Abstract
Background:
The benefits offered by noise reduction (NR) features on a hearing aid had been studied
traditionally using test conditions that set the hearing aids into a stable state
of performance. While adequate, this approach does not allow the differentiation of
two NR algorithms that differ in their timing characteristics (i.e., activation and
stabilization time).
Purpose:
The current study investigated a new method of measuring noise tolerance (Tracking
of Noise Tolerance [TNT]) as a means to differentiate hearing aid technologies. The
study determined the within-session and between-session reliability of the procedure.
The benefits provided by various hearing aid conditions (aided, two NR algorithms,
and a directional microphone algorithm) were measured using this procedure. Performance
on normal-hearing listeners was also measured for referencing.
Research Design:
A single-blinded, repeated-measures design was used.
Study Sample:
Thirteen experienced hearing aid wearers with a bilaterally symmetrical (≤10 dB) mild-to-moderate
sensorineural hearing loss participated in the study. In addition, seven normal-hearing
listeners were tested in the unaided condition.
Data Collection and Analysis:
Participants tracked the noise level that met the criterion of tolerable noise level
(TNL) in the presence of an 85 dB SPL continuous discourse passage. The test conditions
included an unaided condition and an aided condition with combinations of NR and microphone
modes within the UNIQUE hearing aid (omnidirectional microphone, no NR; omnidirectional
microphone, NR; directional microphone, no NR; and directional microphone, NR) and
the DREAM hearing aid (omnidirectional microphone, no NR; omnidirectional microphone,
NR). Each tracking trial lasted 2 min for each hearing aid condition. Normal-hearing
listeners tracked in the unaided condition only. Nine of the 13 hearing-impaired listeners
returned after 3 mo for retesting in the unaided and aided conditions with the UNIQUE
hearing aid. The individual TNL was estimated for each participant for all test conditions.
The TNT index was calculated as the difference between 85 dB SPL and the TNL.
Results:
The TNT index varied from 2.2 dB in the omnidirectional microphone, no NR condition
to −4.4 dB in the directional microphone, NR on condition. Normal-hearing listeners
reported a TNT index of −5.7 dB using this procedure. The averaged improvement in
TNT offered by the NR algorithm on the UNIQUE varied from 2.1 dB when used with a
directional microphone to 3.0 dB when used with the omnidirectional microphone. The
time course of the NR algorithm was different between the UNIQUE and the DREAM hearing
aids, with the UNIQUE reaching a stable TNL sooner than the DREAM. The averaged improvement
in TNT index from the UNIQUE directional microphone was 3.6 dB when NR was activated
and 4.4 dB when NR was deactivated. Together, directional microphone and NR resulted
in a total TNT improvement of 6.5 dB. The test–retest reliability of the procedure
was high, with an intrasession 95% confidence interval (CI) of 2.2 dB and an intersession
95% CI of 4.2 dB.
Conclusions:
The effect of the NR and directional microphone algorithms was measured to be 2–3
and 3.6–4.4 dB, respectively, using the TNT procedure. Because of its tracking property
and reliability, this procedure may hold promise in differentiating among some hearing
aid features that also differ in their time course of action.
Key Words
directional microphone - noise reduction - reliability - tolerable noise level - tracking
noise tolerance
