A major discussion in the scientific community is the effect of the acoustic propagation direction being relative to the mean flow direction on the acoustic boundary condition posed by perforated liners. The reason being that the results from liner-impedance-eduction measurements show acoustic propagation upstream or downstream to the flow direction giving different resulting acoustical impedances. This paper contributes to this continuing effort to gain confidence in results obtained under different acoustical excitation and flow configurations. Instead of a traditional two-port configuration, by placing a perforate sample in a T-junction, this paper presents a three-port measurement technique. The transfer impedance of the perforate is determined under grazing as well as under normal incidence. Moreover, to study the effect of acoustic incidence relative to the flow directions, transfer impedance is also determined under the presence of grazing flow. A comparison of the measurement results with existing analytical and semi empirical models is also presented. An attempt to determine the nature of the transfer impedance under normal acoustic incidence is carried out and an analogous behavior between an empty T-junction and the perforated sample is proposed. 

The effect of high-level excitation on the acoustic properties of perforates, and formulation of the non-linear part of the impedance is under scientific discussion. Analytical models including the non-linear properties, as well as various experimental studies give varying results for the acoustic impedance. This paper aims to provide detailed results obtained under high-level excitation with different acoustic wave incidence and flow configurations. Contrary to the well-established two-port configuration, here, a three-port measurement technique is used to observe the acoustic impedance of the perforated plate using excitation from the three different directions. Plane wave propagation is considered and physical quantities such as in-hole particle velocity, which is calculated at the perforate sample, are used as the controlling parameters. This paper is an attempt to study the effect of high-level excitation on the acoustic behavior of the perforates with grazing and normal acoustic incidence. Moreover, the non-linear behavior of the perforate determined in presence of an external grazing flow is also discussed.

Multiple analytical, experimental, and numerical studies have been carriedout on perforates to study their properties under operating conditions, resultingin varying hypothesis and models to predict their performance. Theongoing effort of providing experimental results using multiform test setups iscontinued in this study. Incorporating the three-port technique, the passiveacoustic response of a perforated plate is studied under acoustic excitationfrom three directions in presence of grazing flow and high-level excitation.Similar to the in-situ method, usage of the three-port technique has an advantageof being a direct method for impedance determination and is not boundby any boundary conditions traditionally considered in presence of grazingflow. Extending the observations of previous studies, a semi-empirical modelis determined for the real part of the transfer impedance of a perforate, wherethe characterisation of the determined impedance on the testing parameterslike the Strouhal number, particle velocity, flow velocity and shear numberis displayed.