Abstract
Background Nonlinear frequency compression (NFC) is a signal processing technique designed to
lower high-frequency inaudible sounds for a listener to a lower frequency that is
audible. Because the maximum frequency that is audible to a listener with hearing
loss will vary with the input speech level, the input level used to set NFC could
impact speech recognition.
Purpose The purpose of this study was to determine the influence of the input level used
to set NFC on nonsense syllable recognition.
Research Design Nonsense syllable recognition was measured for three NFC fitting conditions—with
NFC set based on speech input levels of 50, 60, and 70 dB SPL, respectively, as well
as without NFC (restricted bandwidth condition).
Study Sample Twenty-three adults (ages 42–80 years old) with hearing loss.
Data Collection and Analysis Data were collected, monaurally, using a hearing aid simulator. The start frequency
and frequency compression ratios were set based on the SoundRecover Fitting Assistant.
Speech stimuli were 657 consonant–vowel–consonant nonwords presented at 50, 60, and
70 dB SPL and mixed with steady noise (6 dB signal-to-noise ratio) and scored based
on entire word, initial consonant, vowel, and final consonant. Linear mixed effects
examined the effects of NFC fitting condition, presentation level, and scoring method
on percent correct recognition. Additional predictor variables of start frequency
and frequency–compression ratio were examined.
Results Nonsense syllable recognition increased as presentation level increased. Nonsense
syllable recognition for all presentation levels was highest when NFC was set based
on the 70 dB SPL input level and decreased significantly when set based on the 60
and 50 dB SPL inputs. Relative to consonant recognition, there was a greater reduction
in vowel recognition. Nonsense syllable recognition between NFC fitting conditions
improved with increases in the start frequency, where higher start frequencies led
to better nonsense word recognition.
Conclusion Nonsense syllable recognition was highest when setting NFC based on a 70 dB SPL presentation
level and suggest that a high presentation level should be used to determine NFC parameters
for an individual patient.
Keywords
hearing aids - hearing loss - deafness - speech perception
