Aided growth of masking for speech and nonspeech signals

Growth of masking (GOM) functions for speech and nonspeech signals were obtained in normal and impaired ears, with an emphasis on aided listening conditions. The purpose of the investigation was to determine whether electroacoustic contrasts between hearing aid circuits would manifest themselves in aided GOM functions and to determine whether amplification alters the slopes of GOM functions in impaired ears.

Five normal-hearing and 10 hearing-impaired listeners with sloping high-frequency losses participated. GOM functions were obtained in the sound field using a two-interval forced choice forward masking paradigm. Four sets of test signals consisted of narrowband noise maskers and sinusoidal probes. Two additional stimulus sets consisted of phonemes used as masker and probe. Both on-frequency and off-frequency masking conditions were examined. Test circuits included wide dynamic range compression (low compression threshold and low compression ratio) and output compression limiting (high compression threshold and high compression ratio), each adjusted to the DSL[i/o] prescriptive target. GOM slopes were calculated for normal-hearing and hearing-impaired subjects unaided, and for impaired subjects aided, in an effort to determined whether amplification compensates for abnormal growth of masking and whether electroacoustic contrasts between circuits differentially affect level-dependent masker effectiveness.

Results indicated large differences between normal and impaired functions unaided. Under aided conditions, narrowband noise masked thresholds were reduced re: unaided at lower masker levels but not at higher masker levels. Masked thresholds for phoneme maskers were reduced at all masker levels. In general, aided GOM slopes changed in the direction of normal but did not match normal slopes. Aided results were highly dependent on stimulus test conditions and were consistent with the compression thresholds and release times of the aids under test. Circuit contrasts were more apparent when phonemes were used as masker and probe than when conventional signals were used.

Results suggest that GOM functions can be altered by amplification in ways that are consistent with the electroacoustic characteristics of the circuits being used. Although these data address only a small number of conditions, they suggest a possible application for aided growth of masking in the evaluation of hearing aid performance.