Green roofs can provide significant noise reduction. A Belgian study tested the effects of green roofs in mitigating undesired noise. Singly diffracted sound waves were reduced by more than 10 dB, and doubly diffracted sound waves by up to 10 dB over a large frequency range.
Van Renterghem, Timothy, and Dick Botteldooren. (2011), In-situ measurements of sound propagating over extensive green roofs. Building and Environment 46(3), 729-738.