A Study of Resonant-cavity and Fiberglass-filled Parallel Baffles as Duct Silencers
National Aeronautics and Space Administration, Scientific and Technical Information Branch, 1982 - Acoustical engineering - 68 pages
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 m/sec. 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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10-Foot Wind Tunnel 4.9 STRAIGHT 406 mm thick 8000 ONE-THIRD OCTAVE acoustic impedance ACOUSTIC SECTION acoustic source Ames Research Center baffle length BAND CENTER FREQUENCIES cavity damping cavity resonance cavity shapes CONFIG configuration cross modes diagonal septa duct axis duct blockage duct lining Duct resonances DUCT SILENCERS duct width equation exhaust simulation experimental fiberglass baffles fiberglass silencer fiberglass-filled baffles flow speed Hz Figure Hz third-octave bands i i i i i i impedance inlet simulation Kurze m/sec measured microphone midfrequencies Moffett Field Mylar NASA OCTAVE BAND CENTER ONE-THIRD OCTAVE BAND open area PARALLEL BAFFLES parameters perforated skin reactance rectangular cavities resonant-cavity baffles resonant-cavity silencer Scharton shown in figure silencer attenuation silencer insertion loss silencer performance skin porosity Soderman solid skin sound attenuation sound field sound level sound propagation tail test section test-section tion transverse mode triangular cavities upstream vortex shedding walls wind speed