Re: Head movement and ASA

[John Neuhoff <jneuhoff@xxxxxxxxxxx>]

Hi Pierre, 

Check out Bill Chapin's company (Ausim Inc.) for some incredibly compelling
simulations of moving sounds.  The strikingly realistic auditory motion
simulations that we recently used included absolute delay (to account for
the speed of sound and the changing distance between the listener and the
source), Doppler shift, atmospheric filtering, gain attenuation due to
atmospheric spreading, ground reflection attenuation, and  dynamic HRTFs
from the KEMAR data set  We've just completed some work on the perception of
looming and receding auditory motion that they simulated for us.  

Dear Jim & Al, 

The physics (and perception) of the Doppler effect depend on the velocity of
the source and the distance from the listener to the patch of the source.  A
source travelling at a high velocity, but on a collision course with the
listener will not have a perceptible Doppler effect.  The observed frequency
will be higher than the emitted frequency, but there will be no frequency
change on approach.  So, if the listener doesn't know the source frequency,
the Doppler shift will go undetected (except for the influence of the
intensity change, see Neuhoff & McBeath, 2002).  

The case of an ice skater (or a slower rotating head) is interesting.  As
one ear moves toward the source (causing an increase in frequency) the other
moves away (causing a corresponding decrease).  But, the pattern is reversed
after 1/2 rotation, causing a modulation effect.  This is much like the old
rotating Leslie speakers that came with Hammond organs, only a rotating head
instead of loudspeaker.  Regular head turns typically do not produce
perceptible Doppler shifts, probably for two reasons.  First, the velocity
of the head turn is pretty slow relative to the speed of sound.  Second, as
mentioned above, frequency  rises in one ear and falls in the other. If you
simulated high-velocity head turns, or a source rotating quickly around the
head, this actually creates some interesting situations for ASA.  The pitch
of a single source sound going in opposite directions based on the direction
of the rotation. I wonder if you could manipulate the stimulus in such a way
that the differential pitch cues would override the spatial unity of the
source and get one source to sound like two based on Doppler cues.  

Christian wrote:

"...head movements for the improvement of SSL are quite a nice example of
the role of action in perception, up to the point where some say "Perception
is a behavior, a specific kind of action aiming at the driving home of a
maximum amount of information on the object of interest." (Is this
Gibsonian?)"

Christian, 
Yes, this is quite ecological and quite Gibsonian. Although "Neo-Gibsonian"
might be a better term.  It's doubtful that Gibson would be much enamoured
with any of the top-down cognitive aspects of ASA, but head movements,
absolutely.  It is striking to me that so much work has been done on
localization and the role of spatial separation in ASA while restraining
head movements. We've evolved in an environment in which head movements
toward a source are adaptive.  If we examine the head movements of newborns
toward a source (or hemifield) one could even make the argument that they
are somewhat innate. This leaves little doubt in my opinion of their
importance.  

Best, 

-John 

 ________________________________
John G. Neuhoff
Department of Psychology
The College of Wooster 
Wooster, OH 44691
Phone: 330-263-2475 

http://jneuhoff.com 
-----Original Message-----
From: AUDITORY Research in Auditory Perception
[mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Pierre Divenyi
Sent: Sunday, December 18, 2005 7:48 PM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: Head movement and ASA

Dear Al,

The major problem with the complex (=multi-source) moving source experiments
you are talking about is procedural: as far as I know, there is no algorithm
that would generate good dynamically changing HRTF's and generating even a
single moving source in free field requires quite an undertaking.

Pierre