Nonlinear Acoustic Phenomena in the Presence of Absorptive Liners in Circular Ducts

Abstract

High acoustic intensities are often encountered in acoustically lined ducts, such as in jet aircraft intake ducts and with gas turbine power-generating installations. Many acoustically absorptive materials have characteristics at high intensity which differ from those in the low-amplitude linear region. There are two basic nonlinear phenomena which are encountered with lined ducts at high intensities. First, the absorption properties of the material become nonlinear and, second, the propagation of the acoustic wave in the duct can no longer be described by linear acoustic theory. This is, of course, true even for a hard-walled duct where there is no liner present. A hard-walled circular duct has been employed which is driven by a horn-coupled high-intensity sound source and is anechoically terminated. Sound pressure measurements, waveform photographs, and spectra were obtained at a number of locations in the duct for the unlined case and for several absorptive liners. A previously reported high-order harmonic ringing effect has been further studied at various intensities and for different linear lengths. Possible theoretical explanations for the effect will be presented along with the experimental data and the additional complications caused when higher-order duct modes are present will also be discussed. [This work is supported under NSF Grant No. GK-32584.]