Absorption Performance of an Anechoic Layer with a Steel Plate Backing at Oblique Incidence

Abstract

The sound absorption of steel-plate-backed anechoic coatings at oblique incidence is investigated theoretically and numerically. An analytical expression for the absorption coefficient is derived using elasticity theory. A 2D numerical periodic model suitable for inhomogeneous coatings under oblique incidence is built using the finite-element method. To interpret the absorption mechanism, the dispersion curves for guided elastic waves in a uniform coating and a steel plate are calculated using the spectral method. The absorption coefficients are analyzed with respect to the incident angle and frequency using elastic plate resonance theory. The effects of the steel plate backing and the attenuation of the coating are investigated. For the Alberich anechoic coating with a steel plate backing, the absorption coefficients are calculated numerically, and the absorption mechanism is investigated by structural displacement vectors and deformations. The following absorption characteristics can be found. In the frequency-angle spectra for absorption coefficients, the bright areas resulting from low-order Lamb waves in the coating are recognizable at low frequencies. A broad, strong anechoic area appears at large incident angles and low frequencies; under these conditions, the displacements of the steel plate and the 2D cavity are the main components of structural vibration. The resonance moves to low frequency when the thickness of the steel plate is incorporated and the cavity height is increased. At relatively high frequencies, some new absorption areas result from the resonance of periodic cavities and cover the contribution from the high-order Lamb waves in the coating caused by attenuation. The corresponding absorption coefficients decrease with an increasing angle of incidence.