Wide-band sound-absorbing structures for aircraft engine ducts

With a view to providing sound absorption in a wide frequency range, one-and two-layer sound-absorbing structures, in which nets are used as absorbing layers, are investigated. A semiempirical theory of calculating the impedance characteristics of these structures is proposed. The theory takes into account the interaction between the net layers for a wide range of their perforation factors. A good agreement between theory and experiment is observed. It is shown that two-layer net structures are vastly superior to two-layer structures with perforated panels from the viewpoint of the sound absorbtion bandwidth.