Background: Frequency transposition has gained renewed interest in recent years. This type of
processing takes sounds in the unaidable high-frequency region and moves them to the
lower frequency region. One concern is that the transposed sounds mask or distort
the original low-frequency sounds and lead to a poorer performance. On the other hand,
experience with transposition may allow the listeners to relearn the new auditory
percepts and benefit from transposition.
Purpose: The current study was designed to examine the effect of linear frequency transposition
on consonant identification in quiet (50 dB SPL and 68 dB SPL) and in noise at three
intervals—the initial fit, after one month of use (along with auditory training),
and a further one month of use (without directed training) of transposition.
Research Design: A single-blind, factorial repeated-measures design was used to study the effect of
test conditions (three) and hearing aid setting/time interval (four) on consonant
identification.
Study Sample: Eight adults with a severe-to-profound high-frequency sensorineural hearing loss
participated.
Intervention: Participants were fit with the Widex m4-m behind-the-ear hearing aids binaurally
in the frequency transposition mode, and their speech scores were measured initially.
They wore the hearing aids home for one month and were instructed to complete a self-paced
“bottom-up” training regimen. They returned after the training, and their speech performance
was measured. They wore the hearing aids home for another month, but they were not
instructed to complete any auditory training. Their speech performance was again measured
at the end of the two-month trial.
Data Collection and Analysis: Consonant performance was measured with a nonsense syllable test (ORCA-NST) that
was developed at this facility (Office of Research in Clinical Amplification [Widex]).
The test conditions included testing in quiet at 50 dB SPL and 68 dB SPL, and at 68
dB SPL in noise (SNR [signal-to-noise ratio] = +5). The hearing aid conditions included
no transposition at initial fit (V1), transposition at initial fit (V2), transposition
at one month post-fit (V3), and transposition at 2 months post-fit (V4). Identification
scores were analyzed for each individual phoneme and phonemic class. Repeated-measures
ANOVA were conducted using SPSS software to examine significant differences.
Results: For all test conditions (50 dB SPL in quiet, 68 dB SPL in quiet, and 68 dB SPL in
noise), a statistically significant difference (p < 0.05 level) was reached between
the transposition condition measured at two months postfitting and the initial fitting
(with and without transposition) for fricatives only. The difference between transposition
and the no-transposition conditions at the 50 dB SPL condition was also significant
for the initial and one-month intervals. Analysis of individual phonemes showed a
decrease in the number of confusions and an increase in the number of correct identification
over time.
Conclusions: Linear frequency transposition improved fricative identification over time. Proper
candidate selection with appropriate training is necessary to fully realize the potential
benefit of this type of processing.
Key Words
Linear frequency transposition - nonsense syllable test - training effect
