Green Roofs - Living Architecture Monitor - Fall 2008 - (Page 27)

“The results of these tests [at BCIT] showed that up to half of the sound volume which transmitted through the research facilities roof was eliminated by the addition of the green-roof materials.” level in the interior of the research facility. An array of five loudspeakers was used in the interior of the research facility to create an approximately diffuse sound field. The space-averaged sound pressure level was measured below each of the three roofs. The radiated sound intensity was measured with an intensity probe at 12 discrete points on each of the three roofs. The Center’s field tests indicated an increase performance of the extensive green roofs of five to 13 dB in transmission loss over the low- and midfrequency range (50 to 2000 Hz), and a two to eight dB increase in transmission loss in the higher frequency range, relative to the transmission loss of the reference roof. Generally speaking, a six to 10 dB reduction in sound level is experienced as a 50 per cent reduction in loudness or “volume.” The results of these tests showed that up to half of the sound volume which transmitted through the research facilities roof was eliminated by the addition of the green-roof materials. As part of this research, existing research on the transmission loss of green roofs and non-green roofs was reviewed, highlighting two summarizing concepts: 1) The use of additional materials to mass load and add damping to the roof can potentially eliminate the coincidence effect and increase transmission loss at low frequencies 2, 3, 4; and 2) Increased TL was achieved by the addition of a ceiling; this addition to the roof assembly increased tl only in the mid-and high-frequency ranges, not in the low frequency range of potentially disturbing noise from ground traffic and aircraft 5, 6. It is suspected that transmission losses in the substrate layer of a green roof will be of major importance. Research findings suggest that the physical properties of soils affect the attenuation of sound as it passes through the depth of the soil 7. The most relevant physical properties affecting the acoustic characteristics across the range of in-situ soils include particle-size distribution, bulk density and porosity, flow resistivity and tortuosity, and the soil conditions of moisture and compaction. Our preliminary findings suggest that, within the range of substrates designed to support viable green-roof plants, the variations of physical properties will not significantly affect the acoustical characteristics of the overall extensive green-roof system. Low frequency sound waves transmit through roofs and walls easier than higher frequency sound waves. As you can see in Figure 1, all three roofs exhibited the typical increase in tl (increased sound attenuation) with increasing frequency. This means less high frequency sound transmits through the roof than low frequency sound. gr1 (75 mm substrate) and gr2 (150 mm substrate) exhibited increases in TL above that of the reference roof http://www.jakob-usa.com

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Green Roofs - Living Architecture Monitor - Fall 2008 Contents From the Founder: A Robust Economy Letters Strata - People, Products & Projects: Austin's Hotspot for Habitat & Baltimore Hilton Goes Green A Green Roof for a County Courthouse Research Grant for Design Tool On the Roof With... A New Vue on Downtown Open Space Sound Transmission Loss of Extensive Green Roofs On Target Optimizing for Sustainability First GRHC Green Roof Symposium in Florida Welcome New Corporate Members Professional Calendar Experts Reflect on the Value of the GRP

Green Roofs - Living Architecture Monitor - Fall 2008

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