Influence of Flow and High Sound Levels on the Attenuation in a Lined Duct

Abstract

The attenuation of the fundamental mode in a lined duct can be calculated as a function of the frequency and the impedance of the lining with a finite difference approximation of the differential equations. A recently developed formula is extended to the influence of flow with and without shear. Theoretical results are compared with measured data in a narrow duct of 1-in. square open area for a turbulent flow with Mach numbers −0.5 ⩽M⩽0.5. The influence of the flow on the wavelength of the sound in the duct, which decreases the attenuation for positive Mach numbers and increases the attenuation for negative Mach numbers, dominates the influence of the turbulent-flow profile, which has the opposite effect. The impedance of the duct lining, consisting of a fiber metal or a perforated plate in front of a honeycomb structure, varies at high sound-pressure levels, when the flow resistance of the fiber metal or the perforated plate becomes nonlinear. The nonlinear attenuation of sound with levels up to 160 dB is measured and compared with the biasing effect of a turbulent flow in the duct. [Research supported by Brunswick Corporation, Needham, Massachusetts.]