Measurements of the air-borne and structure-borne sound power transmission through a quiet 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 where gear meshing noise from the transmission has the most impact on speech intelligibility within the cabin. The roof section, framed by a grid of ribs, 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 frequency of the panel system to less than 500 Hz (well below the 1-4 kHz target range). Thin layers of viscoelastomer were included in the facesheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. The sound power transmitted through the panel due to a random incidence acoustic field, as well as due to a transverse force at a joint at a corner of the rib frame, was measured in NASA Langley's SALT facility. The optimized panel provides 6-11 dB of acoustic transmission loss improvement, and 6-15 dB of structure-borne sound reduction at critical rotorcraft transmission tonal frequencies. Reductions in sound power transmission agree well with simulations made using both simple infinite panel theory, as well as more detailed finite element/boundary element models.