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Hean Movements and Sound Source Segregation

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,

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