The influence of the stimulus level used to prescribe nonlinear frequency compression on speech perception.

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 nonlinear frequency compression could impact speech recognition.

Purpose: The purpose of this study was to determine the influence of the input level used to set nonlinear frequency compression on nonsense syllable recognition.

Research design: Nonsense syllable recognition was measured for three NFC fitting condition (i.e., with nonlinear frequency compression set based on speech input levels of 50-, 60-, and 70-dB SPL, respectively), as well as without nonlinear frequency compression (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 SNR) 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 nonlinear frequency compression 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.

Conclusions: Nonsense syllable recognition was highest when setting nonlinear frequency compression based on a 70 dB SPL presentation level and suggest that a high presentation level should be used to determine nonlinear frequency compression parameters for an individual patient.