Psychology
Interaural Time Difference (ITD)
Quick fact
For a sound directly to the side, the ITD is about 0.6 milliseconds—roughly the time it takes for sound to travel the width of a human head.
Why this is interesting
Close your eyes and listen. How do you know that a whisper came from your left, not your right? The answer lies in a delay measured in microseconds.
Read the full explanation
Understanding Interaural Time Difference (ITD)
Imagine two microphones placed 20 cm apart. If a sound source is to the left, it reaches the left microphone slightly earlier than the right one. That tiny time difference—the interaural time difference—is exactly what your ears and brain exploit. For low‑frequency sounds (below about 1500 Hz), the wave’s phase is long enough that the brain can detect the delay between the two ears. High‑frequency sounds are handled differently (by level differences), but for lower pitches, ITD is the main directional cue.
A deeper explanation
Your brain decodes ITD in the brainstem, specifically in the medial superior olivary nucleus. There, neurons act as coincidence detectors: they fire only when signals from both ears arrive at the same time. The axons from each ear have different lengths, creating a delay line. A sound with a specific ITD will cause coincidence at a specific set of neurons, creating a ‘place map’ of azimuth. This Jeffress model (with refinements) explains how microsecond differences are encoded into a neural code. ITD is also critical for the cocktail party effect—the ability to focus on one speaker in a crowd—because it helps separate sound sources based on location.