Impedance Theory and Measurements on Porous Acoustic Liners

The impedance of a point-reacting liner consisting of a thin, porous surface sheet backed by cavities filled with air or with porous materials has been predicted mathematically for a duct with flow. The mathematical model has been compared with experimental results obtained by using the two-microphone technique for various liners mounted in a 1-ft duct with flow at Mach numbers of 0 to 0.6. The predicted and the measured sound pressures were found to be in good agreement for liners consisting of a single layer of either air-filled or porous cavities covered by a thin porous sheet. As expected, the porous-cavity liners have a wider attenuated bandwidth and a higher attenuation at high frequencies than the air-cavity liners. Since porous cavities might be considered to be the equivalent of a large number of layers of air spaces separated by layers of porous plates, several such multilayer liners were constructed and tested. However, the multilayer liners did not perform like a porouscavity liner, and a full understanding of the operation of the multilayer liners is yet to be attained.