Psychoacoustic Principles

Interpretation of sound applied to spatial-audio compression and mixing

Modern psychoacoustics is the field that studies the interaction between the physical properties of sound and human perception, bringing together disciplines such as acoustics, psychology, and neuroscience. Unlike traditional acoustics, psychoacoustics explores how the human brain interprets sound waves to construct the perceptual experience of sound.

Sound is defined physically as a vibration that propagates through an elastic medium such as air, captured by the auditory system and processed in the brain. The frequency of these vibrations, measured in hertz (Hz), determines the perceived pitch, while amplitude —measured in decibels (dB)— is associated with the sound intensity.

Timbre allows us to distinguish two sources with the same frequency and intensity. This phenomenon arises from the spectral structure of the sound, where harmonics add complexity to the fundamental wave—an essential aspect in system design.

Humans, under normal conditions, perceive frequencies from 20 Hz to 20 kHz, with maximum sensitivity in the 2000 to 5000 Hz range where the auditory system is attuned to subtle variations.

Perception

The spatial perception of sound is another core area of psychoacoustics. The human brain uses multiple cues to determine the location of a sound source, including interaural time differences (ITD) and interaural level differences (ILD), enabling the interpretation of spatial position.

ITD cues are effective for low frequencies, while ILD cues—more relevant at higher frequencies—emerge from the attenuation produced by the acoustic shadow of the head.

Spectral cues, derived from the interaction of sound with the shape of the ears, provide information about elevation and distance. The Haas effect (precedence effect) describes how the brain consistently prioritizes the first sound arriving at the ear when multiple acoustic waves originate from the same source but reach from different directions.

This phenomenon enables localization in complex environments.

The duration of sounds also influences perception. Very short sounds, with durations below 20 milliseconds, may not be detected by the auditory system, while longer intervals can generate sensations such as reverberation or spatial ambience.

These characteristics are essential in designing surround-audio systems, allowing perception to be modeled within virtual spaces. Spatial-audio systems rely on localization and masking simulations to create immersive experiences that replicates real-world acoustic behavior.