Comparison of growth of masking functions and speech discrimination abilities in younger and older adults.

This study examined the possibility that age-related differences in speech discrimination abilities may reflect individual differences in the amount of masking and in the rate of growth of on- and off-frequency masking. Young (mean age = 26 years) and older (mean age = 60 years) adult listeners were selected, all of whom had hearing thresholds equal to or better than 30 dB HL at audiometric frequencies < or = 2000 Hz and equal or better than 40 dB HL at audiometric frequencies from 3000 to 6000 Hz. Listeners were tested on a consonant identification task in which nonsense words were presented in quiet, high-pass-filtered, low-pass-filtered and in wide-band noise. Despite their good hearing thresholds, the older listeners made significantly more errors in all four test conditions. Masked thresholds and growth of masking functions were obtained for all listeners at signal frequencies of 750, 1000, and 1500 Hz, in the presence of a one-third-octave band of noise centred at 1000 Hz, with four noise levels from 50 to 80 dB SPL. The older listeners had higher masked thresholds overall, when compared to younger listeners, even though their audiometric thresholds at these frequencies were within normal limits. However, the slope of the growth of masking functions at and above the masker frequency did not differ with age. These results show that older listeners show reduced speech discrimination abilities both in difficult listening conditions and in quiet, even when their pure-tone thresholds are within normal limits. Moreover, these effects cannot be attributed to differences in masked thresholds or in the rate of growth of masking.

The relation between loudness and intensity difference limens for tones in quiet and noise backgrounds.

Recent studies of the relation between loudness and intensity difference limens (DLs) suggest that, if two tones of the same frequency are equally loud, they will have equal relative DLs [R. S. Schlauch and C.C. Wier, J. Speech Hear. Res. 30, 13-20 (1987); J.J. Zwislocki and H.N. Jordan, J. Acoust. Soc. Am. 79, 772-780 (1986)]. To test this hypothesis, loudness matches and intensity DLs for a 1000-Hz pure tone in quiet and in a 40-dB SPL spectrum level broadband noise were obtained for four subjects with normal hearing. The DLs were obtained in both gated- and continuous-pedestal conditions. Contrary to previous reports, equally loud tones do not yield equal relative DLs at several midintensities in the gated condition and at many intensities in the continuous condition. While the equal-loudness, equal-relative-DL hypothesis is not supported by the data, the relation between loudness and intensity discrimination appears to be well described by a model reported by Houtsma et al. [J. Acoust. Soc. Am. 68, 807-813 (1980)].

Locus of selective adaptation in speech perception.

Voiced (/ba/ or /da/) and voiceless (/pa/ or /ta/) consonants seem to affect different auditory system loci. On a voice-onset-time continuum (/ba/ to /pa/ or /da/ to /ta/) the selective adaptation effects produced by voiceless consonants are largely ear-independent and endure over delays of at least 1 min. However, voiced adapters produce selective adaptation effects that are highly ear-specific and relatively short-lived (less than 15 s). These differences suggest that specific cues to voiced and voiceless consonant sounds are processed by distinct auditory mechanisms and that these processes occur at different levels of the auditory system. One mechanism, which processes cues to voiced consonants, is peripheral and ear-specific. The second mechanism, which processes cues to voiceless consonants, is central and ear-independent.