Attenuation of sound in lined ducts

Abstract

Extensive computations have been carried out of the attenuation characteristics of resonator and porous type duct liners in rectangular and circular ducts. First the frequency dependence of the attenuation constant and the phase velocity of the fundamental duct mode are obtained for a large number of duct and liner parameters. Then, assuming that the fundamental mode is dominant in the lined duct element, the octave band transmission losses have been computed. The effect of the shape of the input spectrum is discussed and shown explicitly for three different spectra, namely, a "flat" spectrum and spectra with slopes of + 6 dB per octave and - 6 dB per octave, respectively. Finally the effect of the length of the duct liner on the octave band transmission loss has been computed. It is found that the octave band transmission loss does not increase linearly with the length of the duct liner, particularly in regions where the attenuation varies strongly with frequency.