PHOTO F. Noise-control pergola. Photo courtesy of Kinetics Noise Control Inc. PHOTO G. Noise-control stack. Photo courtesy of Kinetics Noise Control Inc. PHOTO H. Rigid perforated sound absorbers. Photo courtesy of Kinetics Noise Control Inc. PHOTO I. Quilted sound absorbers. Photo courtesy of Kinetics Noise Control Inc. barriers and enclosures. With densities far from what is considered " critical, " these systems are transparent to noise; as a result, noise propagates through them. Noise-control pergolas/stacks. A noisecontrol pergola (Photo F) or stack (Photo G) is designed to perform like a silencer, offering noise reduction and very low pressure loss. A common application is an aircooled chiller located in the alcove of a high-rise offi ce building with noise propagating upward and breaking into windows or encroaching upon balconies. In such cases, a noise-control pergola can be used in conjunction with a double-walled barrier with fi xed-blade-acoustic-louver 32 2023 AMCA inmot ion skirt. When a concrete equipment-yard wall already is installed, a noise-control pergola can be used in combination with rigid or quilted absorbers to control refl ective and direct-line-of-sight mechanical-equipment noise. Sound absorbers. Often, equipment yards are constructed of cinder block or precast concrete. These hard surfaces create a reverberant sound fi eld that magnifi es and redirects noise. Sound absorbers soften wall surfaces and reduce adverse reverberant effect. Panels can be rigid (Photo H) or quilted (Photo I). Another consideration is the design of the structural support for a noise-control pergola or stack. Mechanical www.amca.orghttp://www.amca.org