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Nonlinear frequency compression is a signal processing technique used to increase the audibility of high frequency speech sounds for hearing aid users with sloping, high frequency hearing loss. However, excessive compression ratios may reduce spectral contrast between sounds and negatively impact speech perception. This is of particular concern in infants and young children, who may not be able to provide feedback about frequency compression settings. This study explores use of an objective cortical auditory evoked potential that is sensitive to changes in spectral contrast, the auditory change complex (ACC), in the verification of frequency compression parameters.
We recorded ACC responses in adult listeners to a spectral ripple contrast stimulus processed with a range of frequency compression ratios (1:1 to 4:1). Vowel identification, consonant identification, speech recognition in noise (QuickSIN), and behavioral ripple discrimination thresholds were also measured under identical frequency compression conditions. In Experiment 1, these measures were completed in ten adult normal hearing individuals to determine the effects of this type of signal processing in individuals with optimal hearing. In Experiment 2, these same measures were repeated in ten adults with sloping, high frequency hearing loss, which is the clinical population for whom this signal processing technique was intended.
No significant relationship of compression ratio and vowel identification was found in the normal hearing listeners, though a significant negative effect of increasing frequency compression ratio was observed in the hearing impaired group. Significant effects of compression ratio on ACC amplitude, consonant identification, ripple discrimination threshold, and speech perception in noise were found for both the normal hearing and hearing impaired groups.
These results indicate that the ACC response, like speech and non-speech perceptual measures, is sensitive to frequency compression ratio. Further investigation of this technique with hearing impaired individuals is needed to determine optimal stimulus and recording parameters for the ACC in both adults and children.