Abstract: Using modal coupling theory, the transmission loss of a rectangular panel into a rectangular enclosure in a low frequency random incidence sound field has been calculated. Three random incidence sound field models were considered:(1) single frequency random incidence sound field is stochastic processes in space. (2) every point in single frequency random incidence sound field is a delta function, no relation with each other. (3) single frequency random incidence sound field is sound field, in which plane waves are incidented from all directions with equal probablity and with random phase. The frequency response of the coupling system in single frequency random incidence sound field is given by the theoritical results and computer simulation.