Acoustic Isolation of Fans in Ducts

Abstract

A method of reducing the sound power radiated downstream from axial flow fans is described and investigated. The normal parallel duct fan housing is replaced by a contoured arrangement. With one type of contour and the use of porous absorbent material the reduction in the downstream overall sound power level is 5 dB, that is, the total sound power is reduced by a factor of three. The attenuation is effective in this case over the full audio frequency range but is largest at frequencies above about 1000 c/s. The total pressure losses in each of the several contoured fan housings investigated are practically identical with those in the usual parallel housing and the fan absorbs no significant additional power. Although commercially available duct noise reduction devices may give higher attenuations than those obtained by the contouring method they usually do so at the expense of quite high total pressure losses. For example, the fan used in this investigation gives a total pressure rise of about 1.4 in water gauge and one commercial type of duct noise attenuator, suitable for use with this fan, introduces a total pressure loss of 0.25 in water gauge. Two basic types of fan housing are investigated. One housing has contoured sections upstream and downstream of the fan rotor and the other is a composite arrangement with the normal parallel duct intake and a contoured downstream section. The effect of the presence of absorber on the performance of these basic types is determined. The acoustic performance of each type of fan housing duct is compared by the use of an ‘in duct’ method of sound pressure measurement. It is stressed that the design of low noise fan/duct systems is economically feasible provided sufficient effort is exerted to find a satisfactory compromise between the required aerodynamic and acoustic performances.