Characterization of Compliant-Backplate Helmholtz Resonators for an Electromechanical Acoustic Liner

Passive acoustic liners are currently used to reduce the noise radiated from aircraft engine nacelles. This study is the first phase in the development of an actively-tuned electromechanical acoustic liner that potentially offers improved noise suppression over conventional multi-layer liners. The underlying technical concept is based on the idea that the fundamental frequency of a Helmholtz resonator may be adjusted by adding degrees of freedom (DOF) via substitution of a rigid wall with a piezoelectric composite diaphragm coupled to a passive electrical shunt network. In this paper, a Helmholtz resonator containing a compliant aluminum diaphragm is investigated to provide a fundamental understanding of this two DOF system, before adding complexity via the piezoelectric composite material. Using lumped elements, an equivalent circuit model is derived, from which the transfer function and acoustic impedance are obtained. Additionally, a mass ratio is introduced that quantifies the amount of coupling between the elements of the system. The theory is then compared to experiment in a normal-incidence impedance tube. The experimental results confirm the additional DOF and overall acoustic behavior but also suggest the need for a more comprehensive analytical model to accurately predict the acoustic impedance. Nevertheless, the experiments demonstrate the potential benefits of this approach for the reduction of aircraft engine noise.