strange effect in auditory localization

[Barbara Muller <muller@xxxxxxxxxxxxxx>]

Dear List,

I might first introduce myself shortly.
I am working as an assistant in the laboratory of cognitive psychology at
the University of Geneva. My research deals with auditory localization. I
finished my Master's Thesis on binaural localization in the horizontal plane
(pinnae and head movement effects) and I am actually beginning my Ph.D. on
monaural localization.

I found a strange result concerning frequency (I used pure tones, with a
frequency of either 500Hz or 4Khz) in binaural localization in the
horizontal plane. I used eight loundspeakers in a circle around the subject.
Results show that frequency is ONLY significant for the free localization
condition ("free pinnae" and head movements allowed) where low tones are
better localized than high tones.
Another effect concerns the sound sources placed at 45 degres in the front
of the subject. In this case, and is this case ONLY, localization with
pinnae is significantly better with low tones whereas without pinnae,
localization is significantly better with high tones. Blauert (1983) notes
that the localization threshold for pure tones depends on their frequency.
It increases with laterality of the sound source, and also with frequency.
The author also observes that when pinnae are eliminated, the subjects'
performances are better for high then for low tones. If our subjects are
allowed to use pinnae cues, they make more errors with high frequency tones
whereas without pinnae cues, subjects make more errors with the low tones.

Has someone found a similar result, or may someone have an idea why this
occurred?

Sincerely
Barbara Muller

*******************************************************************

Barbara Muller
Laboratory of Cognitive Psychology
University of Geneva
9, rte de Drize
1227 Geneva
SWITZERLAND

Phone (+41 22) 705 97 03
Fax (+41 22) 705 98 55
email: muller@fapse.unige.ch