perilus v - Stockholms universitet

Inc identally, let us note that, if it ex ists, acoust ic
invar iance is a strange not ion since talkers can only
mon itor it through their senses and listeners can only
access it through the ir hear ing system. Why should sensory
and aud itory transduct ion be assumed to have a transfer
funct ion of one impos ing no transformat ion? Is it the case
that what people really mean when they talk about acoustic
invar iance is in fact Uauditory· invar iance? Let us look at
some psycho-acoust ic results.
IS PHONETIC INVARIANCE AUDITORY?
We ment ioned earl ier a perceptual result that offers a
rather cur ious parallel to Schulman's find ings. It is the
uTraunmCller effect· wh ich is a demonstrat ion of the
transforms requ ired to preserve the perceptual constancy of
vowel quality under changes in (i) vocal effort and (i i)
vocal tract size. It is also somewhat rem inscent of the
find ings on FO-F1 interrelat ionsh ips in soprano vowels
(Sundberg 197).
Effort and vocal tract var iat ions can be dramat ically
illustrated by synthetically modify ing a naturally spoken
IiI. When all formants and FO are sh ifted equally along a
Bark scale an IiI-l ike vowel is perce ived but the voice
changes from an adult's to a ch ild's. When both F1 and FO
are var ied in such a way that F1-FO is kept constant on a
Bark scale - and the upper formant complex is left unchanged
- an IiI-l ike vowel is perceived. Th is is remarkable in view
of the fact that F1 reaches a value more typical of a lowpitched
I /. One's impress ion is that the speaker rema ins
the same but that she ·shouts·.
Note the parallel between Schulman's and TraunmCller's
results. Are the find ings causally related? Do we expla in
the lack of formant compensat ion in loud speech in terms of
the TraunmCller effect? Or do we account for the vowel
qual ity results in terms of the ·Schulmanu effect?
Of importance for the present discuss ion is the fact
that behav ioral constanc ies have been demonstrated and that
they imply that at least in this case phonet iC invar iance
must be def ined at a level of auditory representat ion.
Let us return for a moment to the alleged invar iance of
the release spectra of stop consonants. Diana Krull
collected perceptual responses from Swed ish listeners to
burst fragments obtained from V1C:VZ words (Krull 1987). One
hundred test words were generated by constructing all
poss ible combinat ions of V1 or Vz = short Ii e a 0 ul with
C: · Ib: d: rd: g:/. Confus ion matr ices for the burst
st imul i demonstrate the drastic coart iculat ion effects. By
and large, listener responses can be accounted for in terms
of the acoust ic propert ies of the st imul i. Th is is shown in
her attempts to predict the confus ions from aud itor ily based
·perceptual distance- computat ions.
A related study has been carr ied out by Lacerda (1986).
W. can characterize one part of his research as var ia tions
on the theme struck by Flanagan in his .arly -difference
limen· exper iments on vowel formant frequenc ies (Flanagan
19). Lac.rda's quest ion was: How well can listeners
discr im inate four-formant st imuli that differ solely in
terms of the frequency of F2. His work perm its us to compare
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