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Re: Hean Movements and Sound Source Segregation

Also, in the late 1930s Hans Wallach gave a geometrical argument for
the use of motion in localization: Say that you hear a sound and can
localize it using iteraural timing and level differences to the median
plane.  If you turn your head to the left, a sound source in front of
you will move towards your right ear, while one behind you will move
towards your left ear.

Furthermore, if the sound source is elevated above or below the
horizontal plane, it will move less than if it were in the plane.  In
the extreme, when the sound source is directly above you its cues
don't change at all as you rotate your head.

See e.g. 
Wallach, H. "On Sound Localization." JASA 10(1), 1938, p83


On Sun, Dec 18, 2005 at 01:10:40PM +0100, Christian Kaernbach wrote:
> Dear Al,
> This is an interesting question. I know of no work directly addressing 
> head movements and auditory scene analysis. The role of head movements 
> for sound source localization has certainly been well studied quite some 
> time ago (see postscript). However, whether head movements are only 
> relevant to localization, or whether they help to separate sound 
> sources, would be an interesting field of research. I would reckon that 
> in a typical cocktail party situation the listener would move the head 
> until he/she found the optimal SNR between the desired signal and the 
> rest of the sound field.  Once this position found, it should not be 
> helpful to move the head further. Sure, it would make the desired sound 
> source a moving target, but with all the other sound sources moving 
> around in the same way. The first approach should be to monitor what 
> listeners actually do in difficult auditory scenes. I could imagine that 
> in case of repetitions (the important phrase comes twice, e.g. because 
> the speaker realized that it did not come through) the listener might be 
> inclined (sic!) to try a different head position, so as to reduce 
> redundancy between the two communications.
> Best regards,
> Christian
> PS: Let me write on the role of head movements for sound source 
> localization (SSL) in a postscript, a) because I am not a real expert on 
> this issue, and b) because many listers might know plenty about it. I 
> could not tell from where I have this knowledge, most probably from oral 
> communication early in my career. The two primary cues for SSL are 
> intensity differences and delay differences. These two cues are, 
> however, quite ambiguous: all sound sources on the famous "cone of 
> confusion" induce the same delay and intensity difference. Think of zero 
> delay and intensity difference: this is true for all sound sources on 
> the median plane, i.e. from ahead, top, behind, below, etc. Nevertheless 
> it has early been noted that humans can well discriminate between sound 
> sources from ahead and from behind. I was told the anecdote that in the 
> early days of SSL research this performance was attributed to (from 
> today's viewpoint) weird supposed mechanisms, such as a sound pressure 
> sensitivity of the chest. Later on one started to use head fixation, and 
> much of the ahead/behind discrimination performance went away. Another 
> mechanism involved in this performance is the spectral filtering by the 
> outer ear (head related transfer functions, HRTF), but this mechanism 
> can only be helpful if the sound (or its supposed spectrum) is known to 
> the listener. So if using sine tones of varying loudness, the 
> ahead/behind discrimination depends critically on the participant's 
> ability to move his/her head. I suppose that much of this can be found 
> in Jens Blauert's book "Spatial Hearing". ... Note that 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 Kaernbach
> Institut für Psychologie
> Karl-Franzens-Universität Graz
> 8010 Graz
> Austria
> www.kaernbach.de fechner.uni-graz.at