A Study of Resonant-Cavity and Fiberglass-Filled Parallel Baffles as Duct Silencers.
NATIONAL AERONUATICS AND SPACE ADMINISTRATION MOFFETT FIELD CA AMES RESEARCH CENTER
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Acoustical performance and pressure drop were measured for two types of splitters designed to attenuate sound propagating in ducts-resonant-cavity baffles and fiberglass-filled baffles. Arrays of four baffles were evaluated in the 7- by 10-Foot Wind Tunnel Number 1 at Ames Research Center at flow speeds from 0 to 41 msec. The baffles were 2.1 m high, 305 to 406 mm thick, and 3.1 to 4.4 m long. Emphasis was on measurements of silencer insertion loss as affected by variations of such parameters as baffle length, baffle thickness, perforated skin geometry, cavity size and shape, cavity damping, wind speed, and acoustic field directivity. An analytical method for predicting silencer performance is described and compared with measurements. Unlike small, single-orifice resonators, the undamped, resonant-cavity baffles attenuated sound over a broad frequency range. With the addition of cavity damping in the form of 25-mm foam linings, the insertion loss above 250Hz of the resonant-cavity baffles was improved 2 to 7dB compared with the undamped baffles the loss became equal to or greater than the insertion loss of comparable size fiberglass baffles at frequencies above 250Hz. Variations of cavity size and shape showed that a series of cavities with triangular cross-sections i.e., variable depth were superior to cavities with rectangular cross sections i.e., constant depth. In wind, the undamped, resonant-cavity baffles generated loud cavity-resonance tones the tones could be eliminated by cavity damping. Duct-resonance tones were also generated by configurations that had solid skin over portions of the baffle surfaces. The effects of skin porosity, baffle length, and baffle thickness are documented. Author
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