Re: stereovision question (means of perceiving depth)

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 9 Apr 1996 15:15:40 -0500
>From: P3D Eric Goldstein  <egoldste@xxxxxxxxxx>
>Subject: Re: stereovision question (means of perceiving depth)

>John W Roberts wrote:
>>  - audio effects, and combination of visual and audio effects
>>     (A simple example is determining the distance of a flash of lightning,
>>     but the human auditory system is capable of perceiving much shorter
>>     distances - there's circuitry in there capable of handling time intervals
>>     of hundreds of microseconds, or perhaps better.

>Excellent post... lots of information to sort through and think about.

Thanks.

>Regarding the audio effects you mention, we always worked with 30 
>milliseconds as the lower auditory limit for discrimination between 
>sounds and directional discrimination, at least as experimental evidence 
>accumulated through self report goes. I'd be interested in any additional 
>information or references you may know of which takes this interval down 
>to lower limits.

The only reference I can think of is a Scientific American article that was
printed in the last several years, which stated that 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.
I was surprised by this because a kids' science show I saw back in first
or second grade gave the impression that relative loudness is the sole
determining factor. After having read the article, it seemed reasonable - 
location of sources seems to be much more accurate than would be accounted 
for by differences in volume alone.

If this is the case, then there should be a fairly simple way to get a good
idea of the minimum time differentiation of the human auditory system:
with reasonable precautions in designing the experiment to prevent unintended
cues, 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.)

As I mentioned in the dolphin post, the type of source used may have a lot
to do with how accurately its position can be determined.

Note 1: the "three inches at ten feet" example was pulled out of thin air,
but I wouldn't be surprised if it's in the ballpark.

Note 2: in the post on mechanisms of determining depth, I didn't come up
with the item on texture gradients myself - it was suggested a year or
two back that I add it. I notice that texture gradients have received some
recent coverage on the list.

John R


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