Green Roofs - Living Architecture Monitor - Fall 2008 - (Page 25)

SOUNDS LIKE? KEY ACOUSTICAL TERMS AS THEY RELATE TO GREEN ROOFS ATTENUATION Refers to reduction in sound level, measured in decibels. DECIBEL dB The unit of measurement for sound pressure level, sound intensity level or sound power level; decibel is the term used to indentify 10 times the logarithm of the ratio of the corresponding sound pressure, intensity or power to a reference pressure, intensity or power. The decibel can also describe the change in a quantity; then it is 10 times the logarithm of the ratio of the quantity after and before the change. In general a person can just recognize a one to two dB change in sound pressure level; a six to 10 dB change level is recognized as a 50 per cent change in loudness or “volume”. COINCIDENCE EFFECT The effect produced by an incident sound on a panel when the wave length of sound equals the wavelength of the bend- ing wave in the panel. At coincidence the panel’s transmission loss is greatly reduced at the coincidence frequency. DAMPING The dissipation of vibration by the energy-dissipative properties of a structure or material; damping can be added to a material or struc- ture by materials that are referred to as damping materials. DIFFUSE SOUND FIELD A sound field in which the sound arrives at a position (usually in a room) with equal intensity from all directions. FREQUENCY (ƒ): The number of cycles per second of the pressure fluctuations of a sound. The unit of frequency is Hertz (Hz). In music we perceive frequency in terms of pitch. TRANSMISSION LOSS TL : A decibel measure of the sound insulation of a partition, which generally varies with the frequency of sound. SOUND ABSORPTION The process of dissipating sound energy striking a surface or material by converting sound to heat. SOUND PRESSURE The amplitude of the fluctuating pressure of sound superimposed on the static air pressure. ignificant research has determined green roofs can reduce stormwater runoff and lower a building’s energy demand for cooling/heating through improved thermal performance. It is commonly thought green roofs also provide mitigation of unacceptable noise levels affecting the health, safety and well-being of urban populations though the acoustical benefits of green-roof technologies have not yet been fully investigated. Recent acoustics research (summarized here) reviews applicable literature as well as reporting new empirical findings on the transmission loss of green roofs. (Key acoustical terms used in this article are defined in the glossary, above.) The first question is, of course, how exactly do green roofs mitigate sound? Green roofs have the potential to provide excellent external and internal sound isolation due to their high mass, low stiffness and damping effect and, through absorption at the surface, to reduce noise pollution in the community from aircraft, elevated transit systems, industrial sites and noise build-up in urban areas. Current construction practices, driven in part by cost, aesthetics and sustainable building-rating programs such as leed®, have led to an increased use of lightweight metal roof assemblies and a decreased use of additional ceiling materials, such as drywall or tiles. The underside of the roof deck is commonly exposed as the finished ceiling. Ceiling materials and ceiling plenum spaces effectively reduce mid- and high-frequency sound transmission; thus, only low- S frequency sound transmitting through light-weight roofs is a problem. These research findings support the proposition that green roofs improve transmission loss throughout the full frequency range of concern in building design. Initial research was conducted at the bcit Centre for Architectural Ecology 1 in Vancouver, British Columbia. The Centre’s dedicated green-roof research facility has three roof sections separated by parapets; the two extensive green roof sections and one non-green reference roof section allow direct comparison of measurements. The roof sections have been fully instrumented to measure stormwater runoff characteristics and energy efficiency since 2003; at the time of the acoustical measurements both were planted with sedums: gr1 had 75 mm of substrate, gr2 had 150 mm of substrate. The roofs are nominally sloped (two per cent), and each is 33 m2 (approximately 108 square feet) in area. There is no additional ceiling in the research facility. A reverse, indoor-to-outdoor testing method was employed to measure sound transmission through the two green roofs and the reference roof. This avoided having to generate disturbing high sound signals outside the building, repeatedly, at every possible angle of sound incidence. Potential sound flanking paths through roof drains (which lead to internal meters) and the roof jack conduits (containing the thermal performance and weather station wiring) were eliminated with sand-filled bags and 12 mm steel plates. Through this method, the transmission loss is deduced from the measured transmitted acoustic intensity radiated to the outside of each roof, and from the measured sound pressure LIVING ARCHITECTURE MONITOR FALL 25

Table of Contents Feed for the Digital Edition of Green Roofs - Living Architecture Monitor - Fall 2008

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.