We examined in this paper time fluctuations of sound field diffusibility in transient response by measuring the short-term directional diffusing factor and running power spectrum with direction. As a result, we found that transitions in the values of directional diffusing factor until attaining the diffused state, the time required until essentially attaining the diffused state and attenuation of sound energy at that time all indicated differing characteristics in each of the halls. In addition, it was also found that a virtual state of diffusion was attained within at least approximately 300ms in all of the halls surveyed at this time and the short-term directional diffusing factor, thereafter, indicated essentially identical values regardless of room configuration. We understood from this that sound field diffusibility is related to the time required to attain a diffused state and to sound energy attenuation, but not to the values of directional diffusing factor subsequent to the transition to the diffused state.
