The perception of noise in vowels and vowel-like stimuli

Breathiness is a perceptual characteristic of voice that is associated with the presence of aspiration noise in the speech signal. Previous studies investigated how changes in the noise-to- harmonics ratio (NHR) and in the slope of the glottal spectrum affected noise discrimination. However, the effects of spectral properties (such as formant frequencies and fundamental frequency, fo) on breathiness have remained largely unexplored. The first study of this thesis (Chapter 2) investigated noise discrimination in synthesized vowels as a function of vowel spectrum (/æ/ and /i/ for different speakers) while the amount of aspiration noise and the glottal spectrum’s slope was fixed. The fo was allowed to covary. Findings indicated that both vowel spectrum and fo affected noise discrimination. Additional studies of this thesis investigated noise perception using harmonic series (maskers) with equal-amplitude components so that the effects of spectral factors could be manipulated. The effect of masker level and fo on noise perception was investigated using band-limited maskers in three non-overlapping frequency bands. These bands were set such that listeners predominantly relied on spectral cues (resolved frequency components), temporal cues (unresolved components), or a combination of both (resolved and unresolved components). Experiments with wideband maskers were also performed. The effects of frequency band, level and fo were investigated using noise detection Chapter 3 and noise discrimination tasks (Chapter 4 and Chapter 5). The results showed that fo affected noise detection and discrimination in bands that contained spectral cues, but not temporal cues. Masker level had a large effect on the detection thresholds when temporal cues were used, but the effect of masker level on discrimination thresholds were inconclusive. Detection thresholds for different frequency bands depended on fo and masker level, but discrimination performance was consistently poor in the highest frequency band where temporal cues were used for a given NHR. In conclusion, the effect of fo on noise cues depended on the auditory processing mechanism (spectral vs temporal), and whether the noise was near threshold or suprathreshold. Level differences had a co-modulating effect on the results that were explained by way of the non- linear amplification of the basilar membrane.Medicine, Faculty ofAudiology and Speech Sciences, School ofGraduat