Experimental investigation of the sound transmission of vegetated roofs

The purpose of this research was to determine the phenomenological parameters that impact the sound transmission of vegetated roofs. A reverse indoor-to-outdoor testing method involving propagating sound from an interior diffuse field to an exterior free field was first implemented at an existing field site. The findings from the field work supported the development of a purpose-built field laboratory specifically designed and commissioned as part of this research. Transmission loss measurements were completed on 2 non-vegetated reference panels, on gradients of substrate depth (25-mm increments), and on 2 plant communities established in 150 mm of substrate depth. Increased transmission loss, resulting from the installation of vegetated roof material layers on the reference roof, at the field site (wood frame construction) and at the field laboratory (light-weight metal) generally aligned in the low and mid frequency ranges. The increased transmission loss of the wood frame roof was 5–13 dB in the 50–2000 Hz frequency range, and up to 8 dB above 2000 Hz. For the light-weight metal deck, the increased transmission loss was up to 10 dB, 20 dB, and >20 dB in the low, mid, and high frequency ranges, respectively. Field mass law, using an effective mass to describe the composite roof deck, predicted the transmission loss of non-vegetated reference roofs. A gradient increase in substrate depth (equated in terms of mass) incrementally increased transmission loss, but not as predicted by mass law. A variation in the moisture content of the substrate did not translate to a measurable change in transmission loss. The deep roots of the coastal meadow community contributed to an increase in transmission loss relative to the shallow-rooted sedums community. The results of this research confirm that vegetated roofs increase transmission loss over non-vegetated roofs and have a beneficial application towards architectural situations requiring high transmission loss and specifically mitigation of low frequency noise.