Effects of a human sound-based index on the soundscapes of urban open spaces

The ratio of the perceived extent of natural sounds to the perceived extent of traffic noise in the environment has been demonstrated to be important for soundscapes, whereas research on the influence of human sounds has been limited. To examine this influence, this study proposes a human sound-based index named the red soundscape index (RSI), which is defined as the ratio of the perceived extent of human sounds to the perceived extent of other sounds. Sound pressure levels and crowd density were collected at 41 sites in 9 urban parks, and pedestrian streets in Harbin, China, and the perceived extent of various sounds was investigated by a questionnaire survey. The results confirmed a significant positive correlation between crowd density and RSI, and the A-weighted sound pressure level increased linearly with increasing RSIn (the ratio of human sounds to natural sounds) and decreased with increasing RSIt (the ratio of human sounds to traffic noises). Interestingly, the overall soundscape assessment linearly decreases with the increase in RSIn in the range of (0.8–1.5). The relationship with RSIt first shows an increase and then a decrease in a parabolic form, in which the axis of symmetry is RSIt = 2. Correspondingly, urban open spaces can be divided into three categories based on the variation trend, and different types have significant differences in overall soundscape assessment, pleasantness, and calmness. Among these, pleasantness is the highest in the sites of natural sound predominance perception. At the same time, this factor becomes the lowest in the sites of human sound predominance perception and middle in the site of balanced perception. Consequently, RSI is expected to be useful in soundscape prediction in urban open spaces.