The purpose of this study was to compare the effects on speech acoustics of a wide variety of speaking instructions that have been used across different studies on clear speech, and to investigate the acoustic and articulatory changes that occur in response to these instructions and in different talking environments. Five young adult females were recorded speaking under different instructions meant to elicit more intelligible speech, and measures of speaking rate, speaking F0 and intensity were found to distinguish instructions to speak "as if to someone with hearing loss" from instructions to speak "clearly" or "slowly", which produced different results from instructions to speak "loudly" or as if in noise. Preliminary acoustic and articulatory data are described for a sixth talker who spoke under a subset of these instructions, and in both a quiet and a noisy talking environment.

The purpose of this study was to compare the effects on speech acoustics of a wide variety of speaking instructions that have been used across different studies on clear speech, and to investigate the acoustic and articulatory changes that occur in response to these instructions and in different talking environments. Five young adult females were recorded speaking under different instructions meant to elicit more intelligible speech, and measures of speaking rate, speaking F0 and intensity were found to distinguish instructions to speak "as if to someone with hearing loss" from instructions to speak "clearly" or "slowly", which produced different results from instructions to speak "loudly" or as if in noise. Preliminary acoustic and articulatory data are described for a sixth talker who spoke under a subset of these instructions, and in both a quiet and a noisy talking environment.

The change a speaker makes in response to background noise is known as the Lombard Effect (LE). This study investigated the acoustic changes that are undergone in the presence of broadband noise and two-talker babble. Of particular interest were vocal fundamental frequency (F0) and formant frequency vowel space measures across sex. Forty participants (20 male, 20 female) were recruited and asked to read phrases in quiet and in the presence of two-talker babble and broadband noise. These masker conditions were presented at 50 and 70 dB HL. The phrases were recorded and acoustically analysed. The results showed a significant sex difference for both F0 and vowel space. A masking condition effect was not displayed for either F0 or vowel space. A significant effect was however shown for F0 according to intensity level, suggesting a LE. While the sex difference in F0 values can be explained on the basis of differences in vocal anatomy, the sex difference in vowel space was indicative of a sociophonetic influence on speech production.

The use of personal hearing protection devices (HPDs) is often recommended to protect workers' hearing from noise-induced damage when no other means of reducing noise levels at the source is effective. The effects of HPDs on speech communication cannot be neglected in spite of their benefit in reducing the risk of hearing loss. While much research has been directed at speech perception, much less is known on how HPDs affect speech production. The tendency of talkers to raise their vocal effort in noise, known as the Lombard effect, is often disrupted by HPDs due to their occlusion effect and the lower noise at the ears as well as the attenuated feedback from one’s own voice. Three main knowledge gaps are addressed in this thesis. The first gap is to characterize speech produced by talkers with or without HPDs under realistic acoustic conditions while immersed in an external noise field. The second gap is to evaluate more comprehensively speech production under protected and unprotected talker and listener ear conditions in different types of fluctuating and continous noises. The third gap is to assess the alterations in the characteristics of speech produced by talkers wearing level-dependent HPDs set at different transmission gain settings and in comparison with passive HPDs. This thesis extends methods used to recover Lombard speech elicited in an external noise field. For this purpose, two noise suppression methods, direct waveform subtraction (DWS) and adaptive noise cancellation (ANC), were found to adequately remove noise from speech recorded for SNRs as low as −10 dB. Moreover, this work contributes new knowledge on the effects of conventional passive HPDs on speech production. When talker wears HPD in noise then speech level were found to decrease by up to 9 dB in continuous noises and by 7 dB in fluctuating noises compared to open ears, while speech levels were found to increase by about 5 dB in all noises when the listener wears HPD. Furthermore, changes in pitch and spectral levels were consistent with changes in speech levels. The effects of level-dependent HPD on speech production, depending on the chosen transmission gain setting, revealed that it led to smaller decrease in talkers’ speech levels in noise compared to conventional passive HPD. These findings indicate that the level-dependent HPDs may impede communication less than conventional passive HPDs, while providing protection against high levels of noise.

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The speech of individuals with Parkinson's disease (PD) is typically characterized as lacking in proper prosody because of its monopitch and monoloud quality, in addition to its reduced intensity. These qualities make it difficult for others to understand speakers with PD. The purpose of the current study was to identify what individuals with PD would do vocally, if anything at all, to improve speech production following a simulated misunderstanding of what they had just said. The study evaluated the performance of 5 individuals with PD and compared their performance to 5 age- and sex-matched controls. Specifically, measures of vocal intensity (loudness), fundamental frequency (pitch), and utterance duration were made for repetitions of a ‘misheard’ phrase. In one experimental condition noise was presented through headphones to induce the Lombard effect. Both individuals with PD and controls used increased duration as a means of enhancing clarity in a repetition. Fundamental frequency (F0) and sound pressure level (SPL) were not consistently modified in repetitions for clarification. Under most speaking conditions, individuals with PD and controls had similar F0 and SPL. Individuals with PD, like the controls, responded to the presentation of masking noise by increasing their fundamental frequency and their intensity. Therefore, not all individuals with PD exhibit difficulty using prosody.