Acoustic behavior of sandwich plates

Abstract

Sandwich structures of honeycomb cores and carbon laminate face sheets are ideal to meet low-weight and high-stiffness specifications. However, these structures are susceptible to noise problems. They tend to radiate structure-borne sound very efficiently or be transparent for air-borne sound. The problem’s root is the coincidence frequency, at which the wavelengths of beam and air coincide. In the vicinity of the coincidence frequency, observing wave fields with slanted incidence, the air’s trace wavelength can match the beam’s wavelength. Unlike homogenous structures, where wave trace matching only occurs in a narrow frequency band, sandwich structures can display this effect in a more than two decade wide frequency region. This facilitates noise transmission in a wide frequency band. The first focus of this paper is how to avoid this problem in the first place by designing the sandwich adequately. To check the region of wave trace matching, dispersion curves are used. A simplified but sufficiently accurate dispersion curve is proposed. The second focus lies in the interpretation of the dispersion curve, i.e. what is physically happening in the beam structure. The third focus is placed on the experimental identification of the sandwich beam structure’s shell parameters including its damping. Special attention is placed on which measures are necessary and which not in the experimental setup and experimental procedure to achieve sufficiently accurate results.