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A sound attenuator, or duct silencer, sound trap, or muffler, is a noise control acoustical treatment of Heating Ventilating and Air-Conditioning (HVAC) ductwork designed to reduce transmission of noise through the ductwork, either from equipment into occupied spaces in a building, or between occupied spaces. In its simplest form, a sound attenuator consists of a baffle within the ductwork. These baffles often contain sound-absorbing materials. The physical dimensions and baffle configuration of sound attenuators are selected to attenuate a specific range of frequencies. Unlike conventional internally-lined ductwork, which is only effective at attenuating mid- and high-frequency noise, sound attenuators can achieve broader band attenuation in relatively short lengths. Certain types of sound attenuators are essentially a Helmholtz resonator used as a passive noise-control device. Generally, sound attenuators consist of the following elements: An inner perforated layer of light gauge sheet metal (baffle) The baffle is then filled with sound-absorptive insulation In high velocity systems, or when there is a concern for particulate matter in the air stream, a bagged or mylar-faced insulation is used. Packless sound attenuators do not include sound-absorptive insulation. As a result, the high-frequency insertion loss of a packless sound trap is greatly reduced. Bagged insulation or packless sound attenuators are typically referred to as "hospital grade" attenuators. An outer non-perforated layer of sheet metal. The outer layer is typically heavy gauge sheet metal (18ga or stiffer) to minimize duct break-out and break-in noise. The gauge of circular sound attenuators is typically less of a consideration, as circular ductwork is considerably stiffer than rectangular ductwork and less prone to duct breakout noise. Sound attenuators are available in circular and rectangular form factors. Prefabricated rectangular sound attenuators typically come in 3, 5, 7, or 9-ft lengths.