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very much for all the responses!
quite alot of useful information sent to me as well as the list (actually,
a torrent might be a better description), so I thought I might summarise
and send to the list in an attempt to "give something back"... Hope this
is useful to someone and not repetition - I'm learning rapidly from a fairly
comments fell into three main categories:
had several people suggest (sensibly) that I just avoid the whole problem and
use speakers. I forgot to mention in my initial email that it's
imperitive that headphones be used as there will be space constraints,
otherwise this is exactly what I would do!
it was pointed out several times that it was the mismatch between a
"standard" HRTF and the real HRTF for an individuial that might result in poor
externalisation. I downloaded the CIPIC (Center for Image Processing and
Integrated Computing at the University of California) database which contains
HRTF data for 43 human subjects and two KEMAR dummies (large/small
pinnae). I did indeed find that (just me listening with headphones, no
flashing lights) some subjects' data worked better for me than others, with the
sense of "externalisation" or distance being the main thing that varied
(although the left/right alignment seemed to vary quite a bit as well). However,
as was pointed out, having the synchronised flashing lights would probably help
in this reguard (ventriloquist effect).
adding uncorrelated (left/right) reverberation to "standard" HRTF filtered sound
may help avoid having to measure HRTF for individuals, yet possibly help to
externalise sounds with only a minum effort.
person pointed out the Simon Effect, where responses are speeded on the
ipsilateral side of stimulation. This is a good point: the Simon Effect
can usually be defeated by appropriately mixing/randomising the design within
pointed out the impoprtance of preventing head movements, to reduce the
possibility of divergence between the auditory and visual display when
headphones are used.
that I need only an approximate solution for this application, what I'm going to
investigate is the following: I'll use the CIPIC database (probably one of
the KEMAR dummy sets) and investigate the judicious application of uncorrelated
reverb, remembering that I have the advatage of having synchronised flashing
lights to assist with the front/back confusions that might normally result from
using only an approximate HRTF filter. Hopefully that will give me enough
of a sense of the sound being outside the head while still using
headphones. ANd of course the head will be restrained (also eye-movements
monitored but thats another story), and the design appropriately mixed
once again for all the comments