An improved model for parallel-element liner impedance prediction

Abstract

Improved acoustic liner impedance prediction models and measurement methods are needed to achieve more efficient duct treatments for acoustic noise suppression. A parallel-element model has been developed to predict normal-incidence surface impedances for a variety of locally reacting liner configurations. In addition, a 2-D finite element method, which uses grazing incidence acoustic pressure measurements to educe the acoustic impedance of a test liner, has been developed. This method has been used for grazing incidence impedance measurements in non-progressive wave fields in the presence of mean flow. The parallel-element model was previously demonstrated to provide accurate comparisons to data measured in a normal incidence impedance tube for tubular materials. This paper documents the ability of this parallel-element model to predict normal incidence acoustic impedances of liner configurations which contain high aspect ratio channels. Results are provided for two types of acoustic liners: one ceramic tubular configuration (aspect ratio of one) and three uniform depth slot liner configurations (aspect ratio of nine). The predicted impedances for the ceramic tubular liner compare favorably with the impedances educed using the finite element method, with the exception of the frequency region surrounding the resonance. The impedances educed using the finite element method are also well matched to the parallel-element model predictions for the three slot liner configurations. In addition, impedances measured with a normal incidence impedance tube are demonstrated to be nearly identical to those educed with the finite element method. cv