Quieting a rib-framed honeycomb core sandwich panel for a rotorcraft roof

A rotorcraft roof sandwich panel has been redesigned to optimize sound power transmission loss (TL) for frequencies between 1 and 4 kHz. The roof section, framed by a grid of ribs, was originally constructed of a single honeycomb core/composite facesheet panel. The original panel, characterized in [1], has coincidence frequencies near 700 Hz, leading to poor TL across the frequency range of 1 to 4 kHz. To quiet the panel, the section was split into two thinner sandwich subpanels separated by an air gap. The air gap was sized to target the fundamental mass-spring-mass resonance of the panel system to less than 500 Hz. The panels were designed to withstand structural loading from normal rotorcraft operation, as well as 'man-on-the-roof' static loads experienced during maintenance operations. Thin layers of VHB 9469 viscoelastomer from 3M were included in the facesheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. The optimized panel is expected to provide more than 10 dB transmission loss improvement at critical rotorcraft transmission tonal frequencies.