An EEG test of subjects' brains showed that they perceived recorded sounds as being less noisy than they actually were while watching a video of vegetation but not while watching a video of a busy road. This study at Zhejiang Forestry University in China also included a survey, in which 90% of subjects believed that plants contribute to noise reduction and 55% overrated the plants’ actual ability to attenuate noise.
Yang, Fan, Zhi Yi Bao, and Zhu Jun Zhu. (2011). An assessment of psychological noise reduction by landscape plants. International journal of environmental research and public health, 8(4), 1032-1048.