Effects of a secondary high amplitude stimulus on the impedance of perforated plates

High sound pressure levels cause impedance changes in orifices and perforated plates due to vortex shedding and jet formation at the orifices. The effects of an additional high amplitude stimulus, unrelated in terms of frequency and phase, on the impedance of perforated plates received little attention. This work experimentally studies the impedance changes of perforated plates at various primary frequencies when an additional unrelated high-level single tone actuation is applied. It is shown that the impedance, the primary sound field faces, is altered dependent on the particle velocity induced in the orifices by the secondary actuation. Dimensionless quantities correlating the change of impedance with the secondary excitation are identified from the measurements and an empirical model for the change of resistance at quasi-steady flow conditions is derived. The results show that for low amplitude primary sound fields, the change of impedance is completely dependent on the secondary sound field. In case of a high amplitude primary sound field, the impedance is dependent on the particle velocities induced by both sound fields, whereas the larger induced particle velocity is the main contributor to the impedance changes. For unsteady flow conditions, a dependency on the frequency of the secondary actuation is found.