Hean Movements and Sound Source Segregation

[Christian Kaernbach <auditory@xxxxxxxxxxxx>]

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