Time-Domain Evaluation of Acoustic Liner Impedance from LDV Measurements

Acoustic liners are passive sound absorbing materials ubiquitous to many domains of engineering, and are typically characterized by a quantity called impedance. When a liner is subject to a strong sound pressure level or in the presence of a shear grazing flow, the impedance changes due to nonlinear effects. The measurement of an impedance is often performed through inverse methods that rely on the effect of the impedance onto some other measurable quantities, such as the acoustic pressure or velocity above the sample. In the presence of such nonlinear behaviors in the liner, a time-domain approach could bring additional understanding into the complex intricacies of flow and noise interactions in the vicinity of the liner. One such approach is developed in the present work, where laser Doppler velocimetry measurements capable of capturing both the turbulence flow and acoustic fields above the liner are used in conjunction with a time-domain impedance model that does not rely on a wave propagation model to evaluate the effective impedance of the liner.