Volume 8 Issue 3 (pdf) - Andrew John Publishing Inc

| paneL disCussion froM the 4th seMinars on audition
Signal Processing Techniques in Hearing Aids
Fourth Annual Seminars on Audition
February 25, 1989 (Toronto)
Co-ordinator: Marshall Chasin, AuD., Reg.
CASLPO (far left)
Speaker: Harry Levitt, PhD, City University of
New York (middle)
Speaker: Edgar Villchur, MS Ed., Foundation for
Hearing Aid Research (left)
Question: Could you please give some
information on the redundancy of
speech
E. Villchur: The consonants are
identified not only by their spectral
makeup, but also by their temporal
pattern. A [t] starts out with a sharp jump
in amplitude and tapers off. Also, the
consonant is affected by the vowel
environment – it is preceded or followed
by one sound or another. If interference
destroys on or two of these cues, the third
one may be enough to identify it. One of
the cues that allows us to understand
speech is the context or meaning of the
speech. If I say “I fell out of the boak,” we
are going to change that [k] to a [t],
because it doesn’t make sense otherwise.
But if I also miss the [b] or the [o], I won’t
have the additional cue.
H. Levitt: Another example of
redundancy is to stress a syllable. In the
word “confuse” – we change the stress
pattern and the meaning is changed.
There are cues that are correlated with
stress, such as the lengthening of the
stressed syllable, the intensity of the
voiced syllable, and the increasing of the
voice pitch of the stressed syllable. All of
these cues depend on the stress, and that
is a redundant situation. If only one of
those cues is heard, such as may be the
case with a hearing impaired person,
then the redundancy is reduced so that
the meaning may not be apparent.
Question: What are your experiences
with frequency displacing hearing aids
which transpose the high frequencies
and impose them on the lower
E. Villchur: Work by Johanssen, in
Sweden, has tried to do this, and indeed
they came out with a commercial
product (under the name of Oticon in the
1970s). There was a modification of this
which was published in an IEEE journal
within the last decade, where instead of
folding the entire high frequency band
onto the low frequency band where they
feared interference effects, he only folded
the energy above 5000 Hz back down, in
effect only affecting the fricatives. I don’t
know of any application of this in any
hearing aid.
H. Levitt: There have been a number of
experimental devices along these lines,
but I’m not familiar with any one of them
which has reached the marketplace other
than the Johanssen device.
E. Villchur: One problem with these
devices is that you have to learn a new
language. You have to learn to recognize
new sounds. The thing I liked about the
synthetic fricatives, which followed
surrogate fricatives (Levitt), is that you
don’t have to learn a new language.
H. Levitt: These transposition devices
can be broken up into three groups (1)
which transposes everything from the
high frequencies to lower ones, (which
have not been particularly successful), (2)
the phonetic transposition devices which
first decides whether it’s a fricative or
another sound, and only that sound is
transposed down, (and that reduces the
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