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Close your eyes and have someone snap their fingers to your left. Instantly, without thinking, your brain knows the sound's direction to within two degrees.
Your ears don't have eyes, so your auditory system calculates spatial position using tiny timing gaps, volume drops, and spectral reflections.
How does the brain turn two separate audio streams into a 3D soundstage?

Rayleigh's Duplex Theory

In 1907, British physicist Lord Rayleigh discovered that human spatial hearing relies on two distinct mechanisms split by frequency, known as Duplex Theory.
Low frequencies use time differences between ears, while high frequencies use volume differences caused by the acoustic shadow of your head.
Why is the crossover point around 1.5Β kHz1.5\text{ kHz}? It comes down to basic wave physics and the physical size of your head.

Interaural Time Difference (ITD)

Interaural Time Difference (ITD) is the difference in arrival time of a sound wavefront between your left and right ears.
The average human head width is roughly d=0.20Β md = 0.20\text{ m}. With sound traveling at v=343Β m/sv = 343\text{ m/s}, the maximum delay across the head is calculated with time equals distance divided by speed:
Ξ”tmax=dv=0.20Β m343Β m/sβ‰ˆ0.00058Β s=580Β ΞΌs\Delta t_{\text{max}} = \frac{d}{v} = \frac{0.20\text{ m}}{343\text{ m/s}} \approx 0.00058\text{ s} = 580\text{ }\mu\text{s}
For frequencies below 1.5Β kHz1.5\text{ kHz}, wavelengths are longer than 23Β cm23\text{ cm}, allowing the brain to track phase differences across the cycles without ambiguity.