Sustainable Building Technical Manual - Etn-presco.net

White Noise❑ Consider a white noise or sound-masking system to maintain a constant level ofspeech privacy.If office spaces are very quiet (less than NC-35 to 40 in prime speech frequencies), conversationsare readily audible to adjacent occupants, especially in open-plan offices.This, in turn, reduces concentration and interferes with productivity. A white noisesystem ensures a constant background noise level to maintain speech privacy. In thesesystems an evenly distributed array of speakers concealed above the ceiling artificiallyraises the background noise level in the space. The sound is unobtrusive to occupantsand is similar to that of an HVAC system. Central white sound systems for large officesincorporate amplifiers and equalizers that can adjust the spectrum shape of sound andintensity levels to best suit the objectives. Smaller systems use individual speaker canswith inboard amplification and equalization facilities. White noise systems are typicallydesigned to provide an even background noise in the range of NC-38 to 42, dependingupon whether a cellular or open-plan office arrangement is used.Active Noise Control❑ Consider using newly developing active noise-control systems.Active noise-control systems are currently being developed for ducted HVAC systems,but to date their use is limited. Active noise cancellation (ANC), used in today’s systems,employs a series of microphones to detect the noise occurring in the airstream ofan HVAC duct. A speaker creates an identical noise sound field 180 degrees out ofphase from the original sound waves processed in the controller. The result is a soundfield reduced (not actually canceled) through the interaction of primary and activelycontrolled secondary sound sources. Currently, economic considerations limit theapplication of ANC to frequencies of 500 Hz and lower because passive noise methods(such as sound attenuators and acoustical duct lining) are effective and more cost-efficientat higher frequencies.I➜ R E S O U R C E SIAmerican Society for Heating, Refrigerating, and Air Conditioning Engineers. “Soundand Vibration,” Chapter 7 in 1993 Handbook: Fundamentals. Atlanta: ASHRAE, 1993.———. “Sound and Vibration Control,” Chapter 42 in 1991 Handbook: HVACApplications. Atlanta: ASHRAE, 1991.Fazio, Paula C., ed. Thermal Insulation: Environmental Acoustics, vol. 04-06 of Annual Bookof American Society for Testing and Materials (ASTM) Standards. Philadelphia: ASTM,1 9 9 2 .Harris, Cyril M. Handbook of Acoustical Measurements and Noise Control. New York:McGraw-Hill, 1991..N OT E SI1 American Society for Heating, Refrigerating, and Air-Conditioning Engineers, “Sound and Vibration,” chap. 7in 1993 ASHRAE Handbook: Fundamentals (Atlanta: ASHRAE, 1993), 7.9.2 ASHRAE, ASHRAE Handbook, 7.9.3 Cyril M. Harris, Handbook of Acoustical Measurements and Noise Control (New York: McGraw-Hill, 1991), 2.15,31.3.4 Harris, Handbook, 31.15.