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RichP714

4: Sound localization

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This post would be huge if it provided a comprehensive analysis; hopefully, I'll cover enough to spark interest in further explorations
 
The human auditory system uses several cues for sound source localization, including time- and level-differences between both ears, spectral information, timing analysis, correlation analysis, and pattern matching.
 
All of these techniques are important for accurate sound source localization, and most are used in some way during playback of pre-recorded events.
 
A situation that may surprise some is that the ear-brain mechanism doesn't continuously update and refresh its opinion of a sound source's location; much like the judicial system, (and inertia) a stimulus has to change appreciably to trigger a 'recalculation' event
 
This response to external sound stimulus makes it possible for a human to discern location, for example, in enclosed spaces (where an abundance of reflections would otherwise overwhelm the brain's processing power allotted to auditory processing)
 
This sample and hold technique is demonstrated by the Franssen effect (and Bose acoustimass like loudspeakers
 
Sounds emanating from different locations around an observer will be perceived to have a frequency spectrum that varies with sound source position; several surround sound techniques make use of this aspect of audio location
 
While the ear has adapted to ignore (for the purposes of location) reverberant echoes in enclosed spaces, it does make use of some secondary reflections (sound waves impacting the ear from nearby reflective surfaces) in order to locate a sound's position in space within its surroundings
 
The primary location cues for the human ear, however, are variances in sound level between two ears (inter-aural level differences (ILD)) and variances in the sound arrival time between two ears (inter-aural time differences (ITD))
 
ILDs occur due to the head acting as an acoustic shadow, reducing the sound level at the side of the head that is opposite the side of the sound stimulus;  The amount of ILD helps an ear determine in which direction (on a horizontal plane described by the position of both ears) a sound is coming from; ILDs have a stronger influence at higher frequencies, since at lower frequencies (whose period is below the distance between the ears (typically 5-7 inches) acoustic waves will no longer be shadowed by the head
 
ITDs occur due to the distance in space between human ears; sounds arriving from one side of the head arrive at that ear slightly before the opposite ear
 
It's thought that Below 800 Hz, mainly inter-aural time differences are evaluated (phase delays), for frequencies above 1600 Hz mainly inter-aural level differences are evaluated, and that Between 800 Hz and 1600 Hz there is a transition zone, where both mechanisms play a role.
 
Localization accuracy is 1 degree for sources in front of the listener and 15 degrees for sources to the sides. Humans can discern interaural time differences of 10 microseconds or less.
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Worth mentioning, the listening chair can have a major impact in localization. A high wing back, for instance, will block many of the side reflections and much of the delay information bouncing off the rear of the room. You'll still hear it all, but the positioning will be off.
 
I compromised with a small lazyboy. Back is tall enough to cradle the neck, but the head sticks above it for unimpeded reflections. A lot of what I hear is also first point generation as I run "sexophonic" sound, with two pairs of rear speakers positioned directly to the sides and slightly behind overhead.
 
Just to complicate things, both rear sets are Bose (known for their fun with reflections) and did I mention I also use a Carver C9? <G>
 
The REAL trick is making the speakers disappear while still getting reliably consistent positional information on instrument placement. 

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