were developed by the same procedure to maintain a limited range of reverberation time. With these sound fields, subjective balance was evaluated by the paired-comparison method. A relationship between the scale value of the balance and the acoustical parameters was determined using multiple regression analysis. Finally, the effect of IACC on balance was examined by the subjective test in a simulated sound field. IACC was controlled by changing the energy ratio between the direct sound and the lateral reflections. IACC values for the vocal and the instruments were controlled independently. In these experiments, the effects of acoustical parameters on the balance were investigated while the sound level difference between the stage and the pit sources was fixed.