Re: stereovision question (means of perceiving depth)

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Wed, 10 Apr 1996 10:28:23 -0500
>From: P3D Eric Goldstein  <egoldste@xxxxxxxxxx>
>Subject: Re: stereovision question (means of perceiving depth)

>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.

Perhaps not - see the comments posted later on Wednesday regarding Braille.

>P3D John W Roberts wrote:

>> 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.

Are you sure that's not 30 *microseconds*? 30 milliseconds in air is a little
over 30 feet - you'd have to have an extremely large head to ever experience
that much difference in time between the arrivals of sounds at the two ears.

>> 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.

I should have been more clear - I was talking about distinguishing between
sound source A and sound source B, where A is 10 feet directly in front of
you, and B is 10 feet in front of you and three inches to the right, so B is
3 inches away from A, and only a fraction of an inch further from you than A.
A is an equal distance from both ears, so a sound from A reaches both ears
at the same time. B is about 0.2 inches closer to the right ear than it is
to the left ear, so a sound from B will reach the right ear about 16
microseconds earlier than it reaches the left ear. Since the distance from
B to the right ear is only about 599/600 of the distance from B to the left
ear, and assuming the inverse square law, a sound at B will sound about
0.014 dB louder to the right ear than the left ear. Given the logarithmic
response of the ear, that's such a fantastically tiny difference in volume
that I would venture to say that *if* a person can distinguish between A and
B, the primary factor is almost certainly the detection of a timing
difference rather than a difference in loudness. This is also supported by
the observation that comparing loudness levels is not a very good way to
locate sound sources from an evolutionary viewpoint, since many people have
different sensitivity in their left and right ears, and it would be a
considerable hardship to have to relearn the mapping whenever the relative
acuity changes. Putting an inefficient ear plug in one ear could serve as
a test - do the sound sources around you seem to make a sudden tremendous
shift in position?

I should further clarify that by "perception", I don't mean conscious 
perception - your number of 30 ms sounds about right for that. I'm
talking about some hard-wired phase detector that compares the input from
the two ears in a method possibly analogous to the way visual processing
compares input from the two eyes. Since the eyes are primarily a spatial-
domain sensor while the ears are a time-domain sensor, that would be
consistent. Certainly the auditory system can do things with sounds, such
as differentiating between two very similar tones, without consciously
perceiving the difference in cycle time of the two tones (which could be
a fraction of a millisecond).

If I get a chance, I'll try to find that article. In the meantime - anyone
willing to try the experiment? 

John R


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