Initial Time Delay Gap (ITDG)
Definition
Initial Time Delay Gap (ITDG)
The Initial Time Delay Gap (ITDG) is the time interval between the arrival of the direct sound and the first significant early reflection, measured from the room impulse response. Shorter ITDG (below 20 ms) creates a sense of acoustic intimacy, while longer ITDG (above 30 ms) creates a sense of spaciousness. ITDG is a key design parameter for concert hall acoustics. SonaVyx identifies ITDG from measured impulse responses.
How It Is Measured
ITDG is measured from the room impulse response by identifying the direct sound peak and the first reflection peak that exceeds a threshold (typically -20 dB relative to the direct sound). SonaVyx automatically detects the direct sound arrival and scans for the first significant reflection, computing the time gap in milliseconds. The reflection arrival time indicates the distance to the nearest reflective surface.
Practical Example
A concert hall impulse response shows the direct sound at 25 ms and the first significant reflection (from the stage floor) at 33 ms, yielding an ITDG of 8 ms. This short ITDG contributes to the hall's reputation for intimacy — listeners feel close to the performers despite the large volume. A larger hall with the first reflection at 55 ms (ITDG = 30 ms) feels spacious and distant.
Perception and ITDG
Research by Barron and Marshall showed that ITDG strongly influences the subjective impression of intimacy in concert halls. ITDG below 20 ms creates a sense of closeness to the performers. Between 20 and 40 ms, the room feels of moderate size. Above 40 ms, the space feels large and distant. The best concert halls achieve short ITDG through careful placement of early reflection surfaces (lateral walls, ceiling reflectors) near the stage.
Lateral Reflections
The direction of early reflections matters as much as their timing. Lateral reflections (from side walls) arriving within 5 to 40 ms of the direct sound create apparent source width (ASW) and spaciousness (LEV — Listener Envelopment). Ceiling reflections arriving in the same time frame reinforce loudness but do not contribute as strongly to spaciousness. SonaVyx impulse response analysis identifies the timing of early reflections, guiding reflector placement.
ITDG in Small Rooms
In small rooms (studios, home theaters), the nearest reflective surface is typically close, producing very short ITDG (1 to 5 ms). These close reflections can cause comb filtering rather than beneficial reinforcement. Acoustic treatment (absorption or diffusion) on the nearest surfaces increases the effective ITDG by either eliminating or temporally spreading the first reflection.
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