Multi-cell Digital Feedback Control for Noise Reduction Through Hybrid Absorbers

Abstract

This paper deals with the control aspect of a new type of hybrid acoustic absorbent developed in the LMFA Center for Acoustics. This hybrid cell aims to achieve large noise reduction over a wide frequency range by combining passive and active control. Active control is performed by using an adaptive digital feedback controller with the FXLMS algorithm applied to the IMC architecture. For extended liner surfaces, that is to say for MIMO structures, the main conclusion of the theoretical study is that acoustic coupling occurring between the hybrid cells can produce a global unstable behavior even if the optimal filter is obtained in each cell. An adaptive bandpass filter is therefore used to prevent the development of instabilities. This multi-cell algorithm has been experimentally validated for a 4-cell system located on a duct wall in the presence of flow.

Passive Layer Optimization for Active Absorbers in Flow Duct Applications

“…In this study only the passive part optimization process is considered. The active control system development can be carried out independently and is described in a complementary AIAA paper (see reference 5 ). The final objective of the hybrid cell concept is to provide a new absorbent treatment technology to reduce aircraft engine noise.…”

“…For a complete description of the active control system, see the complementary AIAA paper. 5 Finally several passive and active noise control measurements on Matisse flow duct are presented.…”

Experimental implementation of acoustic impedance control by a 2D network of distributed smart cells

“…The transmission loss is defined as the difference in decibels between the levels of incoming acoustic power upstream and outgoing power downstream when the acoustic treatment is turned on. If reflections of acoustic waves are neglected and only plane waves are assumed, the sound transmission loss can be written as [16] TL = 20 log…”