Ingrid Noordhoek: thesis announcement for the auditory list

[Dan Ellis <dpwe@xxxxxxxxxxxxxxxxx>]

Dear List -

I was sent the enclosed announcement of a new thesis to circulate on
the list.

Best,

-- DAn Ellis
   Auditory List Administrator

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From: "Noordhoek, Ingrid" <Noordhoek@TPD.TNO.NL>
Subject: thesis announcement for the auditory list
Date: Tue, 8 Aug 2000 11:44:41 +0200

[...]

Recently, I have finished my thesis, entitled "Intelligibility of
narrow-band speech and its relation to auditory functions in
hearing-impaired listeners". I would like to inform the aditory list
that my thesis may be downloaded from my web site at

http://www.casema.net/~astuijt/ingrid/graduation.html

I am also pleased to mail paper copies to those who are interested as
long as they are available. Below you will find the abstract of my
dissertation.

Sincerely,
Ingrid Noordhoek.

TNO Institute of Applied Physics
www.tpd.tno.nl
Acoustics Division
Phone +31 15 2692412
Fax   +31 15 2692111
email: noordhoek@tpd.tno.nl


Listeners with sensorineural hearing losses often experience difficulties in
understanding speech, even when the speech is presented well above their
hearing thresholds. In this thesis, the origin of these difficulties was
investigated by examining the relations between speech intelligibility and
basic properties of the auditory system. To enhance the chances of obtaining
clear correlations, the investigations were all performed in a limited
frequency region around 1 kHz.
A novel intelligibility test (the SRBT test) to measure intelligibility of
speech bandpass filtered with a fixed center frequency of 1 kHz was
developed in chapter 2. In this test, the minimum speech bandwidth required
for a 50% intelligibility score is determined (speech-reception bandwidth
threshold or SRBT). The narrowband speech is presented in complementary
bandstop-filtered noise to ensure that the speech is only audible within the
desired frequency band. The bandwidth of the speech signal is varied in an
adaptive up-down procedure using a step size of a factor of 1.37 for the
bandwidth (in Hz). On average, the SRBT of normal-hearing listeners is 1.4
octave (600-1600 Hz) under optimal conditions, i.e., when the entire speech
dynamic range is above the hearing threshold, but not so loud that
audibility is affected by excessive upward spread of masking.
In chapters 3 and 4, the performance of, in total, 22 normal-hearing
listeners and 53 hearing-impaired listeners was measured using the SRBT
test, as well as using more common broadband SRT (speech-reception
threshold) tests, namely the SRT in quiet and the SRT in noise. For most
hearing-impaired listeners, the scores on these intelligibility tests
deviated from those of the normal-hearing listeners. The speech
intelligibility index (SII) model was applied to separate the origin of an
elevated SRT or a broader-than-normal SRBT into (1) reduced audibility, and
(2) suprathreshold deficits. Reduced audibility includes all those factors
that would affect audibility in normal-hearing listeners under the same
conditions when the hearing threshold of the hearing-impaired listener is
simulated by presenting a masking noise. The effect of suprathreshold
deficits on speech perception is quantified by the additional amount of
speech information needed for an intelligibility score of 50% (as compared
to normal-hearing listeners operating in threshold-simulating noise). For
short sentences, normal-hearing listeners require an SII of about 0.3 for a
50% intelligibility score. This means that normal-hearing listeners need 30%
of the speech information to understand 50% of the sentences correctly. When
the SII value that a hearing-impaired listener requires for a 50%
intelligibility score is normal, it is assumed that a possible deviant SRT
or SRBT is due only to inaudibility of a part of the speech spectrum. On the
other hand, a higher-than-normal SII value indicates that speech
intelligibility was affected by suprathreshold deficits.
The results of chapters 3 and 4 show that the presence and size of the
effects of suprathreshold deficits on speech perception depend on the type
of intelligibility test. The SRT test in quiet shows the smallest
sensitivity to suprathreshold deficits in speech perception, while the SRBT
test shows the largest sensitivity. Only a weak relation is observed between
suprathreshold deficits and hearing loss. This shows that it is not possible
to predict whether an individual listener suffers from a suprathreshold
deficit from only a consideration of the size of the sensorineural hearing
loss.
In chapter 4, the relations between suprathreshold speech-perception
deficits and basic auditory functions were investigated. The
auditory-function tests included detection efficiency, temporal resolution
(i.e., forward and backward masking), spectral resolution (i.e., upward and
downward spread of masking), temporal and spectral integration, and
discrimination of intensity, frequency, rhythm, and spectro-temporal shape.
All auditory functions were measured at or around 1 kHz. Because several of
these auditory functions were correlated, the thresholds on the
auditory-function tests were subjected to a principal-components analysis.
This resulted in three uncorrelated "auditory factors" (i.e., linear
combinations of the auditory functions). The first factor is related to
temporal resolution and frequency discrimination, the second factor is
associated with spectral resolution, and the third factor is associated with
detection efficiency, and temporal and spectral integration.
Multiple regression was used to predict suprathreshold speech perception
from the auditory factors for each of the intelligibility tests. The total
variance accounted for by the auditory factors was largest for the results
on the SRBT test (62%). This was in accordance with our expectations,
because all auditory functions were measured around 1 kHz which is also the
center frequency used in the SRBT test. The first and second auditory
factors were most closely related to suprathreshold deficits in speech
perception. This leads to the main conclusion of this thesis: the
suprathreshold deficits that affect speech perception are (1) reduced
temporal resolution, (2) reduced frequency discrimination, and (3) reduced
spectral resolution.


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