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Design

Immersive audio glossary

See below for a series of technical immersive audio definitions.

Term	Definition
Anechoic	Producing no echoes; having very low or no reverberation.
Ambisonic	A 360-degree sound format that is independent of speakers. Meta Quest uses AmbiX B-format ambisonics with four channels ordered WYZX.
Attenuation	A loss of energy; in acoustics, typically a reduction in volume.
Direct sound	Sound that travels directly to the listener without reflecting (versus reverberant sound).
Early reflections	Reflected sounds that arrive relatively soon at a listener’s location (i.e., before Late Reflections).
Head-Related Impulse Response (HRIR)	A formal characterization of the effect of sound interacting with the geometry of a particular human body. Used to create head-related transfer functions.
Head-Related Transfer Function (HRTF)	A transformation of an acoustic signal using a head-related impulse response. Used to simulate the effects of interaction of a sound originating from a specific direction with the geometry of a particular human body.
Head shadowing	The attenuation of sound caused by the head lying between an ear and the sound source.
Initial Time Delay	The interval between the arrival of a direct sound and its first reflection.
Interaural Level Difference (ILD)	The difference in a sound’s level or volume between the two ears.
Interaural Time Difference (ITD)	The length of the interval between when a sound arrives at the first ear and when it arrives at the second ear.
Late reflections	Reflected sounds that arrive relatively late at a listener’s location (i.e., after early reflections).
Motion parallax	When moving objects are farther from a perceiver than other objects, their apparent speed of travel appears to decrease. For example, a moving airplane on the horizon appears to be traveling more slowly than a nearby car. The apparent rate of travel of an object can therefore be used as a distance cue.
Pinnae	The visible portion of the ear that lies outside the head.
Reverberant sound	Sound that has reflected or reverberated before arriving at a listener’s location (versus direct sound).
Reverberation	The reflection of sound off a surface, or the temporary persistence of sound in a space caused by reverberation.
Sound localization	1. The process of determining the location of a sound’s origin; or 2. the suggestion of an object’s location based on the manipulation of auditory cues.
Sound spatialization	The representation of a sound within three-dimensional space.