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The Influence of Lombard Effect on Speech Recognition

The origin of Lombard effect dates back one hundred years. In 1911 Etienne Lombard discovered the psychological effect of speech produced in the presence of noise (Lombard, 1911). The Lombard effect is a phenomenon in which speakers increase their vocal levels in the presence of a loud background noise and make several vocal changes in order to improve intelligibility of the speech signal (Anglade & Junqua, 1990; Bond et al., 1989; Dreher & O'Neill, 1957; Egan, 1971; Junqua, 1996; Junqua & Anglade, 1990; Van Summers et al., 1988). In nowadays speech recognition applications appearance of Lombard effect can be expected in various domains, where spontaneous and conversational speech communication will take place in uncontrolled acoustic environments. Two main interpretations of the Lombard effect have been proposed. The first argues that the effect is a physiological audio-phonatory reflex (Lombard, 1911), the second that Lombard changes are motivated by compensation on the part of the speaker for decreased intelligibility (Lane & Tranel, 1971). Some authors have also argued that both mechanisms may contribute to the changes made by the speaker in noisy environments (Junqua, 1993). Detailed surveys of the literature on the Lombard effect phenomenon was made in (Lane & Tranel, 1971) and more recently in (Junqua, 1996). The conducted research showed that Lombard speech is different from normal speech in a number of ways. The main changes of characteristics of Lombard speech can be seen in increase in voice level, fundamental frequency and vowel duration, and a shift in formant center frequencies for F1 and F2 (Anglade & Junqua, 1990; Applebaum et al., 1996; Junqua, 1996; Junqua & Anglade, 1990). It was also reported in (Hanley & Steer, 1949) that speaking rate may be reduced when speech is produced in a noisy environment. A detailed acoustic and phonetic analysis of speech under different types of stress including the Lombard effect was carried out also in (Hansen, 1988). The studies showed that under the Lombard effect, duration of vowels increase while that of unvoiced stops and fricatives decrease. Also, spectral tilt decreases implying an increase in high-frequency components under the Lombard effect. An increase in pitch and first formant location also occurs in both cases. Also, energy migration from low and high frequency to the middle range for vowels, and movement from low to higher bands for unvoiced stops and fricatives was observed. In addition to the above, differences between male and female speakers was noted in (Junqua, 1993). Lombard changes are on the other hand greater in adults than in children and in spontaneous speech than in reading tasks (Amazi & Garber, 1982; Lane & Tranel, 1971).