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This study explored the effects of sound loudness on human nervous system from four aspects: regional spontaneous brain activity, cognition performance, sympathovagal balance of the autonomic nervous system and subjective feelings. Functional magnetic resonance imaging and electrocardiogram were measured in 12 volunteered students under three noise conditions. Mental performance, fatigue, noise sensation, annoyance and subjective distraction were also investigated. Earplugs were found invalid to improve annoyance and distraction in a common office condition (45 dB(A)), although noisy feeling was alleviated. Possible under-stimulated effects were observed with enhanced stress according to functional magnetic resonance and heart rate variability results. Annoyance, distraction, heart rate and heart rate variability increased significantly in a louder noise condition (65 dB(A)). Correlation analysis shows that heart rate is significantly correlated with either noise sensation, annoyance or distraction while heart rate variability presents significant correlations only with annoyance. Mental performance of calculation and reaction is moderately correlated with annoyance, while no significant differences of performance were observed between the three noise conditions. Constant office work with noise exposure will cause changes in the limbic gyrus and adherent regions. Parahippocampal gyrus is the only region with significantly different brain activity in three noise conditions. [ABSTRACT FROM AUTHOR]