How a tiny timing gap steers a sound
When a sound comes from your right, it reaches your right ear slightly before your left. Your brain compares those arrival times, along with differences in loudness, to estimate where the sound came from. Because the ears are only a short distance apart, the timing clue is extremely small—often measured in millionths of a second for ordinary listening. A delay of one millisecond is therefore much larger than the natural timing difference for many sounds, and can strongly bias the perceived direction.
The brain does not need to hear two separate sounds to react. If nearly identical signals reach the ears at slightly different times, the earlier arrival often dominates the sense of location. This is part of the precedence effect: in many listening situations, the first sound helps determine where the source seems to be, while a later copy is fused with it rather than heard as a distinct echo.
When the delay becomes an echo
Stereo recordings can exploit this sensitivity by sending a sound to one channel a little earlier than the other. The listener may hear one sound pulled toward the earlier side, even though no physical source sits there. As the gap grows, the delayed copy becomes easier to distinguish; under suitable conditions, it can sound like a second arrival or an echo. There is no single cutoff: the result depends on the sound, the delay, and the listening environment.
This is why a millisecond is not merely a technical detail in audio. A very small change can alter the apparent position of a sound before it becomes long enough to feel like a separate event.
