Initial Time Delay Gap ( ITDG)
The Initial Time Delay Gap describing the time difference between arrival of the direct wave and first strong reflection at the listener. Nearby sources are causing a long ITDG. In case of the source are far away, hardly run time differences arise:
Our listening experience let perceive signals by long ITDG's as nearby sources therefore. The ITDG is one of the most important cues for estimating the distance regarding the sound source. The ITDG isn´t a constant of the room; it is dependent from source and listener position. By loudspeaker reproduction in normally dwellings too early reflection of the loudspeakers radiation often unavoidable. That caused a lack of intimacy for the reproduction. Thus hardly remain possible producing nearby sound sources in such case. But also at many recordings isn't considering importance of the cue. Best values for ITDG in concert halls are somewhere between 12 and 25 milliseconds.
link: listening examples