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

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bands. I ran a series of curves with my 2
channel compressor using ratios of 2:1
and 3:1 showing that at low levels it had
a contradiction between level dependent
frequency response and frequency
dependent amplitude response, but that
they are indeed the same. On the other
hand, a level dependent frequency
response can be achieved in another way
which Mead Killion is currently working
on using a single channel compressor
which he feels will be useful for mild to
moderate deficits. Mead feels that a 2
channel approach is not needed for these
more mild losses.
Question: The current calibration
method for evaluating hearing aids
uses a swept signal across the
frequency range, but this would not be
useful for dual channel systems.
H. Levitt: What is needed is a new
standard which would specify the
calibration and evaluation of these level
dependent hearing aids. There are several
methods being proposed. One method is
to use a broadband signal and then
analyze the resultant spectrum. The
other is to have at least two tones – one
to get the compressor working and the
other to sweep across the frequency
range. (Editor’s note: A problem can
occur if the first tone to get the
compressor working is too low in
frequency [e.g., 300 Hz], then difference
tones can erroneously enter the
frequency response).
E. Villchur: The most common is to use
a family of curves – each one at a
successively higher level. Instead of using
one frequency response curve you will
use a series, perhaps spaced every 10 dB
from 50 to 80 dB input.
Comment: You can’t characterize a nonlinear
system with a swept sinusold and
there is a proposal before the ANSI
committee which will allow you to use a
complex stimulus and to define the
characteristics of that spectrum as speech
spectrum noise. The analysis means will
be a swept spectral analysis at the output
or a digital technique. There is some
likelihood that that standard will come
through in the next couple of years.
Question: In the cochlear implants
mentioned today, what are the factors
that limit the frequency of
stimulation
H. Levitt: You first have to characterize
the implant. There is a single channel
implant with a single electrode, and there
are two multi-electrode cochlear
implants. One multi-electrode system is
like a vocoder where you have several
contiguous frequency bands and each
band drives a pair of electrodes. The
second type has an array of 22 electrodes
and each one electrode plus a round
electrode is stimulated. You don’t have
much frequency resolution with the
single channel cochlear implant, and that
implant is on the way out. The multichannel
cochlear implant is the one that
allows for coding of frequency
information in various ways. There are
two essential types of implants. One type
of cochlear implant is a multi-channel
system where there is a correspondence
to different frequency bands. The design
considerations are what frequency range
bandwidths are required to encode
speech. The other type of implant is
where you extract the features of speech
such as the voice fundamentals and
encode that. This stimulates the
electrodes in the cochlea. The question is
which characteristics of speech ought to
be encoded.
Question: For the benefit of those who
do not fit hearing aids, could you
comment on the relative effectiveness
of hearing aids, including some of
those discussed here.
E. Villchur: Neither Dr. Levitt nor I are
clinicians. The models discussed here
have not as yet been implemented in
commercial hearing aids. 3M is just
coming out. The Resound aid will be out
in the spring of 1989, but I’m not quite
sure.
H. Levitt: Regarding the Zeta Noise
Blocker and similar hearing aids, there is
generally not good clinical follow-up, so
we only have information on those who
are dissatisfied and that is not the best
way to measure the degree of satisfaction.
However, even using that crude measure,
and by published return rates, there have
been a fair amount of returns of the Zeta
Nose Blocker. We should have more
formal success/failure information on
these systems.
E. Villchur: I would like to say a word
about the ASP system. One of the two
that Dr. Levitt described used a
compressor in the low channel to reduce
the noise and nothing in the high
frequency channel. The compressor will
reduce the noise only if the compression
threshold is engaged by noise which is
intense enough, so this would imply that
it is not a compressor, but actually a
compressor-limiter. With real
compressors, weak sounds are not
reduced but are increased in gain. Once
you look at a compressor as increasing
weak sounds out of the mud, and once
you look at compressor-limiters as
decreasing overly intense sounds, it
becomes important to point out that I
have been talking about compressors and
not compressor-limiters. That is, a
compressor increases gain, not decreases
it. So the Zeta Noise Blocker is more of a
compressor limiter.
Canadian Hearing Report 2012;8(3):46-50.
50 CANADIAN HEARING REPORT | REVUE CANADIENNE D’AUDITION