Psychophysiological effects of low-frequency sound mediated by room size, room shape, and stimulus duration

Music and sounds elicit a wide range of emotions and activate numerous psychological and physiological effects associated with the functioning of the autonomic nervous system (ANS) and involved in the maintenance of homeostasis. As such, sound interventions can play an important role in supporting human wellbeing and improve outcomes in medical patients. Psychophysiological responses are dependent on the stimulus type, frequency, duration, and spatial conditions of the acoustic environment. To arrive at a more articulate understanding of these dependencies, the effects of singing bowl sounds at 40 Hz, 73 Hz, and 110 Hz were investigated by monitoring physiological, behavioral and emotional responses in healthy adult subjects (n = 59). Singing bowl sounds were spatially projected in three virtual room sizes (small, medium, large) and eight virtual room shapes (pyramid, tetrahedron, cube, octahedron, dodecahedron, icosahedron, cuboctahedron, and sphere). Overall, exposure to low-frequency singing bowl sounds resulted in a significant increase in positive emotions and a significant decrease in negative emotions across all conditions. Contrary to other studies, we found no discrete effects related to the stimulus type or frequency. Significant differences in electroencephalography (EEG), heart rate variability (HRV) and electromyography (EMG) were consistently dependent on a combination of stimulus frequency and spatial condition, i.e. the resonance of a sound in a room with a particular size and shape. Significant differences in localized EEG power were strongly correlated to localized amplitude deviations of sound waves, while the phase shift of sound wave frequency was predictive of EEG frequency. Significant differences in HR and EMG were strongly correlated to the mean, variability and standard deviation of phase distortions of sound waves. Discrete effects of room shape were observed in galvanic skin response (GSR). Arousal was significantly decreased in a cube and cubeoctahedron, regardless of the frequency. Discrete effects of stimulus duration were observed in GSR and EMG. Arousal was significantly increased during the first 15 min of exposure and significantly decreased until 40 min, regardless of the frequency and spatial condition. We discuss the implications of these findings for future research and therapeutic practices.