A comparison of measured and computed sound pressure levels in a non-uniform acoustically lined duct

A comparison of measured and numerically calculated acoustical fields is presented for a non-uniform lined duct in the absence of appreciable mean flow. The frequency range investigated includes the “cut-on” frequencies of several transverse modes in certain portions of the duct. Measured pressure fields are compared to those predicted by one and two dimensional numerical models. The validity of the one dimensional model is confirmed for frequencies below cut-on of the first transverse mode. For higher frequencies the one dimensional model is clearly unsatisfactory, as might be expected. The two dimensional model gives reasonable results for frequencies below and above cut-on of the transverse modes, although it indicates a very strong sensitivity, of acoustical fields, to small variations in local wall impedance data.