Hearing Aid Distortion and Consonant Identification

Abstract

An experiment was conducted with 34 normal-hearing listeners to determine the effects of distortion (generated by a single hearing aid) on consonant identification in noise. Five experimental conditions were employed in which measured harmonic distortion ranged from approximately 1% (high fidelity) to 35%. Each listening condition involved playback of recorded test material at a constant sensation level. Results showed that average consonant identification scores, relative to the high-fidelity condition, decreased 15–29% as a function of increased distortion. Initial consonants were discriminated best across all distortion conditions, and progressive increases in distortion tended to most affect final consonants. The data also suggested possible talker/hearing aid interactions. Reduced high-frequency response and altered speech-to-noise ratio, together with harmonic distortion, were postulated as mutually inclusive causes of the observed decrements in consonant differentiation.

Predictors of Hearing-Aid Outcomes

“…The reason is uncertain. Perhaps excessive hearing-aid compression alone or combined with residual cochlear compression at high intensities (65–80 dB SPL) generated excessive distortion that degraded the temporal representation of speech (recall that RECE HI was a contributor to component PC6: temporal processing deficits) and hindered intelligibility ( Bode & Kasten, 1971 ; Boothroyd, Springer, Smith, & Schulman, 1988 ; Marriage, Moore, Stone, & Baer, 2005 ).…”

“…The REIG 50dB and REIG 65dB were included as predictors because they can affect audibility and reduced audibility decreases speech-in-noise intelligibility (e.g., Peters et al., 1998 ). The RECE LO and RECE HI were included as predictors because compression might distort the amplified speech presented to the hearing-aid user and distortion can reduce speech intelligibility ( Bode & Kasten, 1971 ). A second reason was that RECE LO and RECE HI may interact with residual cochlear compression (BMCE) or cochlear amplification loss, and the interaction may influence speech intelligibility.…”

“…Conversely, listeners with the same audiometric loss but greater inner hair cell loss would prefer more linear amplification because they have substantial residual cochlear compression in their ears. Significant residual cochlear compression combined with hearing-aid compression might cause excessive distortion and diminish intelligibility ( Bode & Kasten, 1971 ). If this were the case, both aided speech-in-noise intelligibility and self-reported improvement in hearing ability should be correlated with the loss of cochlear mechanical amplification alone or combined with the preferred hearing-aid settings.…”

The Causes and Effects of Distortion and Internal Noise in Hearing Aids

“…The author also stated that most types of noise, especially those with intense lowfrequency components, have a severely detrimental effect on intelligibility. Bode and Kasten (1971), studying 34 normalhearing listeners under conditions of varying distortion, showed that consonant identification decreased by 15% to 29% as distortion levels increased. The introduction of moderate levels of distortion alone did not significantly decrease intelligibility.…”