Duct modes conversion due to the presence of liners
Keywords:Acoustic impedance, Acoustic wave propagation, Acoustic wave reflection, Acoustic wave transmission, Attenuation, Ducts, Acoustic duct liners
AbstractThis paper deals with the classical problem of transmission-reflection and modes conversion due to the presence of acoustic liners in duct. The model consists of an infinite rectangular duct with an unlined and lined section. The approach is based on the modal calculation of the total acoustic sound power in a duct by the determination of the modal coefficients of the transmitted waves at the impedance discontinuity junction. For a given propagating mode (plane wave or higher order mode), incident from the rigid duct, the total sound power is calculated in the lined section by summing the acoustic power over all generated modes. The example studied here shows how the conversion affects the nature of the incident mode as a function of the admittance of the liner, frequency and mode number. It also allows for the quantification of the attenuation provided by an acoustic duct liner.
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