Humans localise sound using interaural time, level, and spectral cues from the pinnae

The auditory system uses three classes of cues to localise sound sources in the horizontal and vertical planes. For azimuth: (1) Interaural time difference (ITD)—the delay between the sound reaching the two ears, up to ~700 μs for lateral sources. (2) Interaural level difference (ILD)—high frequencies are shadowed by the head, reducing amplitude at the far ear. For elevation: (3) Spectral shaping—the asymmetrical pinnae (outer ears), shoulders, and torso reflect sound differently depending on vertical angle, imprinting direction-dependent spectral coloration captured in the head-related transfer function (HRTF). Distance cues rely on the ratio of direct to reverberant signal and high-frequency air absorption. These cues can be synthesised to place sounds in virtual 3D space over headphones.

Examples

Binaural rendering: a sound panned 90° to the right has ~700 μs ITD and ~10 dB ILD at 4 kHz. Applying the listener’s HRTF allows elevation cues. Ambisonics encodes soundfield information for flexible decoding to any speaker layout.

Assessment

Explain why low-frequency sounds (below ~1.5 kHz) are harder to localise by amplitude alone, and identify which localization cue is primary at those frequencies.