Reflected sound anchors external source localization while its absence pulls sound into the head

Evidence

In-head localization is logical opposite of an enveloping, external and spacious, auditory illusion. Perceptions of sounds originating inside the head, which routinely occur in stereo (not necessarily binaural) headphone listening, can also occur in loudspeaker listening when the direct sound is not supported by the right amount and kind of reflected sound for the perceptual system to make a proper determination—a default localization. The author and his colleagues have experienced the phenomenon many times when listening to stereo recordings in an anechoic chamber, usually with acoustically “dry” sounds hard-panned to center or, less often, to the sides. It prompted an investigation, Toole (1970), the conclusion of which was that there is a continuum of localization experience from external at a distance through to totally within the head. It is often noted with higher frequencies, and it can happen in a normal room with loudspeakers having high directivity, or in any situation where a strong direct sound is heard without the expected reflections. In an anechoic chamber it can occur when listening to a single loudspeaker, especially on the frontal axis, in which case front-back reversals are also frequent occurrences. This phenomenon is so strong that it need not be a “blind” situation. It is very disconcerting to look at a loudspeaker and to hear the sound inside the head or behind. — Toole, Floyd E, Sound reproduction- the acoustics and…, 450

Categorization

• DEFINE: Explains the mechanism by which missing reflections cause in-head localization, a key spatial-perception failure in psychoacoustics.
• QUESTION: Worth chasing: what specific reflection timing and level thresholds prevent in-head localization in loudspeaker reproduction?