Simultaneous and forward masking of vowels and stop consonants: Effects of age, hearing loss, and spectral shaping.

Fluctuating noise, common in everyday environments, has the potential to mask acoustic cues important for speech recognition. This study examined the extent to which acoustic cues for perception of vowels and stop consonants differ in their susceptibility to simultaneous and forward masking. Younger normal-hearing, older normal-hearing, and older hearing-impaired adults identified initial and final consonants or vowels in noise-masked syllables that had been spectrally shaped. The amount of shaping was determined by subjects' audiometric thresholds. A second group of younger adults with normal hearing was tested with spectral shaping determined by the mean audiogram of the hearing-impaired group. Stimulus timing ensured that the final 10, 40, or 100 ms of the syllable occurred after the masker offset. Results demonstrated that participants benefited from short temporal delays between the noise and speech for vowel identification, but required longer delays for stop consonant identification. Older adults with normal and impaired hearing, with sufficient audibility, required longer delays to obtain performance equivalent to that of the younger adults. Overall, these results demonstrate that in forward masking conditions, younger listeners can successfully identify vowels during short temporal intervals (i.e., one unmasked pitch period), with longer durations required for consonants and for older adults.