Although the ratio of direct-to-reverberant sound energy is known to be an important acoustic cue to sound source distance, human sensitivity to changes in this cue is largely unknown. Here, direct-to-reverberant energy discrimination thresholds were measured for six listeners using virtual sound source techniques that allow for convenient and precise control of this stimulus parameter. Four different types of source stimuli were tested: a 50 ms noise burst with abrupt onset/offset, a 300 ms duration noise burst with gradual onset/offset, a speech syllable, and an impulse. Over a range of direct-to-reverberant ratios from 0 to 20 dB, an adaptive 2AFC procedure (3-down, 1-up) was used to measure discrimination thresholds. For all stimuli, these thresholds ranged from 5 to 6 dB. A post hoc fitting procedure confirmed that slopes of the psychometric functions were homogeneous across stimulus conditions and listeners. These threshold results suggest that direct-to-reverberant energy ratio by itself provides only a course coding of sound source distance, because threshold values correspond to greater than 2-fold changes in physical distance for the acoustic environment under examination.