Re: stereovision question (means of perceiving depth)

[P3D Eric Goldstein <egoldste@xxxxxxxxxx>]

Hope I'm not straying too far off topic in exploring audition as it relates to 
stereo; if so, apologies for the inapproporiate use of bandwidth.

P3D John W Roberts wrote:

> gave the impression that relative loudness is the sole
> determining factor.

It's a combination of phase and loudness (and other cues, too), as your example 
will illucidate.

> a major factor in
> determining the angle of the source through stereo hearing is the perception
> of the phase (or time) difference in the sounds coming into the two ears.

To my knowledge, it is, but only to a limit of about 30 milliseconds, which is 
the lower limit of our ability to discriminate phase or time differences.

> set up two sound sources (or a movable sound source) at a significant
> distance from a test subject, and determine the minimum horizontal spacing
> of the two sources for which a difference in position can be distinguished.
> Compute the differences in paths and plug in the speed of sound to determine
> the difference in arrival times. For instance, if sound sources in front of
> the test subject and ten feet away can be distinguished when they are three
> inches apart, the difference in arrival time would be roughly 16
> microseconds (0.2" difference in path length). (And if I got the math right,
> the difference in volume would be about .014 dB, so that would be unlikely
> to have been a factor.)

At ten feet, the time or phase difference our two ears discriminate is one of 
the most important cues in letting us perceive the directional source of the 
sound (or it's nearest reflection). But at three inches, when the timing 
difference is too small to be perceived, the inverse square law (as it relates 
to sound amplitude) steps in and makes the differential in loudness between our 
two ears significant enough to detect.

This is all gross oversimplification, because unlike visual perception, 
reflections of auditory cues play a _hugh_ role in our perception of their 
location, but is my limited understanding of the basics of how this works.


Eric G.
egoldste@xxxxxx


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