L. Cremer's theory of sound transmission for a single-leaf wall is transformed into a more general formulation in which the exciting pressure is expressed in terms of the spatial cross-correlation. The correlation coefficient was measured in a reverberation room just in front of the wall under test. The sound-transmission loss of a single gypsum-board wall was calculated employing these diffusivity measurements and compared with experimental data. Agreement between theory and experiment is good, especially in the frequency range below the coincidence frequency. Finnaly, a few fictitious quasidiffuse sound fields are investigated in further study of the influence of the degree of diffusivity upon transmission loss.