Initial time delay gap and the sense of space

The initial time delay gap (ITDG) is the time between the direct sound and the first strong reflection at a listening position. It is a short interval, usually 10 to 50 ms, and it has an outsize effect on how a room sounds.

Where it comes from

The direct sound travels in a straight line, and the first reflection arrives via a wall, ceiling or floor. The difference in path length divided by the speed of sound is the gap. For a path difference of 6 m, the gap is 17.5 ms.

The gap depends on both the room and the positions:

• A nearby source has a long gap, because the direct sound arrives early and the reflections come much later.
• A distant source has a short gap, because the direct sound and the reflections have similar lengths.

What it means for perception

• Short gaps, below roughly 20 ms, are associated with an intimate sound. Concert hall acoustics research linked an ITDG below about 20 ms at seats to a feeling of intimacy.
• Long gaps give a sense of a larger, more distant space.
• Early reflections within about 15 ms of the direct sound have a tonal effect through comb filtering, which many listening rooms try to avoid, see Comb filtering.

Using it in reverb and mixing

Reverberation effects often have a pre-delay setting. It is the gap before the reverb starts, and works like the ITDG:

• With little or no pre-delay, the sound merges with the reverberation and seems farther from the listener.
• Adding pre-delay, 20 to 60 ms, separates the sound from the reverb. The source remains clear and sounds closer, with the space behind it.

Reverb that is delayed in time to the tempo of the music, see the BPM delay tool, can add space without blurring rhythms.

In studio design

Control rooms aim for a long ITDG, with the first reflections from the walls absorbed or directed away, and a late diffuse sound field. The idea behind live end, dead end is similar: the front of the room is highly absorbent and the rear diffuses, so that early reflections do not reach the listener and the first reflection comes late.

Practical notes

• Moving a loudspeaker or microphone changes the gap, and with it the sound.
• A reflection from a console or table can arrive within a few milliseconds and colour the sound.
• The gap is an overview figure; the exact strength, direction and spectrum of the reflections also matter.

Draft: written by an AI assistant, awaiting technical review.