Designs for construction of quiet composite sandwich structures.

Sandwich structures are comprised of a low-density core sandwiched between two thin, stiff facesheets, often composite laminates. The construction of the sandwich structure is designed to optimize the high-bending stiffness-to-weight ratio necessary for aerospace applications. Although the high stiffness-to-weight ratio of sandwich structures is ideal for bearing mechanical loads, a trade-off exists between mechanical and acoustic performance, in that increases in one generally come at the expense of the other. Improvement of TL of sandwich panels can be divided into three general categories: altering the panel construction by changing the core/facesheet materials, use of active or semi-active vibration suppression, and finally, added treatments such as foams, fibers, or other structures to either the exterior of the panel, or inside the core. TL of sandwich structures using these strategies is presented, in addition to explanation of the physical principals responsible for the decrease in transmitted sound. Featured approaches include design of core and facesheet materials, attachment of layered gasses to the panel facesheet, and insertion of midplane membrane-type metamaterial to the core. Advantages and disadvantages of each strategy are also considered and discussed.