lesson
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Close your eyes and clap your hands in a tiled bathroom, then in a carpeted bedroom. Without looking, your brain instantly draws a 3D mental map of the room's size, shape, and surface materials.
This ability is called psychoacoustics—the study of how our brain perceives and interprets physical sound waves.
The Three Stages of Sound
Every acoustic space breaks an audio event into three distinct chronological stages: direct sound, early reflections, and the late reverberation tail.
📊Interactive diagram
How does your brain treat the sound that hits your ears first versus the reflections that arrive just milliseconds later?
1. Direct Sound and Pre-Delay
Direct sound reaches your ears first along an unobstructed straight path. Your brain calculates horizontal localization from this initial wavefront using Interaural Time Differences (ITD) and Interaural Level Differences (ILD).
Under the precedence effect (or Haas effect), reflections arriving within the Haas integration window (< 35–50 ms for complex sounds) fuse into a single auditory event. The auditory cortex localizes the sound source almost entirely from the direct sound arrival using Interaural Time and Level Differences (ITD and ILD).
The time interval between the direct sound and the very first boundary reflection is known acoustically as the Initial Time Delay Gap (ITDG), or pre-delay.
Room size perception relies heavily on this timing: a short ITDG with rapidly dense reflections signals a tight room, whereas an ITDG exceeding 20ext−−40extms signals a cavernous, large space.
What happens when that sound wave starts bouncing off the floor, ceiling, and side walls?