Fitting Frequency-Lowering Signal Processing Applying the American Academy of Audiology Pediatric Amplification Guideline: Updates and Protocols

Background: Although guidelines for fitting hearing aids for children are well developed and have strong
basis in evidence, specific protocols for fitting and verifying technologies can supplement such guidelines.
One such technology is frequency-lowering signal processing. Children require access to a broad bandwidth
of speech to detect and use all phonemes including female /s/. When access through conventional amplification
is not possible, the use of frequency-lowering signal processing may be considered as a means to
overcome limitations. Fitting and verification protocols are needed to better define candidacy determination
and options for assessing and fine tuning frequency-lowering signal processing for individuals.
Purpose: This work aims to (1) describe a set of calibrated phonemes that can be used to characterize
the variation in different brands of frequency-lowering processors in hearing aids and the verification with
these signals and (2) determine whether verification with these signal are predictive of perceptual
changes associated with changes in the strength of frequency-lowering signal processing. Finally, we
aimed to develop a fitting protocol for use in pediatric clinical practice.
Study Sample: Study 1 used a sample of six hearing aids spanning four types of frequency lowering algorithms
for an electroacoustic evaluation. Study 2 included 21 adultswho had hearing loss (mean age 66 yr).
Data Collection and Analysis: Simulated fricatives were designed to mimic the level and frequency
shape of female fricatives extracted from two sources of speech. These signals were used to verify the
frequency-lowering effects of four distinct types of frequency-lowering signal processors available in
commercial hearing aids, and verification measures were compared to extracted fricatives made in a
reference system. In a second study, the simulated fricatives were used within a probe microphone measurement
system to verify a wide range of frequency compression settings in a commercial hearing aid,
and 27 adult listeners were tested at each setting. The relation between the hearing aid verification measures
and the listener’s ability to detect and discriminate between fricatives was examined.
Results: Verification measures made with the simulated fricatives agreed to within 4 dB, on average, and
tended to mimic the frequency response shape of fricatives presented in a running speech context. Some
processors showed a greater aided response level for fricatives in running speech than fricatives presented
in isolation.Resultswith listeners indicated that verified settings that provided a positive sensation level of /s/
and that maximized the frequency difference between /s/ and /Ð / tended to have the best performance.