Reduced frequency selectivity and the preservation of spectral contrast in noise

Reduced frequency selectivity associated with sensorineural hearing loss may pose particular problems for hearing-impaired listeners in noisy environments. In these situations, broader-than-normal auditory filters may affect the perception of speech by reducing the contrast between spectral peaks and valleys in at least two ways. First, the peaks and valleys in the internal representation of the speech spectrum become smeared, resulting in less precise frequency analysis. Second, there may be a reduction in the signal-to-noise ratio (S/N) at the output of each auditory filter. In order to examine the relationship between frequency selectivity and identification of speechlike stimuli in noise, hearing-impaired and normal-hearing listeners were trained to assign vowel labels to four harmonic complexes which differed in the frequency locations of four elevated ("peak") harmonics. Peak harmonics were chosen to approximate first- and second-formant frequencies in four English vowels. Listeners were then tested to determine the spectral contrast necessary between peak and background components in order to maintain identification accuracy in the presence of various levels of broadband noise. Results indicated that for these stimuli, normal-hearing listeners required about 1 dB of additional spectral contrast for every doubling of the intensity of noise. The required increase in spectral contrast was generally greater for listeners with broader-than-normal auditory filters at 2000 Hz. This finding suggests indirectly that in the internal representations of speech sounds embedded in noise, the signal-to-noise ratio for listeners with abnormal frequency selectivity is poorer than for listeners with normal frequency selectivity. A poorer-than-normal internal S/N may be one factor underlying the common observation that noise often is more degrading to speech understanding by hearing-impaired listeners than by normal-hearing listeners.