A study was conducted to understand the sound characteristics that humans used to evaluate a source distance in a room. The evaluation was relative where the listener needed to know a priori the sound level produced by the source, or he and she only compared different distances assuming a constant source level. The direct-to-reverberant energy (D/R) ratio had been proposed as the underlying cue for this evaluation. This acoustical attribute was obtained by comparing the direct sound energy to the reverberant sound energy. It decreased systematically with source distance, following the decrease of direct sound energy and it was independent of source level. The D/R ratio was calculated from the room impulse response, but listeners were unable to have access to this room response when they listened to sounds which were not impulses.