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

Abstract

The influence of turbulent flow on the sound attenuation in a narrow rectangular duct is calculated by a finite difference approximation of the differential equation for inviscid flow. The absorptive lining at one duct wall is characterized by a flow-independent wall impedance. Results are found in good agreement with experimental data from a duct with 1-in.2 open area lined on one side with a resistive layer of very fine metal fibers in front of a partitioned air backing. For resistive layers of fibers with larger diameter or of perforated plate used in the same configuration, the wall impedance depends greatly upon the flow velocity. A correlation is found between the nonlinearity of resistive layers due to turbulent flow and due to high sound-pressure levels in a duct. The influence of the nonlinearity due to high sound-pressure levels up to 160 dB is studied in the absence of flow. Measured attenuation data agree with those from a theoretical model, which correlates the varying attenuation along the duct with the varying sound-pressure difference across the resistive lining. By comparison of theoretical and experimental results, the influence of nonlinearity due to turbulent flow can be described by an equivalent level of sound-pressure difference across the resistive lining. A biasing effect of flow on nonlinear duct lining material linearizes the attenuation along the duct, if that equivalent level due to flow exceeds the actual sound level.