A new hybrid active-passive absorber with variable surface impedance
Résumé
Hybrid absorber are developed at the LMFA for about 10 years now. They are composed of distinct cells that each consist of a thin porous layer backed by a cavity. The latter contains an error microphone and a secondary source. Active control is used to reduce the acoustic pressure at the rear side of the porous layer at low frequencies. This results in a cancellation of the imaginary part of the surface impedance. (See also the paper “Caract´erisation d'un absorbant hybride actif-passif par V´elocim´etrie Laser Doppler”.) Choosing a porous layer with a resistance that equals the characteristic impedance of air, a normal incident plane wave (of arbitrary frequency) can be perfectly absorbed. In the case of grazing incidence, however, the optimal impedance (i.e. the one that produces the best absorption) is neither purely real nor constant with frequency. As a consequence, the choice of the porous layer represents a tradeoff between low and high frequency performance. A new type of hybrid absorber is therefore proposed. It contains one microphone on either side of the porous layer. In the presence of grazing flow, the front face microphone can be protected by a microperforated plate of high open area ratio. Normal velocity is simply estimated through the pressure gradient, which allows the determination of the surface impedance. This estimation is then subtracted from a target impedance to form a new error signal. The control system is realized as a digital feedback algorithm. Simulations and experiments show that typical optimal impedance functions for liners can be obtained. Measurements in a grazing incidence tube confirm that transmission loss can be significantly increased in respect to the former absorber.
Origine : Accord explicite pour ce dépôt
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