Design and Testing of a Hybrid Passive/Active Acoustic Treatment for Nacelle Inlets

Abstract

This paper deals with the design and testing of a hybrid acoustic treatment combining porous material properties and active control techniques, intended to reduce time-varying tones in flow-duct applications. A dedicated digital adaptive feedback control algorithm was developed to operate independently cell by cell, enabling easy subsequent increase of the liner surface. Large active panels of 54 cells were tested with pure sine waves from 850 Hz to 2 kHz for flow velocities up to Mach 0.3. Convergence was very rapid and stable for all cells. A detailed analysis of the experimental results is given, with comparisons with results for numerical simulation in the no-flow case. In agreement with predictions, active functioning was generally more effective up to 1 kHz, and the hybrid liner provided overall noise reduction that was largely independent of frequency.

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