Two-microphone Technique Research Articles

Do Locally Reacting Acoustic Liners Always Behave as They Should?

BY definition, a liner is locally reacting (or point reacting) when the local response of the liner is dependent only upon the local acoustic pressure and not upon the structure of the acoustic field in the duct. This implies that no sound propagation may take place in the liner parallel to the liner surface. Helmholtz resonator-type acoustic liner materials are normally assumed to be locally reacting. However, it is shown that different behavior arises when neighboring cells are interconnected (e.g., by water drain holes) or when the cell cross dimensions are not small enough with respect to acoustic wavelength. The assumption of local reaction is then violated and the liner intended to be locally reacting will not behave as it should, that is, not as expected. In these cases the twomicrophone technique of in-situ impedance measurement will provide erroneous data. Further, it is shown that when using the two-microphone technique on really locally reacting liners the location of the surface microphone has to be chosen carefully in order to provide accurate data.

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.

Measurements of lining impedance in a duct with flow

The two-microphone technique has been used to obtain data from which values of acoustic impedance have been calculated for perforated-sheet linings. The magnitude and argument of the impedance values have been plotted as a function of frequency, wave amplitude, and duct Mach number for five different lining geometries. These results are compared to theoretical values of lining impedance obtained from a set of equations based on past work [E.J. Rice and C.E. Feiler, NASA TN D-6178 (1971); E.J. Rice, AIAA Paper 73–995 (1973); W.A. Bell, B.R. Daniel, and B.T. Zinn, AIAA Paper 74–61 (1974); A.S. Hersh and T. Rogers, AIAA Paper 75–495 (1975)]. Although the agreement between experiment and theory is good in general, it is not good in detail because the complicated flow field near the orifices is not fully understood nor completely modeled by the theory. Suggestions for possible improvements to the impedance model are discussed.